C#

public abstract class Surface{
  public abstract void DoStuff() ;
}

public interface Surface {
  void DoStuff();
}

dan neely wrote:

There are to differences between an interface and an abstract class is that you can have multiple interface inheritance, but only single class inheritance. The second is that you can't implement any methods in an interface definition, but can implement methods in an abstract class if you wish.

private void button1_MouseDown(object sender, System.Windows.Forms.MouseEventArgs e)
{
    if(e.Button == MouseButtons.Left)	
    {
	if(button1.ContextMenu!=null)	
	{
            button1.ContextMenu.Show(button1, new System.Drawing.Point(e.X, e.Y));		
	}
    }
}