Understanding Polymorphism in C#

Understanding Polymorphism in C# is one of main steps to understand OOP , also understanding polymorphism will help you to understand almost of Design patterns.

First What is Polymorphism?

Polymorphism is the use of virtual methods to make one method (name) produce one of many possible outcomes (behaviors) depending upon the instance.
Polymorphism allow ancestor to reach descendants.

Roles to know polymorphism 

1- Non virtual method is based in object type and it done at compile time.

sample to be clear

public class ThumbnailBase
    {
        public virtual void GetMetadata()
        {
            Console.WriteLine("ThumbnailBase.GetMetadata");
        }
        public void GetThumbnail()
        {
            Console.WriteLine("ThumbnailBase.GetThumbnail");
        }
    }

    public class ThumbnailImage : ThumbnailBase
    {
        public override void GetMetadata()
        {
            Console.WriteLine("ThumbnailImage.GetMetadata");
        }
        public new void GetThumbnail()
        {
            Console.WriteLine("ThumbnailImage.GetThumbnail");
        }
    }

static void Main(string[] args)
        {
            ThumbnailBase obj = new ThumbnailImage();
            obj.GetMetadata();  // This is Virtual Method so result is ThumbnailImage.GetMetadata
            obj.GetThumbnail(); // This is Non Virtual Method so result ThumbnailBase.GetThumbnail
            Console.ReadLine();

        }

2- Virtual method is based in instance type and it done at Run time.

sample to be clear

public abstract class ThumbnailBase
    {
        protected abstract void GetMetadata();

        public void GetThumbnail()
        {
            GetMetadata();
            Console.WriteLine("ThumbnailBase.GetThumbnail");
        }

    }

    public class ThumbnailImage : ThumbnailBase
    {
        protected override void GetMetadata()
        {
            Console.WriteLine("ThumbnailImage.GetMetadata");
        }
        public new void GetThumbnail()
        {
            Console.WriteLine("ThumbnailImage.GetThumbnail");
        }
    }

    public class ThumbnailVideo : ThumbnailBase
    {
        protected override void GetMetadata()
        {
            Console.WriteLine("ThumbnailVideo.GetMetadata");
        }
        public new void GetThumbnail()
        {
            Console.WriteLine("ThumbnailVideo.GetThumbnail");
        }

    } 

  static void Main(string[] args)     
   {
            ThumbnailBase obj = new ThumbnailImage();
            ThumbnailBase obj2 = new ThumbnailVideo();
            Console.WriteLine("---------------ThumbnailImage-----------by EslamF----");
            obj.GetThumbnail();
            Console.WriteLine("---------------ThumbnailVideo---------by EslamF------");
            obj2.GetThumbnail();
            Console.ReadLine();

     }

key of polymorphism is depends on important feature , The ability to identify families of object with identical interfaces.

Polymorphism allow you to manipulate objects without change client code.and get behavior change depend on the instance.

Finally,, Hope that i reach my Goal.