public class HelloToolbarButton : CustomToolBarButton
{
  public HelloToolbarButton()
  {
    Text        = "Hello World";
    ToolTipText = "Display Hello World Message";
  }
  public override Image GetImage()
  {
    return new Bitmap("World.bmp");
  }
  public override void DoAction(object sender, System.EventArgs e)
  {
    MessageBox.Show(iapi.TheMainForm, "World", "Hello");
  }
}

Example 1: The first class inside of the NS namespace declaration.