C# Interview Questions

C# 4.0 New Features

2011-10-13T13:07:00.000-07:00

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What are the new features introduced in c# 4.0?

This is very commonly asked c# interview question. This question is basically asked to check, if you are passionate about catching up with latest technological advancements. The list below shows a few of the new features introduced in c# 4.0. If you are aware of any other new features, please submit those using the from at the end of this post.

1. Optional and Named Parameters
2. COM Interoperability Enhancements
3. Covariance and Contravariance
4. Dynamic Type Introduction

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What's the difference between IEnumerable and List ?

2011-08-27T03:16:00.000-07:00

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1. IEnumerable is an interface, where as List is one specific implementation of IEnumerable. List is a class.

2. FOR-EACH loop is the only possible way to iterate through a collection of IEnumerable, where as List can be iterated using several ways. List can also be indexed by an int index, element can be added to and removed from and have items inserted at a particular index.

3. IEnumerable doesn't allow random access, where as List does allow random access using integral index.

4. In general from a performance standpoint, iterating thru IEnumerable is much faster than iterating thru a List.

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Explicit Interface Implementation

2011-08-07T01:23:00.000-07:00

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This C# Interview Question was asked my a member of this blog. Please refer to the example code below.

How to implement the void Method() of the interface in the following case ?

class B
{
   public void Method()
   {
       // some code here
       //...
   }
}

interface I
{
   void Method();
}

class D : B, I
{
   // how to implement the void Method() of the interface
   // public void I.Method() { ... }
}

To implement void Method, we use explicit interface implementation technique as shown below.

using System;
namespace SampleConsole
{
class Program
{
   static void Main()
   {
      //To Call Class B Method
      D d = new D();
      d.Method();

      //To Call the Interface Method
      I i = new D();
      i.Method();

      //Another way to call Interface method
      ((I)d).Method();
   }
}
class B
{
   public void Method()
   {
      Console.WriteLine("Void Method - B");
   }
}
interface I
{
   void Method();
}
class D : B, I
{
   void I.Method()
   {
      Console.WriteLine("Void Method - I");
   }
}
}

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Difference between EXE and DLL

2011-07-24T11:34:00.000-07:00

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1. .EXE is an executable file and can run by itself as an application, where as .DLL is usullay consumed by a .EXE or by another .DLL and we cannot run or execute .DLL directly.

2. For example, In .NET, compiling a Console Application or a Windows Application generates .EXE, where as compiling a Class Library Project or an ASP.NET web application generates .DLL. In .NET framework, both .EXE and .DLL are called as assemblies.

3. .EXE stands for executable, and .DLL stands for Dynamic Link Library

If you are aware of any other differences please post them using the form below.

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Unit Testing a private static method in C# .NET

2011-07-15T13:23:00.000-07:00

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In the previous article we have seen how to unit test private instance methods. In this article we will see unit testing static private methods. If you have not read the previous article on unit testing private instance methods, I would strongly recomend you to read, before you proceed with this article.

In general, to unit test a static public method, we invoke the respective method using the

class name in our test method. We then simply check for the expected and actual output. However, when it comes to unit testing a static private method, we cannot do the same, as the private members are not available outside the class. To make the process of unit testing static private members easier, microsoft unit testing framework has provided PrivateType class.

In the example below, CalculatePower() is a private static method. The purpose of this method is to calculate the value, when a given number is raised to a certain power. For example 2 to the power of 3 should return 8 and 3 to the power of 2 sholuld return 9. So to unit test this method we create the instance of PrivateType class. To the constructor of the PrivateType class we pass the type of the class that contains the private static method that we want to unit test. We do this by using the typeof keyword. The PrivateType instance can then be used to invoke the private static method that is contained with in the Maths class. The Maths class that contains the static private CalculatePower() method and the unit test are shown below.

public class Maths
{
   private static int CalculatePower(int Base, int Exponent)
   {
      int Product = 1;

      for (int i = 1; i <= Exponent; i++)
      {
         Product = Product * Base;
      }
      return Product;
   }
}

[TestMethod()]
public void CalculatePowerTest()
{
   PrivateType privateTypeObject = new PrivateType(typeof(Maths));
   object obj = privateTypeObject.InvokeStatic("CalculatePower", 2, 3);

   Assert.AreEqual(8, (int)obj);
}

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Unit Test private method in C# .NET

2011-07-15T04:43:00.000-07:00

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This is a very common c# interview question. As a developer all of us know, how to unit test public members of a class. All you do is create an instance of the respective

class and invoke the methods using the created instance. So, unit testing public methods is very straight forward, but if the method that we want to unit test is a private method, then we cannot access it outside the class and hence cannot easily unit test it.

Consider the example class shown below. CalculatePower() method with in the Maths class is private and we want to unit test this method. Also, note that CalculatePower() is an instance private method. In another articel we will discuss the concept of unit testing a private static method. Microsoft's unit testing assembly contains a class called PrivateObject, which can be used to unit test private methods very easily. Microsoft.VisualStudio.TestTools.UnitTesting.PrivateObject is the fully qualified name. Click here, to read an articel on, unit testing a private static method with an example.

public class Maths
{
   private int CalculatePower(int Base, int Exponent)
   {
      int Product = 1;

      for (int i = 1; i <= Exponent; i++)
      {
         Product = Product * Base;
      }
      return Product;
   }
}

To unit test this method, We create an instance of the class and pass the created instance to the constructor of PrivateObject class. Then we use the instance of the PrivateObject class, to invoke the private method. The fully completed unit test is shown below.

[TestMethod()]
public void CalculatePowerTest()
{
   Maths mathsclassObject = new Maths();
   PrivateObject privateObject = new PrivateObject(mathsclassObject);
   object obj = privateObject.Invoke("CalculatePower", 2, 3);
   Assert.AreEqual(8, (int)obj);
}

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C# Interview Questions Index

2011-06-05T06:38:00.001-07:00

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Search Results

2011-06-05T06:37:00.001-07:00

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Why C# does not support multiple class inheritance

2011-05-26T12:40:00.000-07:00

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Why C# does not support multiple class inheritance?
or
What are the problems of multiple class inheritance?

C# does not support multiple class inheritance because of the diamond problem that is associated, with multiple class inheritance. Let us understand the diamond problem of multiple class inheritance with an example.

As shown in the image above:
1. I have 2 classes - ClassB and ClassC
2. Both of these classes inherit from ClassA
3. Now, we have another class, ClassD which inherits from both ClassB and ClassC

So, if a method in ClassD calls a method defined in ClassA and ClassD has not overriden the invoked method. But both ClassB and ClassC have overridden the same method differently. Now, the ambiguity is, from which class does, ClassD inherit the invoked method: ClassB, or ClassC?

In order not to have these problems, C# does not support multiple class inheritance.

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What are the difference between interfaces and abstract classes

2011-05-26T11:29:00.000-07:00

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There are several differences between an abstract class and an interface as listed below.

1. Abstract classes can have implementations for some of its members, but the interface can't have implementation for any of its members.

2. Interfaces cannot have fields where as an abstract class can have fields.

3. An interface can inherit from another interface only and cannot inherit from an abstract class, where as an abstract class can inherit from another abstract class or another interface.

4. A class can inherit from multiple interfaces at the same time, where as a class cannot inherit from multiple classes at the same time.

5. Abstract class members can have access modifiers where as interface members cannot have access modifiers.

Another common C# Interview Question, that is commonly asked is, When do you choose interface over an abstract class or vice versa?
A general rule of thumb is, If you have an implementation that will be the same for all the derived classes, then it is better to go for an abstract class instead of an interface. So, when you have an interface, you can move your implementation to any class that implements the interface. Where as, when you have an abstract class, you can share implementation for all derived classes in one central place, and avoid code duplication in derived classes.

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Where did use delegates in your project - Part 2

2011-05-25T13:58:00.000-07:00

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In Part 1, we have seen how to create our Employee class. If you have not read Part 1, please do so before reading this part.

So now, the client who uses the Employee class has the flexibility of determining the logic on how they want to promote their employees a shown below. First create the employee objects - E1, E2 and E3. Populate the properties for the respective objects. We then create an employeeList to hold all the 3 employees.

Notice, the private Promote method that we have created. This method has the logic on how we want to promote our employees. The method is then passed as a parameter to the delegate. Also note, this method has the same signature, as that of EligibleToPromote delegate. This is very important, because Promote method cannot be passed as a parameter to the delegate if the signature differs. This is the reason why delegates are called as type safe function pointers.

