Extend an existing class C #

To extend an existing class method

using System;

namespace PureCSharpTest
{   
    public class Rubbish
    {
        public void Say()
        {
            Console.Write("Hello");
        }
    }
    public static class RubbishExtensions
    {
        public static void Say(this Rubbish argRubbish, string argString)
        {
            Console.Write(argString);
        }
        public static void Hello(this Rubbish rubbish)
        {
            Console.WriteLine("extension hello");
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            Rubbish theRubbish = new Rubbish();
            theRubbish.Say("Hey");
            theRubbish.Hello();
            theRubbish.OtherFileTestHello();
            Console.ReadKey(false);
        }
    }
}

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

namespace PureCSharpTest
{
    public static class extensionOtherFileTest
    {
        public static void OtherFileTestHello(this Rubbish rubbish)
        {
            Console.WriteLine("other file extension hello");
        }
    }
}

Visible, this method is extended, as long as access to take into account, it can be polymorphic and extension methods, and only you can at the same namespace