DOCERR: How Dialog Functions Return Values Indicate Failure

The information in this article applies to:

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

SUMMARY

Windows has eight functions that are used to create dialog boxes: four are for modal dialog boxes, and four are for modeless dialog boxes.

Modal dialog boxes are created using the DialogBox, DialogBoxIndirect, DialogBoxParam, and DialogBoxIndirectParam functions. These functions return -1 to signal failure.

Modeless dialog boxes are created with the CreateDialog, CreateDialogIndirect, CreateDialogParam, and CreateDialogIndirectParam functions. These functions returns NULL to signal failure.

The "Microsoft Windows Software Development Kit Reference Volume 1" for version 3.0 incorrectly specifies on page 4-44 the failure return value for the CreateDialogParam function to be -1. The documentation should state that the function returns NULL. This documentation error has been corrected on page 91 of the "Microsoft Windows Software Development Kit: Programmer's Reference, Volume 2: Functions" for version 3.1.

MORE INFORMATION

The DialogBox* functions that create modal dialog boxes return the value specified as the second parameter to the EndDialog function, which is used to end the processing in a modal dialog box.

The DialogBox* functions cannot return NULL (==FALSE) to indicate an error because returning the TRUE or FALSE values through the EndDialog function might be useful to an application: possibly to indicate that the user selected an OK or Cancel button.

The CreateDialog* functions that create modeless dialog boxes return a window handle when the dialog box is successfully created. The value -1 (0xFFFF) can be a valid window handle and is not sufficient to indicate a situation in which the function failed and a dialog box was not created. Because NULL is the only invalid window handle, it is used as a return value to indicate the failure of a CreateDialog* function. The NULL return value indicates failure for all Windows functions that return a HANDLE data type.