So if we did not have the concept of delegates, it would not have been possible to pass a function as a parameter. As Promote method in the Employee class makes use of delegate, it is possible to dynamically decide the logic on how we want to promote employees.

In C Sharp 3.0, Lambda expressions are introduced. So, you can make use of lambda expressions instead of creating a function, and then an instance of a delegate and then passing the function as a parameter to the delegate. The sample example rewritten using Lambda expressions is shown below. Private Promote method is no longer required now.

The output of the above program should be as below.
Employee Test 2 Promoted
Employee Test 3 Promoted

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Where did use delegates in your project - Part 1

2011-05-25T13:05:00.000-07:00

Where did you use delegates in your project?
or
How did you use delegates in your project?
or
Usage of delegates in a Real Time Project?

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This is a very common c sharp interview question. Delegates is one of the very important aspects to understand. Most of the interviewers ask you to explain the usage of delegates in a real time project that you have worked on.

Delegates are extensively used by framework developers. Let us say we have a class called Employee as shown below.

Employee Class

The Employee class has the following properties.
1. Id
2. Name
3. Experience
4. Salary

Now, I want you to write a method in the Employee class, which can be used to promote employees. The method should take a list of Employee objects as a parameter, and should print the names of all the employees who are eligible for a promotion. But the logic, based on which the employee gets promoted should not be hard coded. At times, we may promote employees based on their experience and at times we may promote them based on their salary or may be some other condition. So, the logic to promote employees should not be hard coded with in the method.

To achieve this, we can make use of delegates. So, now I would design my class as shown below. We also, created a delegate EligibleToPromote. This delegate takes Employee object as a parameter and returns a boolean. In the Employee class, we have PromoteEmpoloyee method. This method takes in a list of Employees and a Delegate of type EligibleToPromote as parameters. The method, then loops thru each employee object, and passes it to the delegate. If the delegate returns true, then them Employee is promoted, else not promoted. So, with in the method we have not hard coded any logic on how we want to promote employees.

In Part 2 we will see how to consume, the Employee class that we have created.

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C# Interview Questions on Delegates

2011-05-24T14:06:00.000-07:00

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What is a delegate?
A delegate is a type safe function pointer. Using delegates you can pass methods as parameters. To pass a method as a parameter, to a delegate, the signature of the method must match the signature of the delegate. This is why, delegates are called type safe function pointers.

What is the main use of delegates in C#?
Delegates are mainly used to define call back methods.

What do you mean by chaining delegates?
Or
What is a multicast delegate?
The capability of calling multiple methods on a single event is called as chaining delegates. Let me give you an example to understand this further.
1. Create a new asp.net web application
2. Drag and drop a button control and leave the ID as Button1.
3. On the code behind file, add the code shown below.

When you click the Button now, both Method1 and Method2 will be executed. So, this capability of calling multiple methods on a single event is called as chaining delegates. In the example, we are using EventHandler delegate, to hook up Method1 and Method2 to the click event of the button control. Since, the EventHandler delegate is now pointing to multiple methods, it is also called as multicast delegate.

Will the following code compile?

No, the code does not compile. For the code to compile, the signature of Method1 should match the signature of SampleDelegate.

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What are the advantages of using interfaces

2011-05-23T11:36:00.000-07:00

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This is the most commonly asked interview question. This interview question is being asked in almost all the dot net interviews. It is very important that we understand all the concepts of interfaces and abstract classes.

Interfaces are very powerful. If properly used, interfaces provide all the advantages as listed below.

1. Interfaces allow us to implement polymorphic behaviour. Ofcourse, abstract classes can also be used to implement polymorphic behaviour.

2. Interfaces allow us to develop very loosely coupled systems.

3. Interfaces enable mocking for better unit testing.

4. Interfaces enables us to implement multiple class inheritance in C#.

5. Interfaces are great for implementing Inverson of Control or Dependancy Injection.

6. Interfaces enable parallel application development.

If you can think of any other advantages of using interfaces, or if you can add more explanation, please feel free to do so using the form below.

For basic interview questions on interfaces, you can read this article.

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C# Interview Questions by topic

2011-05-22T06:01:00.001-07:00

On this page you can see list of all C# Interview Questions by topic. In most of the interviews, these questions are very frequently asked. If you have an asp.net question that is asked in a previous interview, please post it here.

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Advantages and disadvantages of using generics in C#

2011-05-05T13:28:00.000-07:00

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To better understand the advantages and disadvantages of generics, it is better you read the below 2 articles first.
1. Click here to read about Advantages and disadvantages of Arrays
2. Click here to read about Advantages and disadvantages of System.Collections

In Microsoft.NET version 1.0 there were collections, such as the ArrayList for working with groups of objects. An ArrayList is much like an array, except it could automatically grow and offered many convenience methods that arrays don't have. The problem with ArrayList and all the other .NET v1.0 collections is that they operate on type object. Since all objects derive from the object type, you can assign anything to an ArrayList. The problem with this is that you incur performance overhead converting value type objects to and from the object type and a single ArrayList could accidentally hold different types, which would cause a hard to find errors at runtime because you wrote code to work with one type. Generic collections fix these problems.

A generic collection is strongly typed (type safe), meaning that you can only put one type of object into it. This eliminates type mismatches at runtime. Another benefit of type safety is that performance is better with value type objects because they don't incur overhead of being converted to and from type object. With generic collections, you have the best of all worlds because they are strongly typed, like arrays, and you have the additional functionality, like ArrayList and other non-generic collections, without the problems.

It is always good to use generics rather than using ArrayList,Hashtable etc, found in System.Collections namespace. The only reason why you may want to use System.Collections is for backward compatibility.

I cannot think of any disadvantages of using generics at the moment. Please feel free to comment if you are aware of any disadvantages.

The screen shot below shows, the generics collection classes and their respective non generic counterparts.

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What are the advantages and disadvantages of using collection classes present in System.Collections namespace

2011-03-18T16:51:00.000-07:00

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We will understand the advantages and disadvantages of using collection classes present in System.Collections namespace, using ArrayList collection class. The same advantages and disadvantages apply to all other collection classes like Stack, Queue and Hashtable classes.

Advantages of using ArrayList:
1. ArrayList can grow in size dynamcally. In the example below, the Numbers ArrayList initial size is set 2. But we have added 3 elements. This proves that ArrayList, and the rest of the collection classes like Stack, Queue and Hashtable can grow in size dynamically. If Numbers, was an integer array, then we would have run into Index Out of Range compiler error.

2. ArrayList provide several convinient methods to add and remove elements to the collection. You can use the Add(), Remove() etc which are very handy to add and remove elements respectively. Similarly the Stack, Queue and Hashtable classes have thier respective methods, to add or remove the elements.

Disadvantages of using ArrayList:
ArrayList and all other collection classes like stack, queue and hashtable which are present in System.Collection namespace operate on object and hence are loosely typed. The loosely typed nature of these collections make them vulnerable to runtime errors. Click here for an example on how the loosely typed nature of an ArrayList can cause runtime erros.

Loosley typed collections can also cause performance overhead, because boxing and unboxing happens. In the example below, Numbers is an arraylist. We are stroing 10 and 20 which are integers and value types. Since, arraylist operate on object type, and object type is a

reference type, the value 10 is boxed and converted into a reference type. The same is the case with integer 20. If we store 100 integers in the arraylist. All the 100 intgers are boxed, meaning converted into reference types and then stored in the collection.

When we try to retrieve the elements out of the collection, we covert the object type back to integer type, unboxing happens. So this unnecessary boxing and unboxing happens behind the scenes everytime we add and remove value types to the collection classes present in System.Collections namespace. This can severly affect the performance, especially if your collections are large. To solve this problem, we have generics introduced in dotnet. Click here for the advantages and disadvantages of using generics.

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What are the advantages and disadvantages of using arrays

2011-03-18T16:32:00.000-07:00

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Advantages of using arrays:
1. Arrays are strongly typed, meaning you can only have one type of elements in the array. The strongly typed nature of arrays gives us 2 advantages. One, the performance will be much better because boxing and unboxing will not happen. Second, run time errors can be prevented because of type mis matches. Type mis matches and runtime errors are most commonly seen with collection classes like ArrayList, Queue, Stack etc, that are present in System.Collections namespace.

In the example below, Numbers is an integer array. When we try to store a string in the integer array, a compiler error is reported stating cannot implicitly convert string to integer. This is why we call arrays are strongly typed.

