Subclassing the Desktop and Windows of Other Applications

The information in this article applies to:

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

SUMMARY

The code to subclass the desktop window, or windows that belong to other applications, must be placed in a dynamic-link library (DLL).

MORE INFORMATION

In most cases, the desktop window uses the stack segment (SS) of the currently scheduled task. This is done for performance reasons because it reduces the number of task switches that take place when the desktop receives paint or mouse messages. In some cases, the desktop cannot be switched immediately to the currently scheduled task. In this situation, it will temporarily use the SS of the previously scheduled task.

When the desktop is subclassed, the data segment (DS) that the subclass procedure uses is specified by the hInstance used in a MakeProcInstance() call. However, the subclass procedure's SS depends on the currently scheduled task, as described earlier. Because the currently scheduled task may not be the task that subclassed the desktop, it is possible that SS != DS when the subclass procedure is executed. Therefore, the subclass procedure cannot assume that SS == DS. For this reason, the code to subclass the desktop must reside in a DLL where it is assumed that SS != DS.

When a window belonging to another application is subclassed, the DS that the subclass procedure uses is specified by the hInstance used in the MakeProcInstance() call. The SS used by the subclass procedure is the SS of the application that owns the window. Therefore, the subclass procedure cannot assume that SS == DS. For this reason, the code to subclass another application's window must reside in a DLL where it is assumed that SS != DS.

The DLL in which the subclass procedure is placed should be compiled with the Microsoft C Compiler's -Aw option, which tells the compiler that SS != DS. Using this option causes the compiler to generate a warning message when it detects the improper creation of a near pointer to an automatic variable.