The Windows Marketplace for Mobile Client and your Setup code

The Windows Marketplace for Mobile Client and your Setup code:
The Windows Marketplace for Mobile client provides users a streamlined experience to purchase and install applications on their Windows Mobile 6.5 devices (coming soon for 6.1, and 6.0).

As you will notice when installing applications, the user interface is different from the typical CAB file installation (see below) perhaps you are used to.

It is very important that the Marketplace install experience be uniform and streamlined. To keep it this way, no custom UI (errors, prompts, informational messages, etc.) should be displayed during installation or uninstallation.
If you absolutely must display custom UI, there are some things you should know about the Marketplace client.
The Marketplace client achieves a streamlined experience by suppressing the normal user interface, displaying a client specific progress bar and providing feedback when the installation has completed. This can cause problems for applications that have custom prompts implemented in the setup dll. For example, you may want to prompt the user to start your application after installation has completed. This is easily done by adding the appropriate MessageBox prompt to the Install_Init export of your setup dll. However, there can be problems with this.
The foreground display of your MessageBox or other custom UI, depends on knowing the window handle of the calling process. When your application is installed with the Marketplace client, no window handle is passed to your setup DLL’s exported functions. This can cause your custom UI to not be displayed in the foreground. This causes confusion for the user since it looks like the installation is still running, but there is no UI feedback. The only recourse for the user is to use task manager to locate the prompt window and select it to bring it to the foreground. This is a very poor user experience. 
You can resolve this by using the MB_TOPMOST | MB_SETFOREGROUND flags when calling MessageBox or WS_EX_TOPMOST when creating a window for custom UI.
Have you tried the Marketplace client yet? No? Don’t have a 6.5 ROM update or device yet? No problem. Download the Windows Mobile 6.5 standalone emulator images here. These come with a fully functional Marketplace client you can download and install apps with.
Hope this helps.
Mike