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What is Abstract Class in C#. When, where and why to use it.

Introduction: Abstract Class and Interface is one of the most frequently asked interview question. In this tutorial, I will explain about Abstract class in C#.  To learn about Interface visit What is Interface in C# and Where to Use it

Abstract Class:
Just like class, The Abstract keyword is used to create Abstract classes. Abstract classes are incomplete and hence can’t be instantiated. An Abstract class can only be a base class. An abstract class can have both abstract and non-abstract Methods (concrete methods).

Example: 
abstract class Customer  // Using abstract keyword to create abstract class.
    {
        public void Print1() // Non Abstract method
        {
        }
        public abstract void Print2(); // Abstract method       
    }

If we declare body of abstract method then it will give error because we can’t declare body of Abstract class.

Example:
abstract class Customer  // Using abstract keyword to create abstract class.
    {
        public void Print1() // Non Abstract method
            {
            }

public abstract void Print2(); // Declaring body for Abstract method                                                             // Error-Invalid token '{' in class, struct, 
                                 //   or interface member declaration
             {
             }       
    }


Since Abstract classes are incomplete class So it doesn’t make sense to create instance of Abstract  class. Abstract class can’t be instantiated they can only be used as a base class.

Example:
namespace ConsoleApplication
{
    abstract class Customer  // Using abstract keyword to create abstract class.
    {
        public void Print1() // Non Abstract method
        {
        }
        public abstract void Print2(); // Abstract method
        {
        }       
    }
    class Program
    {
        static void Main(string[] args)
        {
            Customer c = new Customer();  //can't create instance of Abstract
                                         // class or Interface 'Customer'
        }
    }
}

If a class is inherited from Abstract class then that class would be responsible to define that abstract method, otherwise it will give error.

Example: 
abstract class Customer
    {
        public abstract void Print2(); // Abstract method      
    }
    class Program : Customer
    {
        static void Main(string[] args)
        {           
            // Program doesn’t implemented Abstract method  
      }
    }

If a class inherits an abstract class, then there are two options available for that class

Option 1: Provide implementation for all the abstract members inherited from the base abstract class.

Example:

abstract class Customer
    {
        public abstract void Print2(); // Abstract method      
    }
    class Program : Customer
    {
        public override void Print2()
        {
            Console.WriteLine("Print2 method");
        }
        static void Main(string[] args)
        {   
        }
    }

Option 2: If the class doesn’t wish to provide implementation for all the abstract members inherited from the abstract class, then the class has to be mark as abstract.
  
Example: 
   abstract class Customer
    {
        public abstract void Print2(); // Abstract method      
    }

   abstract class Program : Customer // Marked as Abstract
    {
        static void Main(string[] args)
        {        
        }
    }

We can’t create instance of Abstract class but Abstract class reference variable can point to base class object.

Example: 
abstract class Customer
    {
        public abstract void Print2(); // Abstract method      
    }

    class Program : Customer
    {
        public override void Print2()
        {
            Console.WriteLine("Print2 method");
        } 
        static void Main(string[] args)

        {
            Customer objCustomer = new Program();// base class reference variable
                                                  //pointing derived class object
        }
    }


Abstract class can’t be marked as sealed because they contradicted to each other.

Example: 
 abstract class Customer
    {
        public abstract void Print2(); // Abstract method      
    }

Abstract class means Customer class always can be used as base class.
While
Sealed means Customer class cannot used as base class.

abstract sealed class Customer // An Abstract class cannot be sealed or static
    {
        public abstract void Print2();      
    }


An Abstract class may contain abstract members (Method, properties, indexers, and Events), But not mandatory.

When to use Abstract classes:
When you have a requirement where your base class should provide default implementation of certain methods whereas other methods should be open to being overridden by child classes use abstract classes.

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 library to provide their own implementation of the class by creating a derived class.

Real Life example:
Your boss assigned you to create a registration page for your project then he marked 'Create registration page' as Abstract. Now you are responsible to create that page in your own style.

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