Why doesn't my program receive the WM_DWMSENDICONICTHUMBNAIL message when I ask for an iconic representation?

Raymond Chen

A customer was having trouble adding Windows 7 taskbar integration features to their application:

I’m trying to use the new Windows 7 taskbar integration features, but am running into a problem. I’ve made certain that my program has the DWMWA_FORCE_ICONIC_REPRESENTATION and DWMWA_HAS_ICONIC_BUTMAP DWMWA_HAS_ICONIC_BITMAP [corrected 8am] attributes set, yet I never receive a WM_DWMSENDICONICTHUMBNAIL message in my window procedure.

A member of the taskbar team invoked psychic powers:

Is your program running elevated by any chance? If so, then you need to add the WM_DWMSENDICONICTHUMBNAIL message to your UIPI message filter:

ChangeWindowMessageFilter(WM_DWMSENDICONICTHUMBNAIL, MSGFLT_ADD);

Psychic powers once again save the day.

That was it. Thanks!

Note that forcing someone to invoke psychic powers is a bad thing. It means that you withheld crucial information that somebody had to guess at. Here are some crucial pieces of information people tend to forget to mention:

• “Oh, did I mention that my process is running elevated?”
• “Oh, did I mention that my process is a service?”
• “Oh, did I mention that my process is 32-bit, running on a 64-bit machine?”
• “Oh, did I mention that I initialized COM as MTA?”
• “Oh, did I mention that my process is impersonating another user?”
• “Oh, did I mention that I’m running a version of the operating system different from Windows 7?” (or whatever the most current client operating system is)

Added: The reason those items above are crucial pieces of information is that all of them (except the last one) are “game-changers”: If you do any of the things in the list, a whole bunch of rules change. It’s like asking a question about traffic regulations and neglecting to mention that the other vehicle is a fire truck with its siren on. (And presumably when you chose to invoke any of those game-changing conditions, you were aware of the rules which applied to the new situation. Those rules are documented with the game-changing condition, not with the general description of a normally-functioning universe. Imagine if every physics book had the sentence “This rule does not apply to Superman” at the end of every paragraph.)

Raymond Chen

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