Subclassed Combo Boxes

Subclassing is a procedure that allows an application to intercept and process messages sent or posted to a window. By using subclassing, an application can substitute its own processing for certain messages, while leaving most message processing to the class-defined window procedure.

When the operating system creates a window, it saves information about it in an internal data structure that includes the address of the window procedure. To subclass a window, an application calls the SetClassLong function to replace the address of that procedure with the instance address of an application-defined subclass procedure. Thereafter, all messages to the window are sent to the subclass procedure. This procedure then uses the CallWindowProc function to pass unprocessed messages to the original window procedure. For a description of the message processing performed by the COMBOBOX class window procedure, see Default Combo Box Behavior.

When the combo box is outside a dialog box, an application cannot process the TAB, ENTER, and ESCAPE keys unless it uses a subclass procedure. When a simple or drop-down combo box receives the input focus, it immediately sets the focus to its child edit control. Therefore, an application must subclass the edit control to intercept keyboard input for a simple or drop-down combo box. For an example of this, see Subclassing a Combo Box.

If a subclass procedure processes the WM_PAINT message, it must use the BeginPaint function to prepare for painting. Before calling the EndPaint function, it passes the device-context (DC) handle as the wParam parameter for the window procedure. If EndPaint is called first, the class window procedure does no painting because EndPaint validates the entire window.

A technique related to subclassing is superclassing. A superclass resembles any other class except that its window procedure does not call DefWindowProc to handle unprocessed messages. Instead, it passes unprocessed messages to the window procedure for the parent window class. Follow the guidelines in Window Procedures to avoid problems that can occur with subclassing and superclassing.

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