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The case of the missing Mergemod.dll in WiX v3.0.4123

There’ve been a couple of reports that WiX v3.0.4123 doesn’t include mergemod.dll. That’s unfortunate because it’s a really annoying bug and worse, I was the last one to touch mergemod.dll (to fix 1965131) so it was probably my fault.

I couldn’t reproduce the problem, however; mergemod.dll was present in Wix3.msi, Wix3_x64.msi, and I was about to ask for install logs to investigate when a likely cause occurred to me. The bug fix was to revert from a version of mergemod.dll with a bug to a prior one that doesn’t have the bug. Unfortunately, downgrading files is problematic with major upgrades: Windows Installer wants very much to keep higher-versioned files around. After all, the latest version probably has all the latest bug fixes, right?

If you install WiX v3.0.4123 over v3.0.4116, the log will contain a line like this:

MSI (c) (34:D4) [10:47:26:734]: Disallowing installation of component: {AAF02F71-9684-4F4F-8EEA-FC99A61EAA9A} since the same component with higher versioned keyfile exists

Simple workaround: Uninstall the earlier version of WiX, then install v3.0.4123.