abstract (C# Reference)

The abstract modifier indicates that the thing being modified has a missing or incomplete implementation. 
The abstract modifier can be used with classes, methods, properties, indexers, and events. 
Use the abstract modifier in a class declaration to indicate that a class is intended only to be a base class of other classes. 
Members marked as abstract, or included in an abstract class, must be implemented by classes that derive from the abstract class.

abstract class ShapesClass
{
    abstract public int Area();
}
class Square : ShapesClass
{
    int side = 0;

    public Square(int n)
    {
        side = n;
    }
    // Area method is required to avoid
    // a compile-time error.
    public override int Area()
    {
        return side * side;
    }

    static void Main() 
    {
        Square sq = new Square(12);
        Console.WriteLine("Area of the square = {0}", sq.Area());
    }

    interface I
    {
        void M();
    }
    abstract class C : I
    {
        public abstract void M();
    }

}
// Output: Area of the square = 144