In the example below, Numbers is an ArrayList. Collections of type arraylist are loosely typed. This means any type of elements can be added to the collection. ArrayList operate on object type, which makes them loosely typed. No compiler error is reported, but when we run the application, a runtime error is reported as shown. In software development, it is always better to catch errors at compile time rather than at runtime.

Disadvantages of using arrays:
1. Arrays are fixed in size and cannot grow over time, where ArrayList in System.Collections namespace can grow dynamically.
2. Arrays are zero index based, and hence a little difficult to work with. The only way to store or retrieve elements from arrays, is to use integral index. Arrays donot provide convinient methods like Add(), Remove() etc provided by collection classes found in System.Collections or System.Collections.Generics namespaces, which are very easy to work with.

Another common C# interview question, that is asked is, what are the advantages and disadvantages of using collection classes present in System.Collections namespace. Click here to learn about the advantages and disadvantages of using Systen.Collections namespace classes.

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Remoting - Interview Questions

2010-04-26T08:49:00.000-07:00

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What is .NET Remoting?
.NET Remoting allows objects to interact with one another across application domains.

What are the 2 message encoding formats supported by .NET Remoting and when do you choose one over the other?
Message Encoding Formats:
1. Binary encoding.
2. XML encoding.

Applications can use binary encoding where performance is critical, or XML encoding where interoperability with other remoting frameworks is essential.

What are the two types of .NET remote objects?
1. Client-activated objects - Client-activated objects are under the control of a lease-based lifetime manager that ensures that the object is garbage collected when its lease expires.
2. Server-activated objects - In the case of server-activated objects, developers have a choice of selecting either a "single call" or "singleton" model. The lifetime of singletons are also controlled by lease-based lifetime.

What is considered as Remote Object?
Any object outside the application domain of the calling appication is considered remote object, even if the objects are executing on the same machine.

Can you treat every object as a remote object?
Objects that cannot be serialized cannot be passed to a different application domain and are therefore nonremotable.

What are the ways in which an object can be serialized?
1. Mark your class with serializable attribute.
2. Make your class implement ISerializable interface.

How can you change an object into a remote object?
Any object can be changed into a remote object by deriving it from MarshalByRefObject.

What happens when a client activates a remote object?
When a client activates a remote object, it receives a proxy to the remote object. All operations on this proxy are appropriately indirected to enable the remoting infrastructure to intercept and forward the calls appropriately.

What are proxy objects and what is the use of these proxy objects?
Proxy objects are created when a client activates a remote object. The proxy object acts as a representative of the remote object and ensures that all calls made on the proxy are forwarded to the correct remote object instance.

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C# Interview Questions related to Interfaces.

2009-07-09T16:46:00.000-07:00

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Explain what is an Interface in C#?
An Interface in C# is created using the interface keyword. An example is shown below.

using System;
namespace Interfaces
{
interface IBankCustomer
{
void DepositMoney();
void WithdrawMoney();
}
public class Demo : IBankCustomer
{
public void DepositMoney()
{
Console.WriteLine("Deposit Money");
}

public void WithdrawMoney()
{
Console.WriteLine("Withdraw Money");
}

public static void Main()
{
Demo DemoObject = new Demo();
DemoObject.DepositMoney();
DemoObject.WithdrawMoney();
}
}
}

In our example we created IBankCustomer interface. The interface declares 2 methods.
1. void DepositMoney();
2. void WithdrawMoney();

Notice that method declarations does not have access modifiers like public, private, etc. By default all interface members are public. It is a compile time error to use access modifiers on interface member declarations. Also notice that the interface methods have only declarations and not implementation. It is a compile time error to provide implementation for any interface member. In our example as the Demo class is inherited from the IBankCustomer interface, the Demo class has to provide the implementation for both the methods (WithdrawMoney() and DepositMoney()) that is inherited from the interface. If the class fails to provide implementation for any of the inherited interface member, a compile time error will be generated. Interfaces can consist of methods, properties, events, indexers, or any combination of those four member types. When a class or a struct inherits an interface, the class or struct must provide implementation for all of the members declared in the interface. The interface itself provides no functionality that a class or struct can inherit in the way that base class functionality can be inherited. However, if a base class implements an interface, the derived class inherits that implementation.

Can an Interface contain fields?
No, an Interface cannot contain fields.

What is the difference between class inheritance and interface inheritance?
Classes and structs can inherit from interfaces just like how classes can inherit a base class or struct. However there are 2 differences.
1. A class or a struct can inherit from more than one interface at the same time where as A class or a struct cannot inherit from more than one class at the same time. An example depicting the same is shown below.

using System;
namespace Interfaces
{
interface Interface1
{
void Interface1Method();
}
interface Interface2
{
void Interface2Method();
}
class BaseClass1
{
public void BaseClass1Method()
{
Console.WriteLine("BaseClass1 Method");
}
}
class BaseClass2
{
public void BaseClass2Method()
{
Console.WriteLine("BaseClass2 Method");
}
}

//Error : A class cannot inherit from more than one class at the same time
//class DerivedClass : BaseClass1, BaseClass2
//{
//}

//A class can inherit from more than one interface at the same time
public class Demo : Interface1, Interface2
{
public void Interface1Method()
{
Console.WriteLine("Interface1 Method");
}

public void Interface2Method()
{
Console.WriteLine("Interface2 Method");
}

public static void Main()
{
Demo DemoObject = new Demo();
DemoObject.Interface1Method();
DemoObject.Interface2Method();
}
}
}

2. When a class or struct inherits an interface, it inherits only the method names and signatures, because the interface itself contains no implementations.

Can an interface inherit from another interface?
Yes, an interface can inherit from another interface. It is possible for a class to inherit an interface multiple times, through base classes or interfaces it inherits. In this case, the class can only implement the interface one time, if it is declared as part of the new class. If the inherited interface is not declared as part of the new class, its implementation is provided by the base class that declared it. It is possible for a base class to implement interface members using virtual members; in that case, the class inheriting the interface can change the interface behavior by overriding the virtual members.

Can you create an instance of an interface?
No, you cannot create an instance of an interface.

If a class inherits an interface, what are the 2 options available for that class?
Option 1: Provide Implementation for all the members inheirted from the interface.

namespace Interfaces
{
interface Interface1
{
void Interface1Method();
}

class BaseClass1 : Interface1
{
public void Interface1Method()
{
Console.WriteLine("Interface1 Method");
}
public void BaseClass1Method()
{
Console.WriteLine("BaseClass1 Method");
}
}
}

Option 2: If the class does not wish to provide Implementation for all the members inheirted from the interface, then the class has to be marked as abstract.

namespace Interfaces
{
interface Interface1
{
void Interface1Method();
}

abstract class BaseClass1 : Interface1
{
abstract public void Interface1Method();
public void BaseClass1Method()
{
Console.WriteLine("BaseClass1 Method");
}
}
}

A class inherits from 2 interfaces and both the interfaces have the same method name as shown below. How should the class implement the drive method for both Car and Bus interface?
namespace Interfaces
{
interface Car
{
void Drive();
}
interface Bus
{
void Drive();
}

class Demo : Car,Bus
{
//How to implement the Drive() Method inherited from Bus and Car
}
}

To implement the Drive() method use the fully qualified name as shown in the example below. To call the respective interface drive method type cast the demo object to the respective interface and then call the drive method.

using System;
namespace Interfaces
{
interface Car
{
void Drive();
}
interface Bus
{
void Drive();
}

class Demo : Car,Bus
{
void Car.Drive()
{
Console.WriteLine("Drive Car");
}
void Bus.Drive()
{
Console.WriteLine("Drive Bus");
}

static void Main()
{
Demo DemoObject = new Demo();
((Car)DemoObject).Drive();
((Bus)DemoObject).Drive();
}
}
}

What do you mean by "Explicitly Implemeting an Interface". Give an example?
If a class is implementing the inherited interface member by prefixing the name of the interface, then the class is "Explicitly Implemeting an Interface member". The disadvantage of Explicitly Implemeting an Interface member is that, the class object has to be type casted to the interface type to invoke the interface member. An example is shown below.

using System;
namespace Interfaces
{
interface Car
{
void Drive();
}

class Demo : Car
{
// Explicit implementation of an interface member
void Car.Drive()
{
Console.WriteLine("Drive Car");
}

static void Main()
{
Demo DemoObject = new Demo();

//DemoObject.Drive();
// Error: Cannot call explicitly implemented interface method
// using the class object.
// Type cast the demo object to interface type Car
((Car)DemoObject).Drive();
}
}
}

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C# Interview Questions on partial classes, structs and methods.

