using System;
using System.Windows.Forms;
using System.Drawing;

public class MyWindow : Form
{
   const int iWidth      = 600;
   const int iHeight     = 480;
   const int iMargin     = 10;

   private Button   _pbDo;
   private MyWindow _mywin;

   public MyWindow (bool fFirst)
   {
      Control ctrlCurr;

      if (fFirst) {
         Text = "MyWindow 1";
      } else {
         Text = "MyWindow 2";
      }
      ClientSize = new Size (iWidth, iHeight);

      ctrlCurr = _pbDo = new Button ();
      ctrlCurr.Location = new Point (iMargin, iMargin);
      if (fFirst) {
         ctrlCurr.Location = new Point (iMargin, iMargin);
         ctrlCurr.Size = new Size (iWidth/2-2*iMargin, iHeight/2-2*iMargin);
         ctrlCurr.Text = "aus MyWindow 1";
         ctrlCurr.Click += new EventHandler (DoClick);
      } else {
         ctrlCurr.Location = new Point (iWidth/2+iMargin, iHeight/2+iMargin);
         ctrlCurr.Size = new Size (iWidth/2-2*iMargin, iHeight/2-2*iMargin);
         ctrlCurr.Text = "aus MyWindow 2";
      }
      Controls.Add (ctrlCurr);
   }

   protected void DoClick (Object objSender, EventArgs e)
   {
      if (_mywin == null) {
         _mywin = new MyWindow (false);
         _mywin.Show ();
         _mywin.Controls.Add (_pbDo);  // ≤= hier
      }
   }
}

abstract class App
{
   public static void Main (string [] astrArg)
   {
      Application.Run (new MyWindow (true));
   }
}

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;

namespace WindowsApplication10
{
	/// <summary>
	/// Summary description for Form1.
	/// </summary>
	public class Form1 : System.Windows.Forms.Form
	{
		private System.Windows.Forms.Button button1;
		/// <summary>
		/// Required designer variable.
		/// </summary>
		private System.ComponentModel.Container components = null;

		public Form1()
		{
			//
			// Required for Windows Form Designer support
			//
			InitializeComponent();

			//
			// TODO: Add any constructor code after InitializeComponent call
			//
		}

		/// <summary>
		/// Clean up any resources being used.
		/// </summary>
		protected override void Dispose( bool disposing )
		{
			if( disposing )
			{
				if (components != null) 
				{
					components.Dispose();
				}
			}
			base.Dispose( disposing );
		}

		#region Windows Form Designer generated code
		/// <summary>
		/// Required method for Designer support - do not modify
		/// the contents of this method with the code editor.
		/// </summary>
		private void InitializeComponent()
		{
			this.button1 = new System.Windows.Forms.Button();
			this.SuspendLayout();
			// 
			// button1
			// 
			this.button1.Location = new System.Drawing.Point(0, 0);
			this.button1.Name = "button1";
			this.button1.Size = new System.Drawing.Size(128, 64);
			this.button1.TabIndex = 0;
			this.button1.Text = "button1";
			this.button1.Click += new System.EventHandler(this.button1_Click);
			// 
			// Form1
			// 
			this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
			this.ClientSize = new System.Drawing.Size(120, 61);
			this.Controls.Add(this.button1);
			this.Name = "Form1";
			this.Text = "Form1";
			this.ResumeLayout(false);

		}
		#endregion

		/// <summary>
		/// The main entry point for the application.
		/// </summary>
		[STAThread]
		static void Main() 
		{
			Application.Run(new Form1());
		}

		private void button1_Click(object sender, System.EventArgs e)
		{
			Form1 frm=new Form1();
			frm.TopLevel=false;
			frm.Location=new Point(0,this.ClientSize.Height);
			this.ClientSize=new Size(this.ClientSize.Width+frm.ClientSize.Width,this.ClientSize.Height+frm.Height);
			this.Controls.Add(frm);
			frm.Show();
		}
	}
}