Sometimes ShowWindow never calls OnShowWindow and main App hangs

Question

I have a multi-threaded application with several supporting DLLs and several popup dialogs. My Main App loads all DLLs on startup, which creates all popups, but they are kept hidden until needed.

When the user presses a button in the main app, a particular popup is shown (from the DLL) by calling ShowWindow( SW_SHOW ) (modeless)

Sometimes (1-in-10 times?) the popup simply fails to show and the Main App hangs. OnShowWindow of the Popup dialog is never called. I have tried calling ShowWindowASync instead, and it still fails to show the popup sometimes, but this call doesn't lock the Main App.

This problem only affects popups within a single DLL.

If the popup shows the first time ShowWindow is called, it can be closed and re-opened indefinitely throughout the lifetime of the Main App. If (using ShowWindowASync), the popup fails to show, it will never show during the lifetime of the Main App. I can re-run the application (without rebuilding anything) and there is a hit-or-miss chance that it will work or fail. I haven't been able to identify any predictive conditions or properties.

I have used tools to renumber all of my resource elements so that there are no conflicts throughout the solution.

UPDATE:

I used Winspector to get some information about the dialog when it does and does not work.

When the dialog works (shows properly), Winspector reports that my dialog has valid position (10,96, 1015, 514), style attributes that match the resource template, ID of 0 (not sure what ID means), and the "Owner EXE" is "MyApp.exe" - I can see many messages passing in and out of the dialog, including WM_SHOWWINDOW.

When the dialog fails (does not get a show window message), Winspector reports position (-1512, 190, -517, 634), style attributes that do not match the resource template, ID 509290824, and owner EXE is "C:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\devenv.exe" - I also do not see ANY messages in the message viewer within Winspector when the dialog fails to show.

Clearly, the dialog is not getting created correctly. CDialog::Create never fails for me. Additionally, I experience this problem in release (not using Visual Studio) so it's not a VS-specific problem.

Answer 1

The dialog was getting created from a thread that was not executing a message pump. This was the root problem. I had tried to initialize all dialogs (ie call "Create") from an "init" thread, and then use them later on in the winproc/GUI thread. Can't do this apparently.

A pointer to the dialog was then retrieved into the winproc/GUI (which has a message pump), but by that time, the dialog was already "corrupt" and no longer responding to winproc properly