practical example for abstract class

Abstract classes

The abstract modifier is used to indicate that a class is incomplete and that it is intended to be used only as a base class. An abstract class differs from a non-abstract class is the following ways:

• An abstract class cannot be instantiated directly, and it is a compile-time error to use the new operator on an abstract class. While it is possible to have variables and values whose compile-time types are abstract, such variables and values will necessarily either be null or contain references to instances of non-abstract classes derived from the abstract types.
• An abstract class is permitted (but not required) to contain abstract members.
•  An abstract class cannot be sealed.

When a non-abstract class is derived from an abstract class, the non-abstract class must include actual implementations of all inherited abstract members. Such implementations are provided by overriding the abstract members. For example:

abstract class A
{
 public abstract void F();
}

abstract class B: A
{
 public void G() {}
}

class C: B
{

   public override void F() {
    // actual implementation of F
   }

}

the abstract class A introduces an abstract method F. Class B introduces an additional method G, but since it doesn’t provide an implementation of F, B must also be declared abstract. Class C overrides F and provides an actual implementation. Since there are no abstract members in C, C is permitted (but not required) to be non-abstract.

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using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace TestConsoleApp
{
    public abstract class Duck
    {
        public abstract string Fly();

        public void Run()
        {
            Console.Write(Fly().ToString());
        }
    }

    public class WoodenDuck : Duck
    {

        public override string Fly()
        {
            return "Cannot fly";
        }
    }

    public class LiveDuck : Duck
    {
        public override string Fly()
        {
            return "Can fly";
        }
    }

    public class TestApp
    {
        public void Test()
        {
            Duck duck;
            duck = new LiveDuck();
            duck.Run();

            duck = new WoodenDuck();
            duck.Run();
        }
    }
}