2009-07-08T19:09:00.000-07:00

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What is a partial class. Give an example?
A partial class is a class whose definition is present in 2 or more files. Each source file contains a section of the class, and all parts are combined when the application is compiled. To split a class definition, use the partial keyword as shown in the example below. Student class is split into 2 parts. The first part defines the study() method and the second part defines the Play() method. When we compile this program both the parts will be combined and compiled. Note that both the parts uses partial keyword and public access modifier.

using System;
namespace PartialClass
{
  public partial class Student
  {
    public void Study()
    {
      Console.WriteLine("I am studying");
    }
  }
  public partial class Student
  {
    public void Play()
    {
      Console.WriteLine("I am Playing");
    }
  }
  public class Demo
  {
    public static void Main()
    {
      Student StudentObject = new Student();
      StudentObject.Study();
      StudentObject.Play();
    }
  }
}

It is very important to keep the following points in mind when creating partial classes.
1. All the parts must use the partial keyword.
2. All the parts must be available at compile time to form the final class.
3. All the parts must have the same access modifiers - public, private, protected etc.
4. Any class members declared in a partial definition are available to all the other parts.
5. The final class is the combination of all the parts at compile time.

What are the advantages of using partial classes?
1. When working on large projects, spreading a class over separate files enables multiple programmers to work on it at the same time.

2. When working with automatically generated source, code can be added to the class without having to recreate the source file. Visual Studio uses this approach when it creates Windows Forms, Web service wrapper code, and so on. You can create code that uses these classes without having to modify the file created by Visual Studio.

Is it possible to create partial structs, interfaces and methods?
Yes, it is possible to create partial structs, interfaces and methods. We can create partial structs, interfaces and methods the same way as we create partial classes.

Will the following code compile?
using System;
namespace PartialClass
{
  public partial class Student
  {
    public void Study()
    {
      Console.WriteLine("I am studying");
    }
  }
  public abstract partial class Student
  {
    public void Play()
    {
      Console.WriteLine("I am Playing");
    }
  }
  public class Demo
  {
    public static void Main()
    {
      Student StudentObject = new Student();
    }
  }
}

No, a compile time error will be generated stating "Cannot create an instance of the abstract class or interface "PartialClass.Student". This is because, if any part is declared abstract, then the whole class becomes abstract. Similarly if any part is declared sealed, then the whole class becomes sealed and if any part declares a base class, then the whole class inherits that base class.

Can you create partial delegates and enumerations?
No, you cannot create partial delegates and enumerations.

Can different parts of a partial class inherit from different interfaces?
Yes, different parts of a partial class can inherit from different interfaces.

Can you specify nested classes as partial classes?
Yes, nested classes can be specified as partial classes even if the containing class is not partial. An example is shown below.

class ContainerClass
{
  public partial class Nested
  {
    void Test1() { }
  }
  public partial class Nested
  {
    void Test2() { }
  }
}

How do you create partial methods?
To create a partial method we create the declaration of the method in one part of the partial class and implementation in the other part of the partial class. The implementation is optional. If the implementation is not provided, then the method and all the calls to the method are removed at compile time. Therefore, any code in the partial class can freely use a partial method, even if the implementation is not supplied. No compile-time or run-time errors will result if the method is called but not implemented. In summary a partial method declaration consists of two parts. The definition, and the implementation. These may be in separate parts of a partial class, or in the same part. If there is no implementation declaration, then the compiler optimizes away both the defining declaration and all calls to the method.

The following are the points to keep in mind when creating partial methods.
1. Partial method declarations must begin partial keyword.
2. The return type of a partial method must be void.
3. Partial methods can have ref but not out parameters.
4. Partial methods are implicitly private, and therefore they cannot be virtual.
5. Partial methods cannot be extern, because the presence of the body determines whether they are defining or implementing.

What is the use of partial methods?
Partial methods can be used to customize generated code. They allow for a method name and signature to be reserved, so that generated code can call the method but the developer can decide whether to implement the method. Much like partial classes, partial methods enable code created by a code generator and code created by a human developer to work together without run-time costs.

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Nested Types in C#

2009-07-07T17:36:00.000-07:00

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What is a nested type. Give an example?
A type(class or a struct) defined inside another class or struct is called a nested type. An example is shown below. InnerClass is inside ContainerClass, Hence InnerClass is called as nested class.

using System;
namespace Nested
{
  class ContainerClass
  {
    class InnerClass
    {
      public string str = "A string variable in nested class";
    }

    public static void Main()
    {
      InnerClass nestedClassObj = new InnerClass();
      Console.WriteLine(nestedClassObj.str);
    }
  }
}

Will the following code compile?
using System;
namespace Nested
{
  class ContainerClass
  {
    class InnerClass
    {
      public string str = "A string variable in nested class";
    }
  }

  class Demo
  {
    public static void Main()
    {
      InnerClass nestedClassObj = new InnerClass();
      Console.WriteLine(nestedClassObj.str);
    }
  }
}

No, the above code will generate a compile time error stating - The type or namespace name 'InnerClass' could not be found (are you missing a using directive or an assembly reference?). This is bcos InnerClass is inside ContainerClass and does not have any access modifier. Hence inner class is like a private member inside ContainerClass. For the above code to compile and run, we should make InnerClass public and use the fully qualified name when creating the instance of the nested class as shown below.

using System;
namespace Nested
{
  class ContainerClass
  {
    public class InnerClass
    {
      public string str = "A string variable in nested class";
    }
  }

  class Demo
  {
    public static void Main()
    {
      ContainerClass.InnerClass nestedClassObj = new ContainerClass.InnerClass();
      Console.WriteLine(nestedClassObj.str);
    }
  }
}

Can the nested class access, the Containing class. Give an example?
Yes, the nested class, or inner class can access the containing or outer class as shown in the example below. Nested types can access private and protected members of the containing type, including any inherited private or protected members.

using System;
namespace Nested
{
  class ContainerClass
  {
    string OuterClassVariable = "I am an outer class variable";

    public class InnerClass
    {
      ContainerClass ContainerClassObject = new ContainerClass();
      string InnerClassVariable = "I am an Inner class variable";
      public InnerClass()
      {
        Console.WriteLine(ContainerClassObject.OuterClassVariable);
        Console.WriteLine(this.InnerClassVariable);
      }
    }
  }

  class Demo
  {
    public static void Main()
    {
      ContainerClass.InnerClass nestedClassObj = new ContainerClass.InnerClass();
    }
  }
}

What is the ouput of the following program?
using System;
namespace Nested
{
  class ContainerClass
  {
    public ContainerClass()
    {
      Console.WriteLine("I am a container class");
    }

    public class InnerClass : ContainerClass
    {
      public InnerClass()
      {
        Console.WriteLine("I am an inner class");
      }
    }
  }

  class DemoClass : ContainerClass.InnerClass
  {
    public DemoClass()
    {
      Console.WriteLine("I am a Demo class");
    }
    public static void Main()
    {
      DemoClass DC = new DemoClass();
    }
  }
}

Output:
I am a container class
I am an inner class
I am a Demo class

The above program has used the concepts of inheritance and nested classes. The ContainerClass is at the top in the inheritance chain. The nested InnerClass derives from outer ContainerClass. Finally the DemoClass derives from nested InnerClass. As all the 3 classes are related by inheritance we have the above output.

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C# Interview Questions on Destructors

2009-07-06T17:58:00.000-07:00

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What is a Destructor?
A Destructor has the same name as the class with a tilde character and is used to destroy an instance of a class.

Can a class have more than 1 destructor?
No, a class can have only 1 destructor.

Can structs in C# have destructors?
No, structs can have constructors but not destructors, only classes can have destructors.

Can you pass parameters to destructors?
No, you cannot pass parameters to destructors. Hence, you cannot overload destructors.

Can you explicitly call a destructor?
No, you cannot explicitly call a destructor. Destructors are invoked automatically by the garbage collector.

Why is it not a good idea to use Empty destructors?
When a class contains a destructor, an entry is created in the Finalize queue. When the destructor is called, the garbage collector is invoked to process the queue. If the destructor is empty, this just causes a needless loss of performance.

