CComboBox with disabled items

Introduction

The basic derivation has four parts:

1. By making the combo box owner-draw, we can draw the disabled items in a distinct color.
2. Overriding the WM_LBUTTONUP handler of the enclosed list box, we can actually disable clicking on the disabled items.
3. Overriding CharToItem handler, we can disable picking the disabled items by keyboard.
4. And finally, by reacting to reflected CBN_SELENDOK, we can assure that a disabled item was not, by bug in CExtendedComboBox implementation, selected.

The reader is urged to carefully study the code to understand the internals of the CExtendedComboBox class.

The use of WM_CTLCOLORLISTBOX message for subclassing comes from another article.

How to use the class

The behaviour of CExtendedComboBox is customizable by overriding:

virtual BOOL IsItemEnabled(UINT) const;

This function takes index of item as an argument (the function must explicitely check that the argument is not, in fact, out of bounds). It returns TRUE/FALSE. The default implementation uses least significant bit of item data.

This class should work with all combo box styles (CBS_SIMPLE, CBS_DROPDOWN and CBS_DROPLIST), although it has been thoroughly tested only with CBS_DROPDOWN. The combo box must be marked as CBS_OWNERDRAWFIXED (CBS_OWNERDRAWVARIABLE is possible but would require some changes to the code) and CBS_HASSTRINGS.

I am interested in hearing any comments.