using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        object o= new Item();

        DoWork(o);

        DoWork((Item) o);

        DoWork((Itembase) o);

    }

    private static void DoWork(object o){

        Console.WriteLine("DoWork(object o)");

    }

    private static void DoWork(Itembase o){

        Console.WriteLine("DoWork(Itembase o)");

    }

    private static void DoWork(Item o){

        Console.WriteLine("DoWork(Item o)");

    }

}

​

public class Itembase{}

public class Item:Itembase{}

​