Inheritance

Inheritance:
Inheritance is a process of deriving the new class from already existing class
C# is a complete object oriented programming language. Inheritance is one of the primary concepts of object-oriented programming. It allows you to reuse existing code. Through effective use of inheritance, you can save lot of time in your programming and also reduce errors, which in turn will increase the quality of work and productivity. A simple example to understand inheritance in C#.
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 output 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 ();
}
}
Method Hiding and Inheritance We will look at an example of how to hide a method in C#. The Parent class has a write () method which is available to the child class. In the child class I have created a new write () method. So, now if I create an instance of child class and call the write () method, the child class write () method will be called. The child class is hiding the base class write () method. This is called method hiding.
If we want to call the parent class write () method, we would have to type cast the child object to Parent type and then call the write () method as shown in the code snippet below.


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 ();
}
}