using System;
using System.Windows.Forms;

public partial class MainForm : Form
{
    public MainForm()
    {
        this.Menu = new MainMenu(new MenuItem[] {
			  	 new MenuItem("Test", new EventHandler(Test)),
			});
    }

    private void Test(object sender, EventArgs e)
    {
        IChildFormProxy cfa = new ChildFormProxy();
        cfa.SetParent(this.Handle);
    }
}

public interface IChildFormProxy
{
    void SetParent(IntPtr hwnd);
}

public class Win32Window : IWin32Window
{
    public Win32Window(IntPtr hwnd)
    {
        _Handle = hwnd;
    }
    public IntPtr Handle
    {
        get { return _Handle; }
    }
    private IntPtr _Handle;
}

public class ChildFormProxy : IChildFormProxy
{
    public void SetParent(IntPtr hwnd)
    {
        Form cf = new Form();
        cf.ShowDialog(new Win32Window(hwnd));
    }
}