using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        PhotoBase photo = new ExternalPhoto();

        photo.Load();

        photo = new FilePhoto();

        photo.Load();

    }

}

​

public abstract class PhotoBase

{

    public string Path { get; set; }

    public abstract void Load();

}

​

public class FilePhoto : PhotoBase

{

   public override void Load()

   {

       Console.WriteLine("load file photo");

   }

}

​

public class ExternalPhoto : PhotoBase

{

   public override void Load()

   {

       Console.WriteLine("load external photo");

   }

}