I create a combo class as follows. When I drop it onto a user control and compile I get the error
"The type name '_ComboBase' does not exist in the type 'ComboTree.ComboTree'".

namespace ComboTree
{
    public class _ComboBase : ComboBox
    {
        // Event to handle when the down arrow is clicked
        public event EventHandler ArrowClicked;

        // 
        protected virtual void OnArrowClicked(EventArgs e)
        {
            if (this.ArrowClicked != null)
            {
                this.ArrowClicked(this, e);
            }
        }

        // Overwrite the WndProc event to 
        protected override void WndProc(ref System.Windows.Forms.Message m)
        {

            if (m.Msg == 0x201 || m.Msg == 0x203)  //WM_LBUTTONDOWN or WM_LBUTTONDBLCLK 
            {
                // Raise the event
                this.OnArrowClicked(new EventArgs());

                // Pevent the list from opening
                return;

            }
            base.WndProc(ref m);
        }
    }
}