An owner-drawn combo box sends the WM_DRAWITEM message to its parent window or dialog box procedure each time the application must repaint a list item. The lParam parameter points to a DRAWITEMSTRUCT structure that identifies the control and list item. It also contains information needed to paint the item.
The example in Creating a Square Meal Dialog Box displays the list-item text and the bitmap associated with the food group. If the item has the focus, it also draws a focus rectangle. Before displaying the text, the example sets the foreground and background colors, based on the item selected. Because the combo box has the CBS_HASSTRINGS style, the combo box maintains the text for each list item that can be retrieved using the CB_GETLBTEXT message.
The bitmaps used for the list item depend on the food group. InitGroupList uses the CB_SETITEMDATA message to associate a constant with each list item, identifying the corresponding food group. The window procedure uses this value, contained in the itemData member of the DRAWITEMSTRUCT structure, to determine which bitmaps to display. The system uses two bitmaps for each food group symbol: a monochrome bitmap with the SRCAND raster operation to erase the irregular region behind the image, and a color bitmap with the SRCPAINT raster operation to paint the image.