Exposing Owner-Drawn Combo Box Items

Application developers do not need to implement IAccessible to expose the items in an owner-drawn combo box that has the style CBS_HASSTRINGS because Active Accessibility exposes the items in combo boxes with this style. The items in an owner-drawn combo box with the CBS_HASSTRINGS style are displayed as text. However, this style is also used with owner-drawn combo boxes that do not display text so that the combo box items are exposed by Active Accessibility.

To allow Active Accessibility to expose the items in an owner-drawn combo box that does not display text:

  • Use the CBS_HASSTRINGS style when creating the combo box.
  • Create a textual counterpart that names or describes each item in the combo box.
  • When adding items to the owner-drawn combo box, use the CB_ADDSTRING message to add the text that you want Active Accessibility to expose. This text is not displayed, so it must not be part of the owner-drawn data. Add the owner-drawn item data using the CB_SETITEMDATA message.

When using the above method, note the following:

  • If you use the CBS_SORT style, the combo box is sorted using the supplied strings and not the WM_COMPAREITEM callback procedure.
  • With owner-drawn variable combo boxes created with the style CBS_OWNERDRAWVARIABLE, use a global variable or some other mechanism to keep track of when the itemData member of the MEASUREITEMSTRUCT is valid. The global variable is required because the system sends the WM_MEASUREITEM message as soon as the string is added but before the item data is attached, and at this point the itemData member is not valid.
  • To change the string for an item in a combo box with the CBS_HASSTRINGS style, delete the item with the CB_DELETESTRING message and add the new string with the CB_ADDSTRING message.

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