Most Asked C# Interview Questions And Answers

If you need to know other interview questions and answers, I strongly recommend to read my following articles:

Important ADO.NET Interview Questions

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Now in this post we are going to share the interview questions for C# or a Dot Net developer. No matter you are an experienced professional or a fresher, it is important that you must be aware of these questions. So please read it carefully. I hope you will like this article.

Background

C# is one of my favorite programming language. So I am really happy to share you some interview questions related to C#. So shall we now discuss about C# interview questions?

Question: What are Abstract Classes?

The purpose of an abstract class is to define some common behavior that can be inherited by multiple subclasses, without implementing the entire class. In C#, the abstract keyword designates both an abstract class and a pure virtual method.

The purpose of an abstract class is to provide a common definition of a base class that multiple derived classes can share. For example, a class library may define an abstract class that is used as a parameter to many of its functions, and require programmers using that libarary to provide their own implementation of the class by creating a derived class.

Properties

• An abstract class cannot be instantiated.
• It can be inherited.
• It can have method implementations and class members.
• Only abstract classes can have abstract methods.

Syntax

public abstract class A
{
public abstract void DoWork(int i);
}

Question: What are Abstract methods? Give an example?

Abstract Methods

Abstract methods have no implementation, so the method definition is followed by a semicolon instead of a normal method block. (See above example for syntax).

Derived classes of the abstract class must implement all abstract methods.

When an abstract class inherits a virtual method from a base class, the abstract class can override the virtual method with an abstract method.

Example

// compile with: /target:library   
public  class D  
{   
    public  virtual  void DoWork(int i)   
    {  
        // Original implementation.  
    }   
}  
public  abstract  class E: D  
{  
    public  abstract  override  void DoWork(int i);  
}  
public  class F: E  
{  
    public  override  void DoWork(int i)   
    {  
       // New implementation.  
    }  
}

Question: What is Interface? Explain with an example?

An interface is useful when you want to use some common functionality of otherwise unrelated classes- they share no implementation details, only the function signatures. In C#, function declarations within an interface are implicitly pure virtual.

An interface contains only the signatures of methods, properties, events or indexers. A class or struct that implements the interface must implement the members of the interface that are specified in the interface definition. 

Properties

An interface can be a member of a namespace or a class and can contain signatures of methods, properties, events, indexers, etc.

An interface can inherit from one or more base interfaces.

A class that implements an interface can explicitly implement members of that interface. An explicitly implemented member cannot be accessed through a class instance, but only through an instance of the interface.

Example

interface IControl   
{   
    void Paint();   
}  
interface ISurface   
{  
    void Paint();   
}  
public  class SampleClass: IControl, ISurface   
{  
    void IControl.Paint()   
    {  
        System.Console.WriteLine("IControl.Paint");   
    }  
    void ISurface.Paint()   
    {  
        System.Console.WriteLine("ISurface.Paint");   
    }    
} 

Calling the methods

// Call the Paint methods from Main.  
SampleClass obj = new SampleClass();  
//obj.Paint(); // Compiler error.  
  
IControl c = (IControl)obj; c.Paint();  
// Calls IControl.Paint on SampleClass.  
  
ISurface s = (ISurface)obj; s.Paint();  
// Calls ISurface.Paint on SampleClass.   

Question: Explain The Difference Between Abstract Class and Interface?

An Abstract class do not provide full abstraction but an interface does provide full abstraction; i.e. both a declaration and a definition is given in an abstract class but not so in an interface.

Using Abstract we cannot achieve multiple inheritance but using an Interface we can achieve multiple inheritance.