[Gtk-sharp-list] error compiling helloworld

[Akbar]

This is my code.

using Gtk;
using GtkSharp;
using System;

class Hello
{
        static void Main()
        {
                Application.Init();

                Window window = new Window ( "helloworld" );
                window.Show();

                Application.Run();

        }
}

I compiled and installed mono, mcs, and gtk-sharp with the prefix=/usr. 
So I don't understand why
I cann't compile this code.
$ mcs Hello.cs -r:gtk-sharp
error CS0006: Cannot find assembly `gtk-sharp'
Log:

Compilation failed: 1 error(s), 0 warnings

What's the problem? I got gtk-sharp.dll in my /usr/lib/mono/gtk-sharp 
directory.