Is it possible to force garbage collector to run?
Yes, it possible to force garbage collector to run by calling the Collect() method, but this is not considered a good practice because this might create a performance over head. Usually the programmer has no control over when the garbage collector runs. The garbage collector checks for objects that are no longer being used by the application. If it considers an object eligible for destruction, it calls the destructor(if there is one) and reclaims the memory used to store the object.

Usually in .NET, the CLR takes care of memory management. Is there any need for a programmer to explicitly release memory and resources? If yes, why and how?
If the application is using expensive external resource, it is recommend to explicitly release the resource before the garbage collector runs and frees the object. We can do this by implementing the Dispose method from the IDisposable interface that performs the necessary cleanup for the object. This can considerably improve the performance of the application.

When do we generally use destructors to release resources?
If the application uses unmanaged resources such as windows, files, and network connections, we use destructors to release resources.

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C# Interview Questions on constructors

2009-07-01T19:59:00.000-07:00

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What is a constructor in C#?
Constructor is a class method that is executed when an object of a class is created. Constructor has the same name as the class, and usually used to initialize the data members of the new object.

In C#, What will happen if you do not explicitly provide a constructor for a class?
If you do not provide a constructor explicitly for your class, C# will create one by default that instantiates the object and sets all the member variables to their default values.

Structs are not reference types. Can structs have constructors?
Yes, even though Structs are not reference types, structs can have constructors.

We cannot create instances of static classes. Can we have constructors for static classes?
Yes, static classes can also have constructors.

Can you prevent a class from being instantiated?
Yes, a class can be prevented from being instantiated by using a private constructor as shown in the example below.

using System;
namespace TestConsole
{
  class Program
  {
    public static void Main()
    {
      //Error cannot create instance of a class with private constructor
      SampleClass SC = new SampleClass();
    }
  }
  class SampleClass
  {
    double PI = 3.141;
    private SampleClass()
    {
    }
  }
}

Can a class or a struct have multiple constructors?
Yes, a class or a struct can have multiple constructors. Constructors in csharp can be overloaded.

Can a child class call the constructor of a base class?
Yes, a child class can call the constructor of a base class by using the base keyword as shown in the example below.

using System;
namespace TestConsole
{
  class BaseClass
  {
    public BaseClass(string str)
    {
      Console.WriteLine(str);
    }
  }

  class ChildClass : BaseClass
  {
    public ChildClass(string str): base(str)
    {
    }

    public static void Main()
    {
      ChildClass CC = new ChildClass("Calling base class constructor from child class");
    }
  }
}

If a child class instance is created, which class constructor is called first - base class or child class?
When an instance of a child class is created, the base class constructor is called before the child class constructor. An example is shown below.

using System;
namespace TestConsole
{
  class BaseClass
  {
    public BaseClass()
    {
      Console.WriteLine("I am a base class constructor");
    }
  }
  class ChildClass : BaseClass
  {
    public ChildClass()
    {
      Console.WriteLine("I am a child class constructor");
    }
    public static void Main()
    {
      ChildClass CC = new ChildClass();
    }
  }
}

Will the following code compile?
using System;
namespace TestConsole
{
  class BaseClass
  {
    public BaseClass(string str)
    {
      Console.WriteLine(str);
    }
  }
  class ChildClass : BaseClass
  {
    public ChildClass()
    {
      Console.WriteLine("I am a child class constructor");
    }
    public static void Main()
    {
      ChildClass CC = new ChildClass();
    }
  }
}

No, the above code will not compile. This is because, if a base class does not offer a default constructor, the derived class must make an explicit call to a base class constructor by using the base keyword as shown in the example below.

using System;
namespace TestConsole
{
  class BaseClass
  {
    public BaseClass(string str)
    {
      Console.WriteLine(str);
    }
  }
  class ChildClass : BaseClass
  {
    //Call the base class contructor from child class
    public ChildClass() : base("A call to base class constructor")
    {
      Console.WriteLine("I am a child class constructor");
    }
    public static void Main()
    {
      ChildClass CC = new ChildClass();
    }
  }
}

Can a class have static constructor?
Yes, a class can have static constructor. Static constructors are called automatically, immediately before any static fields are accessed, and are generally used to initialize static class members. It is called automatically before the first instance is created or any static members are referenced. Static constructors are called before instance constructors. An example is shown below.

using System;
namespace TestConsole
{
  class Program
  {
    static int I;
    static Program()
    {
      I = 100;
      Console.WriteLine("Static Constructor called");
    }
    public Program()
    {
      Console.WriteLine("Instance Constructor called");
    }
    public static void Main()
    {
      Program P = new Program();
    }
  }
}

Can you mark static constructor with access modifiers?
No, we cannot use access modifiers on static constructor.

Can you have parameters for static constructors?
No, static constructors cannot have parameters.

What happens if a static constructor throws an exception?
If a static constructor throws an exception, the runtime will not invoke it a second time, and the type will remain uninitialized for the lifetime of the application domain in which your program is running.

Give 2 scenarios where static constructors can be used?
1. A typical use of static constructors is when the class is using a log file and the constructor is used to write entries to this file.
2. Static constructors are also useful when creating wrapper classes for unmanaged code, when the constructor can call the LoadLibrary method.

Does C# provide copy constructor?
No, C# does not provide copy constructor.

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C# Interview Questions on Methods / Functions

2009-07-01T19:35:00.000-07:00

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Is the following code legal?
using System;
namespace Demo
{
class Program
{
  public static void Main()
  {

  }
  public void Sum(int FirstNumber, int SecondNumber)
  {
   int Result = FirstNumber + SecondNumber;
  }

  public int Sum(int FirstNumber, int SecondNumber)
  {
   int Result = FirstNumber + SecondNumber;
  }
}
}

No, The above code does not compile. You cannot overload a method based on the return type. To overload a method in C# either the number or type of parameters should be different. In general the return type of a method is not part of the signature of the method for the purposes of method overloading. However, it is part of the signature of the method when determining the compatibility between a delegate and the method that it points to.

What is the difference between method parameters and method arguments. Give an example?
In the example below FirstNumber and SecondNumber are method parameters where as FN and LN are method arguments. The method definition specifies the names and types of any parameters that are required. When calling code calls the method, it provides concrete values called arguments for each parameter. The arguments must be compatible with the parameter type but the argument name (if any) used in the calling code does not have to be the same as the parameter named defined in the method.

using System;
namespace Demo
{
class Program
{
  public static void Main()
  {
   int FN = 10;
   int SN = 20;
   //FN and LN are method arguments
   int Total = Sum(FN, SN);
   Console.WriteLine(Total);
  }
  //FirstNumber and SecondNumber are method parameters
  public static int Sum(int FirstNumber, int SecondNumber)
  {
   int Result = FirstNumber + SecondNumber;
   return Result;
  }
}
}

Explain the difference between passing parameters by value and passing parameters by reference with an example?
We can pass parameters to a method by value or by reference. By default all value types are passed by value where as all reference types are passed by reference. By default, when a value type is passed to a method, a copy is passed instead of the object itself. Therefore, changes to the argument have no effect on the original copy in the calling method.An example is shown below.

using System;
namespace Demo
{
class Program
{
  public static void Main()
  {
   int I = 10;
   int K = Function(I);

   Console.WriteLine("I = " + I);
   Console.WriteLine("K = " + K);
  }
  public static int Function(int Number)
  {
   int ChangedValue = Number + 1;
   return ChangedValue;
  }
}
}

By default, reference types are passed by reference. When an object of a reference type is passed to a method, the reference points to the original object, not a copy of the object. Changes made through this reference will therefore be reflected in the calling method. Reference types are created by using the class keyword as shown in the example below.

using System;
namespace Demo
{
class Program
{
  public static void Main()
  {
   ReferenceTypeExample Object = new ReferenceTypeExample();
   Object.Number = 20;
   Console.WriteLine("Original Object Value = " + Object.Number);
   Function(Object);
   Console.WriteLine("Object Value after passed to the method= " + Object.Number);
  }
  public static void Function(ReferenceTypeExample ReferenceTypeObject)
  {
   ReferenceTypeObject.Number = ReferenceTypeObject.Number + 5;
  }
}

class ReferenceTypeExample
{
  public int Number;
}
}

Can you pass value types by reference to a method?
Yes, we can pass value types by by reference to a method. An example is shown below.

using System;
namespace Demo
{
class Program
{
  public static void Main()
  {
   int I = 10;
   Console.WriteLine("Value of I before passing to the method = " + I);
   Function(ref I);
   Console.WriteLine("Value of I after passing to the method by reference= " + I);
  }
  public static void Function(ref int Number)
  {
   Number = Number + 5;
  }
}
}

If a method's return type is void, can you use a return keyword in the method?
Yes, Even though a method's return type is void, you can use the return keyword to stop the execution of the method as shown in the example below.
using System;
namespace Demo
{
class Program
{
  public static void Main()
  {
   SayHi();
  }
  public static void SayHi()
  {
   Console.WriteLine("Hi");
   return;
   Console.WriteLine("This statement will never be executed");
  }
}
}

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C# Interview Questions on Properties

2009-03-02T18:05:00.000-08:00

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What are Properties in C#. Explain with an example?
Properties in C# are class members that provide a flexible mechanism to read, write, or compute the values of private fields. Properties can be used as if they are public data members, but they are actually special methods called accessors. This enables data to be accessed easily and still helps promote the safety and flexibility of methods.

