A ComboBox control with a TreeView

This UserControl is a ComboBox control that has its dropdown window replaced with a TreeView control. Handling the mouse, the keyboard and the focusing was a bit tricky and the control is not 100% perfect but it works just fine. Two issues are unresolved, but they were not important to me: firstly, since the TreeView is a child control it cannot be drawn outside the form; and secondly, a ComboBox.CloseUp call must be placed on the Form.Click event since the control does not monitor its own parent. For diligent people it should be relatively easy to adjust this control to their own needs, and even to replace the TreeView control with something else and build other types of specialized ComboBoxes.