1. Inheritance
2. Encapsulation
3. Abstraction
4. Polymorphism

using System;

public class BaseClass

{

    public BaseClass()

    {

        Console.WriteLine("Base Class Constructor executed");

    }

    public void Write()

    {

        Console.WriteLine("Write method in Base Class executed");

    }

}

public class ChildClass : BaseClass

{

    public ChildClass()

    {

        Console.WriteLine("Child Class Constructor executed");

    }

    public static void Main()

    {

        ChildClass CC = new ChildClass();

        CC.Write();

    }

}

• In the Main() method in ChildClass we create an instance of childclass. Then we call the write() method. If you observe the ChildClass does not have a write() method in it. This write() method has been inherited from the parent BaseClass.
• The out put of the above program is
  
  OutPut:
    Base Class Constructor executed
    Child Class Constructor executed
    Write method in Base Class executed
  
  This output proves that when we create an instance of a child class, the base class constructor will automatically be called before the child class constructor. So in general Base classes are automatically instantiated before derived classes.
• In C# the syntax for specifying BaseClass and ChildClass relationship is shown below.The base class is specified by adding a colon, ":", after the derived class identifier and then specifying the base class name.

  Syntax :  class ChildClassName : BaseClass 
                {
                     //Body 
                }

• C# supports single class inheritance only. What this means is, your class can inherit from only one base class at a time. In the code snippet below, class C is trying to inherit from Class A and B at the same time. This is not allowed in C#. This will lead to a compile time error : Class 'C' cannot have multiple base classes: 'A' and 'B'.

  public class A

  {

  }

  public class B

  {

  }

  public class C : A, B

  {

  }

• In C# Multi-Level inheritance is possible. Code snippet below demonstrates mlti-level inheritance. Class B is derived from Class A. Class C is derived from Class B. So class C, will have access to all members present in both Class A and Class B. As a result of multi-level inheritance Class has access to A_Method(),B_Method() and C_Method().
  
  Note: Classes can inherit from multiple interfaces at the same time. Interview Question: How can you implement multiple inheritance in C#? Ans : Using Interfaces. We will talk about interfaces in our later article.

  using System;

  public class A

  {

      public void A_Method()

      {

          Console.WriteLine("Class A Method Called");

      }

  }

  public class B : A

  {

      public void B_Method()

      {

          Console.WriteLine("Class A Method Called");

      }

  }

  public class C : B

  {

      public void C_Method()

      {

          Console.WriteLine("Class A Method Called");

      }

                     

      public static void Main()

      {

          C C1 = new C();

          C1.A_Method();

          C1.B_Method();

          C1.C_Method();

      }

  }
• When you derive a class from a base class, the derived class will inherit all members of the base class except constructors. In the code snippet below class B will inherit both M1 and M2 from Class A, but you cannot access M2 because of the private access modifier. Class members declared with a private access modifier can be accessed only with in the class. We will talk about access modifiers in our later article.
  
  Common Interview Question : Are private class members inherited to the derived class?
  Ans : Yes, the private members are also inherited in the derived class but we will not be able to access them. Trying to access a private base class member in the derived class will report a compile time error.

  using System;

  public class A

  {

      public void M1()

      {

      }

      private void M2()

      {

      }

  }

           

  public class B : A

  {

      public static void Main()

      {

          B B1 = new B();

          B1.M1();

          //Error, Cannot access private member M2

          //B1.M2();

      }

  }

using System;

public class Parent

{

    public void Write()

    {

        Console.WriteLine("Parent Class write method");

    }

}

public class Child : Parent

{

    public new void Write()

    {

        Console.WriteLine("Child Class write method");

    }

    public static void Main()

    {

        Child C1 = new Child();

        C1.Write();

        //Type caste C1 to be of type Parent and call Write() method

        ((Parent)C1).Write();

    }