In the example below _firstName and _lastName are private string variables which are accessible only inside the Customer class. _firstName and _lastName are exposed using FirstName and LastName public properties respectively. The get property accessor is used to return the property value, and a set accessor is used to assign a new value. These accessors can have different access levels. The value keyword is used to define the value being assigned by the set accessor. The FullName property computes the full name of the customer. Full Name property is readonly, because it has only the get accessor. Properties that do not implement a set accessor are read only.

The code block for the get accessor is executed when the property is read and the code block for the set accessor is executed when the property is assigned a new value.

using System;
class Customer
{
   // Private fileds not accessible outside the class.
   private string _firstName = string.Empty;
   private string _lastName = string.Empty;
   private string _coutry = string.Empty;

   // public FirstName property exposes _firstName variable
   public string FirstName
   {
      get
      {
         return _firstName;
      }
      set
      {
         _firstName = value;
      }
   }
   // public LastName property exposes _lastName variable
   public string LastName
   {
      get
      {
         return _lastName;
      }
      set
      {
         _lastName = value;
      }
   }
   // FullName property is readonly and computes customer full name.
   public string FullName
   {
      get
      {
         return _lastName + ", " + _firstName;
      }
   }
   //Country Property is Write Only
   public string Country
   {
      set
      {
         _coutry = value;
      }
   }

}
class MainClass
{
   public static void Main()
   {
      Customer CustomerObject = new Customer();
      //This line will call the set accessor of FirstName Property
      CustomerObject.FirstName = "David";
      //This line will call the set accessor of LastName Property
      CustomerObject.LastName = "Boon";
      //This line will call the get accessor of FullName Property
      Console.WriteLine("Customer Full Name is : " + CustomerObject.FullName);
   }
}

Explain the 3 types of properties in C# with an example?
1. Read Only Properties: Properties without a set accessor are considered read-only. In the above example FullName is read only property.
2. Write Only Properties: Properties without a get accessor are considered write-only. In the above example Country is write only property.
3. Read Write Properties: Properties with both a get and set accessor are considered read-write properties. In the above example FirstName and LastName are read write properties.

What are the advantages of properties in C#?
1. Properties can validate data before allowing a change.
2. Properties can transparently expose data on a class where that data is actually retrieved from some other source such as a database.
3. Properties can take an action when data is changed, such as raising an event or changing the value of other fields.

What is a static property. Give an example?
A property that is marked with a static keyword is considered as static property. This makes the property available to callers at any time, even if no instance of the class exists. In the example below PI is a static property.

using System;
class Circle
{
   private static double _pi = 3.14;
   public static double PI
   {
      get
      {
         return _pi;
      }
   }
}
class MainClass
{
   public static void Main()
   {
      Console.WriteLine(Circle.PI);
   }
}

What is a virtual property. Give an example?
A property that is marked with virtual keyword is considered virtual property. Virtual properties enable derived classes to override the property behavior by using the override keyword. In the example below FullName is virtual property in the Customer class. BankCustomer class inherits from Customer class and overrides the FullName virtual property. In the output you can see the over riden implementation. A property overriding a virtual property can also be sealed, specifying that for derived classes it is no longer virtual.

using System;
class Customer
{
   private string _firstName = string.Empty;
   private string _lastName = string.Empty;

   public string FirstName
   {
      get
      {
         return _firstName;
      }
      set
      {
         _firstName = value;
      }
   }
   public string LastName
   {
      get
      {
         return _lastName;
      }
      set
      {
         _lastName = value;
      }
   }
   // FullName is virtual
   public virtual string FullName
   {
      get
      {
         return _lastName + ", " + _firstName;
      }
   }
}
class BankCustomer : Customer
{
   // Overiding the FullName virtual property derived from customer class
   public override string FullName
   {
      get
      {
         return "Mr. " + FirstName + " " + LastName;
      }
   }
}
class MainClass
{
   public static void Main()
   {
      BankCustomer BankCustomerObject = new BankCustomer();
      BankCustomerObject.FirstName = "David";
      BankCustomerObject.LastName = "Boon";
      Console.WriteLine("Customer Full Name is : " + BankCustomerObject.FullName);
   }
}

What is an abstract property. Give an example?
A property that is marked with abstract keyword is considered abstract property. An abstract property should not have any implementation in the class. The derived classes must write their own implementation. In the example below FullName property is abstract in the Customer class. BankCustomer class overrides the inherited abstract FullName property with its own implementation.

using System;
abstract class Customer
{
   private string _firstName = string.Empty;
   private string _lastName = string.Empty;

   public string FirstName
   {
      get
      {
         return _firstName;
      }
      set
      {
         _firstName = value;
      }
   }
   public string LastName
   {
      get
      {
         return _lastName;
      }
      set
      {
         _lastName = value;
      }
   }
   // FullName is abstract
   public abstract string FullName
   {
      get;
   }
}
class BankCustomer : Customer
{
   // Overiding the FullName abstract property derived from customer class
   public override string FullName
   {
      get
      {
         return "Mr. " + FirstName + " " + LastName;
      }
   }
}
class MainClass
{
   public static void Main()
   {
      BankCustomer BankCustomerObject = new BankCustomer();
      BankCustomerObject.FirstName = "David";
      BankCustomerObject.LastName = "Boon";
      Console.WriteLine("Customer Full Name is : " + BankCustomerObject.FullName);
   }
}

Can you use virtual, override or abstract keywords on an accessor of a static property?
No, it is a compile time error to use a virtual, abstract or override keywords on an accessor of a static property.

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C# Interview Questions on Constants

2009-02-27T21:34:00.000-08:00

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What are constants in C#?
Constants in C# are immutable values which are known at compile time and do not change for the life of the program. Constants are declared using the const keyword. Constants must be initialized as they are declared. You cannot assign a value to a constant after it isdeclared. An example is shown below.

using System;
class Circle
{
   public const double PI = 3.14;
   public Circle()
   {
      //Error : You can only assign a value to a constant field at the time of declaration
      //PI = 3.15;
   }
}
class MainClass
{
   public static void Main()
   {
      Console.WriteLine(Circle.PI);
   }
}

Can you declare a class or a struct as constant?
No, User-defined types including classes, structs, and arrays, cannot be const. Only the C# built-in types excluding System.Object may be declared as const. Use the readonly modifier to create a class, struct, or array that is initialized one time at runtime (for example in a constructor) and thereafter cannot be changed.

Does C# support const methods, properties, or events?
No, C# does not support const methods, properties, or events.

Can you change the value of a constant filed after its declaration?
No, you cannot change the value of a constant filed after its declaration. In the example below, the constant field PI is always 3.14, and it cannot be changed even by the class itself. In fact, when the compiler encounters a constant identifier in C# source code (for example, PI), it substitutes the literal value directly into the intermediate language (IL) code that it produces. Because there is no variable address associated with a constant at run time, const fields cannot be passed by reference.

using System;
class Circle
{
   public const double PI = 3.14;
}

How do you access a constant field declared in a class?
Constants are accessed as if they were static fields because the value of the constant is the same for all instances of the type. You do not use the static keyword to declare them. Expressions that are not in the class that defines the constant must use the class name, a period, and the name of the constant to access the constant. In the example below constant field PI can be accessed in the Main method using the class name and not the instance of the class. Trying to access a constant field using a class instance will generate a compile time error.

using System;
class Circle
{
   public const double PI = 3.14;
}
class MainClass
{
   public static void Main()
   {
      Console.WriteLine(Circle.PI);
      Circle C = new Circle();
      // Error : PI cannot be accessed using an instance
      // Console.WriteLine(C.PI);
   }
}

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C# Interview Questions on Fields

2009-02-27T21:14:00.000-08:00

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What are the 2 broad classifications of fields in C#?
1. Instance fields
2. Static fields

What are instance fields in C#?
Instance fields are specific to an instance of a type. If you have a class T, with an instance field F, you can create two objects of type T, and modify the value of F in each object without affecting the value in the other object.

