C#

public class MyControl : UserControl
{
  static readonly IntPtr WM_LBUTTONDOWN = new IntPtr(0x0201);
  const int WM_PARENTNOTIFY = 0x0210;
  // ...
  protected override void WndProc(ref Message m)
  {
    if (m.Msg == WM_PARENTNOFITY)
    {
      if (m.WParam == WM_LBUTTONDOWN)
      {
        // ...
      }
    }
    base.WndProc(ref m);
  }
}

for (int i=0; i < 5; i++)
{
  MenuItem menu = new MenuItem("Menu " + i.ToString());
  menu.Click += new EventHandler(MenuItem_Click);
  contextMenu1.MenuItems.Add(menu);
}
// ...
private void MenuItem_Click(object sender, EventArgs e)
{
  MenuItem menu = sender as MenuItem;
  if (menu != null) MessageBox.Show(menu.Text + " was clicked.");
}

// in the Form class, MyForm.cs
OtherClass theOtherClass = new OtherClass(this);

// in the other class
public class OtherClass
{
   MyForm myFormInstance = null;
   
   public OtherClass(MyForm frmInst)
   {
      this.myFormInstance = frmInst;

      this.myFormInstance.DoSomething();
   }
}

public class MyForm : Form
{
   public static MyForm TheFormInstance = null;

   public MyForm()
   {
      TheFormInstance = this;
   }
}

// in some other class (note that MyForm is the actual name of the class, not a class instance)
MyForm.TheFormInstance.DoSomething();