Someone

using System;
using System.Windows.Forms;

namespace Test
{
	class Tester
	{
		public static void Main (string[] args)
		{
			Form f = new Form ();
			Form f2 = new Form ();	
			f2.TopLevel = false;
			f.Controls.Add (f2);
			Button b = new Button ();
			b.Visible = true;
			f2.Controls.Add (b);
			Console.WriteLine (b.Visible);
		}
	}
}