What is a static field?
A static field belongs to the class itself, and is shared among all instances of that class. Changes made from instance A will be visible immediately to instances B and C if they access the field.

Will the following code compile?

using System;
class Area
{
   public static double PI = 3.14;
}
class MainClass
{
   public static void Main()
   {
      Area A = new Area();
      Console.WriteLine(A.PI);
   }
}
No, a compile time error will be generated stating "Static member 'Area.PI' cannot be accessed with an instance reference; qualify it with a type name instead". This is because PI is a static field. Static fields can only be accessed using the name of the class and not the instance of the class. The above sample program is rewritten as shown below.

using System;
class Area
{
   public static double PI = 3.14;
}
class MainClass
{
   public static void Main()
   {
      Console.WriteLine(Area.PI);
   }
}

Can you declare a field readonly?
Yes, a field can be declared readonly. A read-only field can only be assigned a value during initialization or in a constructor. An example is shown below.

using System;
class Area
{
   public readonly double PI = 3.14;
}
class MainClass
{
   public static void Main()
   {
      Area A = new Area();
      Console.WriteLine(A.PI);
   }
}

Will the following code compile?

using System;
class Area
{
   public readonly double PI = 3.14;
}
class MainClass
{
   public static void Main()
   {
      Area A = new Area();
      A.PI = 3.15;
      Console.WriteLine(A.PI);
   }
}

No, PI is readonly. You can only read the value of PI in the Main() method. You cannot assign any value to PI.

What is wrong with the sample program below?

using System;
class Area
{
   public const double PI = 3.14;
   static Area()
   {
      Area.PI = 3.15;
   }
}
class MainClass
{
   public static void Main()
   {
      Console.WriteLine(Area.PI);
   }
}
You cannot assign a value to the constant PI field.

What is the difference between a constant and a static readonly field?
A static readonly field is very similar to a constant, except that the C# compiler does not have access to the value of a static read-only field at compile time, only at run time.

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C# Interview Questions on Access Modifiers

2009-02-26T15:54:00.000-08:00

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What are Access Modifiers in C#?
In C# there are 5 different types of Access Modifiers.
Public
The public type or member can be accessed by any other code in the same assembly or another assembly that references it.

Private
The type or member can only be accessed by code in the same class or struct.

Protected
The type or member can only be accessed by code in the same class or struct, or in a derived class.

Internal
The type or member can be accessed by any code in the same assembly, but not from another assembly.

Protected Internal
The type or member can be accessed by any code in the same assembly, or by any derived class in another assembly.

What are Access Modifiers used for?
Access Modifiers are used to control the accessibilty of types and members with in the types.

Can you use all access modifiers for all types?
No, Not all access modifiers can be used by all types or members in all contexts, and in some cases the accessibility of a type member is constrained by the accessibility of its containing type.

Can derived classes have greater accessibility than their base types?
No, Derived classes cannot have greater accessibility than their base types. For example the following code is illegal.
using System;
internal class InternalBaseClass
{
   public void Print()
   {
      Console.WriteLine("I am a Base Class Method");
   }
}
public class PublicDerivedClass : InternalBaseClass
{
   public static void Main()
   {
      Console.WriteLine("I am a Public Derived Class Method");
   }
}

When you compile the above code an error will be generated stating "Inconsistent accessibility: base class InternalBaseClass is less accessible than class PublicDerivedClass".To make this simple, you cannot have a public class B that derives from an internal class A. If this were allowed, it would have the effect of making A public, because all protected or internal members of A are accessible from the derived class.

Is the following code legal?

using System;
private class Test
{
   public static void Main()
   {
   }
}

No, a compile time error will be generated stating "Namespace elements cannot be explicitly declared as private, protected, or protected internal"

Can you declare struct members as protected?
No, struct members cannot be declared protected. This is because structs do not support inheritance.

Can the accessibility of a type member be greater than the accessibility of its containing type?
No, the accessibility of a type member can never be greater than the accessibility of its containing type. For example, a public method declared in an internal class has only internal accessibility.

Can destructors have access modifiers?
No, destructors cannot have access modifiers.

What does protected internal access modifier mean?
The protected internal access means protected OR internal, not protected AND internal. In simple terms, a protected internal member is accessible from any class in the same assembly, including derived classes. To limit accessibility to only derived classes in the same assembly, declare the class itself internal, and declare its members as protected.

What is the default access modifier for a class,struct and an interface declared directly with a namespace?
internal

Will the following code compile?

using System;
interface IExampleInterface
{
   public void Save();
}

No, you cannot specify access modifer for an interface member. Interface members are always public.

Can you specify an access modifier for an enumeration?
Enumeration members are always public, and no access modifiers can be specified.

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Why should you override the ToString() method

2009-02-19T17:19:00.000-08:00

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Why should you override the ToString() method?
All types in .Net inherit from system.object directly or indirectly. Because of this inheritance, every type in .Net inherit the ToString() method from System.Object class. Consider the example below.

using System;
public class MainClass
{
  public static void Main()
  {
   int Number = 10;
   Console.WriteLine(Number.ToString());
  }
}

In the above example Number.ToString() method will correctly give the string representaion of int 10, when you call the ToString() method.

If you have a Customer class as shown in the below example and when you call the ToString() method the output doesnot make any sense. Hence you have to override the ToString() method, that is inherited from the System.Object class.

using System;
public class Customer
{
public string FirstName;
public string LastName;
}
public class MainClass
{
public static void Main()
{
  Customer C = new Customer();
  C.FirstName = "David";
  C.LastName = "Boon";
  Console.WriteLine(C.ToString());
}
}

The code sample below shows how to override the ToString() method in a class, that would give the output you want.

using System;
public class Customer
{
  public string FirstName;
  public string LastName;

  public override string ToString()
  {
    return LastName + ", " + FirstName;
  }
}
public class MainClass
{
  public static void Main()
  {
    Customer C = new Customer();
    C.FirstName = "David";
    C.LastName = "Boon";
    Console.WriteLine(C.ToString());
  }
}

Conclusion : If you have a class or a struct, make sure you override the inherited ToString() method.

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C# Interview Questions on polymorphism

2009-02-19T17:12:00.000-08:00

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Explain polymorphism in C# with a simple example?
Polymorphism allows you to invoke derived class methods through a base class reference during run-time. An example is shown below.
using System;
public class DrawingObject
{
public virtual void Draw()
{
Console.WriteLine("I am a drawing object.");
}
}
public class Triangle : DrawingObject
{
public override void Draw()
{
Console.WriteLine("I am a Triangle.");
}
}
public class Circle : DrawingObject
{
public override void Draw()
{
Console.WriteLine("I am a Circle.");
}
}
public class Rectangle : DrawingObject
{
public override void Draw()
{
Console.WriteLine("I am a Rectangle.");
}
}
public class DrawDemo
{
public static void Main()
{
DrawingObject[] DrawObj = new DrawingObject[4];

DrawObj[0] = new Triangle();
DrawObj[1] = new Circle();
DrawObj[2] = new Rectangle();
DrawObj[3] = new DrawingObject();

foreach (DrawingObject drawObj in DrawObj)
{
drawObj.Draw();
}
}
}

When can a derived class override a base class member?
A derived class can override a base class member only if the base class member is declared as virtual or abstract.

What is the difference between a virtual method and an abstract method?
A virtual method must have a body where as an abstract method should not have a body.

Can fields inside a class be virtual?
No, Fields inside a class cannot be virtua. Only methods, properties, events and indexers can be virtual.

