An owner-drawn combo box sends the WM_MEASUREITEM message to its parent window or dialog box procedure so the application can set the dimensions of each list item. Because the example combo box has the CBS_OWNERDRAWFIXED style, the system sends the WM_MEASUREITEM message only once. Combo boxes with the CBS_OWNERDRAWVARIABLE style send a WM_MEASUREITEM message for each list item.
The lParam parameter points to a MEASUREITEMSTRUCT structure that identifies the control and list item. It also contains the default dimensions of the list item. The example in Creating a Square Meal Dialog Box modifies the itemHeight structure member to ensure that the list items are high enough to accommodate the food-group bitmaps.