WiX v2 needs fixed mergemod.dll too

As Rob mentioned, one of the things we’re looking at fixing in a WiX v2 service release is to downgrade the version of mergemod.dll to one that doesn’t have a particular bug. For more information on the bug, see Rob’s original blog post and bugs 1965131 and 1911165.)

Until that release, you can download the latest WiX v3 release, extract mergemod.dll from the .zip file, and replace your copy of mergemod.dll with it. (If you installed via the Votive installer, it will be in Program Files\Windows Installer XML\bin.)

We’ll get that v2 SP1 out to solve the problem instead of just working around it.

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 wix3-binaries.zip. 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.