Registering a Class in a DLL (97758)

SUMMARY

If a class is registered in a dynamic-link library (DLL) with the RegisterClass() function and an application uses this class when creating a window or a dialog box, then the CreateDialog() or CreateWindow() call in the application will fail. The debug version of Windows will produce a Fatal Exit code 0x8800 with the following message:

CreateWindow failed: Window class not found.

Either of the following may generate this message:

The window class in the DLL was not registered correctly with the CS_GLOBALCLASS class style. Without this class style, a class cannot be accessed globally by all the applications that link into the DLL. -or-
The DLL is not loaded into memory and therefore the class is not accessible. This will happen if the application implicitly links to the DLL, and none of the exported functions in the DLL is explicitly called in the application. To work around this problem, the DLL must provide an entry point for the application to call or the application must explicitly load and free the library.

MORE INFORMATION

To register a window class in a DLL and subsequently create a window (or create a dialog box) of that class in any application that links into this DLL, the window class must be registered with the CS_GLOBALCLASS class style. This style specifies that the window class is an application global class, and will allow this class to be accessed globally by all applications. The CS_GLOBALCLASS class style is documented in the Windows version 3.1 SDK "Programmer's Reference, Volume 1: Overview," manual on page 19 under the Class Styles section.

Care must be taken to ensure that an application global class is not accessed after the DLL (that registered it) terminates. Doing so will generate a Fatal Exit 0x001A message under the debug version of Windows. For more information on this topic, query on the following words in the Microsoft Knowledge Base:

fatal exit and 0x001A

After registering the window class using the CS_GLOBALCLASS class style, calling CreateWindow or CreateDialog from an application should work correctly. However, if the intent is to have the DLL own the message queue, this is not possible. The reason is that DLLs, being taskless entities, are designed to be resources or to only provide resources and functions; they are not capable of handling the message processing. This functionality must be implemented in an application.

Also, note that with Windows version 3.0, there is a potential problem in destroying windows whose window class has been registered with the CS_GLOBALCLASS class style by a DLL (or an application). This problem has been corrected in Windows version 3.1. For more information on this problem and a solution, please query on the following words in the Microsoft Knowledge Base:

exit and windows and DLL-instanced

Once the class is registered properly with the CS_GLOBALCLASS style in the DLL, any application can use it, provided the DLL is loaded in memory. This issue is of concern in cases where the DLL is used only to register classes (for example, in LibMain) that can be used by all applications (for example, custom control libraries). Because the DLL does not contain any function entry points that the application can call into, there are no call-dependencies between the application and the DLL, and therefore the DLL will not be loaded implicitly. In such a case, the solution is that the DLL must provide an entry point that the application can call or the application must explicitly load and free the library using LoadLibrary and FreeLibrary application programming interfaces (APIs).