Give an example to show for hiding base class methods?
Use the new keyword to hide a base class method in the derived class as shown in the example below.
using System;
public class BaseClass
{
public virtual void Method()
{
Console.WriteLine("I am a base class method.");
}
}
public class DerivedClass : BaseClass
{
public new void Method()
{
Console.WriteLine("I am a child class method.");
}

public static void Main()
{
DerivedClass DC = new DerivedClass();
DC.Method();
}
}

Can you access a hidden base class method in the derived class?
Yes, Hidden base class methods can be accessed from the derived class by casting the instance of the derived class to an instance of the base class as shown in the example below.
using System;
public class BaseClass
{
public virtual void Method()
{
Console.WriteLine("I am a base class method.");
}
}
public class DerivedClass : BaseClass
{
public new void Method()
{
Console.WriteLine("I am a child class method.");
}

public static void Main()
{
DerivedClass DC = new DerivedClass();
((BaseClass)DC).Method();
}
}

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C# Interview Questions on Abstract and Sealed Class Members

2009-02-19T17:00:00.000-08:00

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What is an abstract class?
An abstract class is an incomplete class and must be implemented in a derived class.

Can you create an instance of an abstract class?
No, abstract classes are incomplete and you cannot create an instance of an abstract class.

What is a sealed class?
A sealed class is a class that cannot be inherited from. This means, If you have a class called Customer that is marked as sealed. No other class can inherit from Customer class. For example, the below code generates a compile time error "MainClass cannot derive from sealed type Customer.
using System;
public sealed class Customer
{
}
public class MainClass : Customer
{
public static void Main()
{
}
}

What are abstract methods?
Abstract methods are methods that only the declaration of the method and no implementation.

Will the following code compile?
using System;
public abstract class Customer
{
public abstract void Test()
{
Console.WriteLine("I am customer");
}
}
public class MainClass
{
public static void Main()
{
}
}
No, abstract methods cannot have body. Hence, the above code will generate a compile time error stating "Customer.Test() cannot declare a body because it is marked abstract"

Is the following code legal?
using System;
public class Customer
{
public abstract void Test();
}
public class MainClass
{
public static void Main()
{
}
}

No, if a class has even a single abstract member, the class has to be marked abstract. Hence the above code will generate a compile time error stating "Customer.Test() is abstract but it is contained in nonabstract class Customer"

How can you force derived classes to provide new method implementations for virtual methods?
Abstract classes can be used to force derived classes to provide new method implementations for virtual methods. An example is shown below.
public class BaseClass
{
public virtual void Method()
{
// Original Implementation.
}
}

public abstract class AbstractClass : BaseClass
{
public abstract override void Method();
}

public class NonAbstractChildClass : AbstractClass
{
public override void Method()
{
// New implementation.
}
}

When an abstract class inherits a virtual method from a base class, the abstract class can override the virtual method with an abstract method. If a virtual method is declared abstract, it is still virtual to any class inheriting from the abstract class. A class inheriting an abstract method cannot access the original implementation of the method. In the above example, Method() on class NonAbstractChildClass cannot call Method() on class BaseClass. In this way, an abstract class can force derived classes to provide new method implementations for virtual methods.

Can a sealed class be used as a base class?
No, sealed class cannot be used as a base class. A compile time error will be generated.

Will the following code compile?
public abstract sealed class Test
{
public virtual void Method()
{
}
}
No, a class cannot be marked as sealed and abstract at the same time. This is because by definition, a sealed class cannot be a base class and an abstract class can only be a base class.

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C# Interview Questions on Inheritance

2009-02-11T18:23:00.000-08:00

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What are the 4 pillars of any object oriented programming language?
1. Abstraction
2. Inheritance
3. Encapsulation
4. Polymorphism

Do structs support inheritance?
No, structs do not support inheritance, but they can implement interfaces.

What is the main advantage of using inheritance?
Code reuse

Is the following code legal?
class ChildClass : ParentClassA, ParentClassB
{
}
No, a child class can have only one base class. You cannot specify 2 base classes at the same time. C# supports single class inheritance only. Therefore, you can specify only one base class to inherit from. However, it does allow multiple interface inheritance.

What will be the output of the following code?
using System;
public class BaseClass
{
public BaseClass()
{
Console.WriteLine("I am a base class");
}
}
public class ChildClass : BaseClass
{
public ChildClass()
{
Console.WriteLine("I am a child class");
}
static void Main()
{
ChildClass CC = new ChildClass();
}
}
Output:
I am a base class
I am a child class
This is because base classes are automatically instantiated before derived classes. Notice the output, The BaseClass constructor executed before the ChildClass constructor.

Does C# support multiple class inheritance?
No, C# supports single class inheritance only. However classes can implement multiple interfaces at the same time.

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C# Interview Questions on structs

2009-02-10T18:32:00.000-08:00

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Will the following code compile?
using System;
public class Example
{
static void Main()
{
TestStruct T = new TestStruct();
Console.WriteLine(T.i);
}
}
public struct TestStruct
{
public int i=10;
//Error: cannot have instance field initializers in structs
}
No, a compile time error will be generated stating "within a struct declaration, fields cannot be initialized unless they are declared as const or static"

Can a struct have a default constructor (a constructor without parameters) or a destructor in C#?
No

Can you instantiate a struct without using a new operator in C#?
Yes, you can instantiate a struct without using a new operator

Can a struct inherit from another struct or class in C#?
No, a struct cannot inherit from another struct or class, and it cannot be the base of a class.

Can a struct inherit from an interface in C#?
Yes

Are structs value types or reference types?
Structs are value types.

What is the base type from which all structs inherit directly?
All structs inherit directly from System.ValueType, which inherits from System.Object.

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Basic C# Interview Questions on classes and structs

2009-02-04T16:26:00.000-08:00

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What do you mean by saying a "class is a reference type"?
A class is a reference type means when an object of the class is created, the variable to which the object is assigned holds only a reference to that memory. When the object reference is assigned to a new variable, the new variable refers to the original object. Changes made through one variable are reflected in the other variable because they both refer to the same data.

What do you mean by saying a "struct is a value type"?
A struct is a value type mean when a struct is created, the variable to which the struct is assigned holds the struct's actual data. When the struct is assigned to a new variable, it is copied. The new variable and the original variable therefore contain two separate copies of the same data. Changes made to one copy do not affect the other copy.

When do you generally use a class over a struct?
A class is used to model more complex behavior, or data that is intended to be modified after a class object is created. A struct is best suited for small data structures that contain primarily data that is not intended to be modified after the struct is created.

List the 5 different access modifiers in C#?
1. public
2. protected
3. internal
4. protected internal
5. private

If you donot specify an access modifier for a method, what is the default access modifier?
private

Classes and structs support inheritance. Is this statement true or false?
False, Only classes support inheritance. structs donot support inheritance.

If a class derives from another class, will the derived class automatically contain all the public, protected, and internal members of the base class?
Yes, the derived class will automatically contain all the public, protected, and internal members of the base class except its constructors and destructors.

Can you create an instance for an abstract class?
No, you cannot create an instance for an abstract class.

How do you prevent a class from being inherited by another class?
Use the sealed keyword to prevent a class from being inherited by another class.

Classes and structs can be declared as static, Is this statement true or false?
False, only classes can be declared as static and not structs.

Can you create an instance of a static class?
No, you cannot create an instance of a static class.

Can a static class contain non static members?
No, a static class can contain only static members.

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C# Interview Questions

2009-01-31T17:41:00.000-08:00

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More C# interview questions on strings

2009-01-29T19:50:00.000-08:00

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Will the following code compile and run?
string str = null;
Console.WriteLine(str.Length);
The above code will compile, but at runtime System.NullReferenceException will be thrown.

How do you create empty strings in C#?
Using string.empty as shown in the example below.
string EmptyString = string.empty;

What is the difference between System.Text.StringBuilder and System.String?
1. Objects of type StringBuilder are mutable where as objects of type System.String are immutable.
2. As StringBuilder objects are mutable, they offer better performance than string objects of type System.String.
3. StringBuilder class is present in System.Text namespace where String class is present in System namespace.

How do you determine whether a String represents a numeric value?
To determine whether a String represents a numeric value use TryParse method as shown in the example below. If the string contains nonnumeric characters or the numeric value is too large or too small for the particular type you have specified, TryParse returns false and sets the out parameter to zero. Otherwise, it returns true and sets the out parameter to the numeric value of the string.

string str = "One";
int i = 0;
if(int.TryParse(str,out i))
{
Console.WriteLine("Yes string contains Integer and it is " + i);
}
else
{
Console.WriteLine("string does not contain Integer");
}

What is the difference between int.Parse and int.TryParse methods?
Parse method throws an exception if the string you are trying to parse is not a valid number where as TryParse returns false and does not throw an exception if parsing fails. Hence TryParse is more efficient than Parse.

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