Excel freezes on Open/Save As if a managed add-in is registered with a “Warmup” registry value, even if the add-in is not even loaded.
The “Warmup” registry value, when set to 1 in an add-in’s registry key, was once supposed to cause Excel to load the VSTO runtime at startup, rather than when the first user interaction with the add-in occurred. Apparently, it was never implemented and has since been removed from Microsoft’s online documentation of the registry entries for VSTO add-ins.
Once I removed the “Warmup” value from my add-in’s registry key, Excel’s Open/Save As dialogs no longer froze, nor did the add-in’s own SaveFileDialog.ShowDialog() calls. I don’t know why this is, but evidently, the “Warmup” value does do something.
I was able to reproduce this with a dummy add-in that does not do anything. If that dummy add-in is registered in HKCU\Software\Microsoft\Excel\Addins without the “Warmup” DWORD, Excel works just fine. As soon as I add the DWORD with a value of 1, Excel freezes when I click Open or Save As — again, the dummy add-in does not do anything at all.
So, if people get here wondering why their Office application freezes after installation of a VSTO add-in, I’d suggest to remove the “Warmup” value from the registry rather than spending hours debugging their code (as I did before I found out about “Warmup”).
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solved How to debug hanging SaveFileDialog