C#

public class MyForm : Form
{
  private Add addControl;
  private Delete deleteControl;
  public MyForm()
  {
    addControl = new Add();
    deleteControl = new Delete();
    // If you want, add them to the Controls collection now.
  }
  private void addMenu_Click(object sender, EventArgs e)
  {
    Controls.Remove(deleteControl);
    Controls.Add(addControl);
    // Or use deleteControl.Hide() and addControl.Show()
  }
  private void deleteMenu_Click(object sender, EventArgs e)
  {
    Controls.Remove(addControl);
    Controls.Add(deleteControl);
    // Or use addControl.Hide() and deleteControl.Show()
  }
}

private void CloseAllChildren()
{
  System.Windows.Forms.Form[] myChildren = this.MdiChildren;
  foreach (Form child in myChildren)
  {
      child.Close();
  }
  return;
}

public void Form1_Closing(object sender, System.EventArgs e)
{
    ... release resources ...
    ... if user did not save see if they really want to throw
        awway their work
    // user wants to save data...do not close this form
    e.CloseCancel = true;
}

using System;
using System.Windows.Forms
public class Test : Form
{
  static void Main()
  {
    Application.Run(new Test());
  }
  protected override void OnKeyDown(KeyEventArgs e)
  {
    Console.WriteLine(e.KeyValue);
    base.OnKeyDown(e);
  }
}

string path = Environment.GetFolderPath(
  Environment.SpecialFolder.Startup);

private const int WM_HSCROLL = 0x0114;
private const int WM_VSCROLL = 0x0115;
protected override void WndProc(ref Message m)
{
  base.WndProc(ref m);
  if (m.Msg == WM_HSCROLL) OnHScroll();
  else if (m.Msg == WM_VSCROLL) OnVScroll();
}
protected virtual void OnHScroll()
{
  // Do something
}
protected virtual void OnVScroll()
{
  // Do something
}