WM_CTLCOLOR Processing for Combo Boxes of all Styles

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.1 and 3.0

SUMMARY

Windows sends a WM_CTLCOLOR message to the parent of a control window to enable the parent to specify the color of the control. A notification code, which is the value of the high-order word of the lParam, accompanies the WM_CTLCOLOR message to indicate the control type for a WM_CTLCOLOR message. Windows does not define a notification code that enables an application to change the color of a combo box control. However, Windows sends WM_CTLCOLOR messages to a combo box control that relate to its component parts: one message for the list box portion and, if applicable, another message for the edit control portion. An application can subclass the control to intercept and process these messages. This article discusses how to perform the subclassing and how to address the problems that arise when an application changes the colors of a combo box under Windows 3.0.

MORE INFORMATION

Under Windows versions 3.0 and later, Windows sends a WM_CTLCOLOR message to a combo box for each of its individual elements. An application can subclass the combo box control to process the message. The text below discusses each of the combo box styles and how the color can be changed for each style.

CBS_SIMPLE Style

Under Windows 3.0, an application must process three WM_CTLCOLOR notifications codes, CTLCOLOR_EDIT, CTLCOLOR_MSGBOX, and CTLCOLOR_LISTBOX, to change the colors for a CBS_SIMPLE-style list box. Under Windows 3.1, the application is not required to process the CTLCOLOR_MSGBOX notification.

Each time the application processes a notification, it must set the foreground and background colors, using SetTextColor and SetBkColor, respectively. The wParam accompanying the WM_CTLCOLOR message contains a handle to the appropriate display context. In addition, the application must return a valid handle to the appropriate background brush that Windows will use to paint those areas not occupied by text.

CBS_DROPDOWN Style

To change the colors of a CBS_DROPDOWN-style combo box under Windows 3.0, process the same three notifications as for the CBS_SIMPLE combo box. However, there is a difficulty with regard to setting the text color. In the drop-down list box, the colors set with SetBkColor and SetTextColor are not used. Instead, the combo box uses the system default colors. As a workaround, change the combo box to the owner-draw style. Process the WM_DRAWITEM message to draw the individual items with the desired text colors.

For more information on using an owner-draw combo box in an application, query on the following words in the Microsoft Knowledge Base:

   prod(winsdk) and owncombo

CBS_DROPDOWNLIST Style

To change the color of a CBS_DROPDOWNLIST-style combo box, process the CTLCOLOR_LISTBOX notification. However, the application must process this notification in the combo box subclass procedure and in the window procedure for the parent window of the combo box.

Under Windows 3.0, the text color problem discussed above for CBS_DROPDOWN-style combo boxes is evident for CBS_DROPDOWNLIST combo boxes. To address this problem, the application must use an owner-draw combo box.

Although it is not necessary to use an owner-draw combo box under Windows 3.1, the application must process the CTLCOLOR_LISTBOX notification in both the combo box subclass procedure and in the window procedure for the parent window of the combo box.

NOTE: WM_CTLCOLOR with codes is obsolete in Win32. Each code now has its own message, such as WM_CTRLCOLORBTN, WM_CTLCOLORLISTBOX, and so forth. Otherwise, things work the way they do in Windows 3.1.