AbstractClasses.cs

using System;

/* //Abstract Classes//
 * 
 * An abstract class is incomplete and CANNOT be instantiated
 * 
 * An abstract class CAN ONLY be used as a base class
 * 
 * An abstract class CANNOT be sealed
 * 
 * An abstract class may contain abstract members (methods, properties, indexers, events, etc) but is not mandatory
 * 
 * A non-abstract class derived from an abstract class MUST provide implementations for all inherited abstract members
 * 
 * !@!@ If a class inherits an abstract class, there are 2 options available for that class !@!@
 *      Option 1: Provide implementation for all the abstract members inherited from the base abstract class
 *      Option 2: If the class does not wish to provide implementations for all the abstract members that are 
 *                inherited from the abstract class, then the class has to marked as abstract
 * 
 */


public abstract class AbstractClass //cannot be instantiated, can only be used as a base class
{
    public abstract void Print();
    //if method is marked as abstract, you CANNOT have an implementation here, but must provide
    //the implementation in any inherited classes

}

public abstract class AnotherAbstractClass : AbstractClass 
//if you do not wish to provide the implementation of the inherited abstract class method
//then you MUST mark the class that is INHERITING, as abstract
{
    //this class is inhereting from 'AbstractClass', which provides an abstract method, but it does not need to be implemented here
    //because this class is also marked as abstract
}

public class Program : AbstractClass
{
    public override void Print() //implementation of 'AbstractClass' Print method
    {
        Console.WriteLine("AbstractClass Print method called");
    }
    
    public static void Main()
    {
        Program P = new Program();
        P.Print();

        Console.ReadKey();
    }
}