InUIThread.cs

/* the helper, built as a Control extension method */

public delegate void Action();

public static void InUIThread(this Control control, Action action) {
  if (control.InvokeRequired)
    control.EndInvoke(control.BeginInvoke(action));
  else
    action();
}

/* some real use from a form */

class MyForm {
  void SomeMethodRunningInABackgroundThread() {
    /* some processing ... */
    this.InUIThread(() => { statusBar.Text = newStatus; }
    /* more processing ... */
    this.InUIThread(() => { listBox.Items.Add(message); }
  }		
}