Sudden build failures on GitHub with "WixSharp attempted to install the extension but did not succeed"

[christophwille]

Our last successful Github build was on Jan 30, 2026, and today we got the following failure:

  Wix project file has been built: D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.wxs
  
  Compiling ILSpy.wxs
  Source base directory: D:\a\ILSpy\ILSpy
WixToolset.UI.wixext : warning WIX6101: Could not find expected package root folder wixext6. Ensure WixToolset.UI.wixext/7.0.0-rc.1 is compatible with WiX v6. [D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.Installer.csproj]
EXEC : error : Cannot find WiX extension 'WixToolset.UI.wixext'. WixSharp attempted to install the extension but did not succeed. Please install the extension manually with: `wix.exe extension add -g WixToolset.UI.wixext` [D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.Installer.csproj]
wix.exe : error WIX0144: The extension 'D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.wxs' could not be loaded because of the following reason: Bad IL format. The format of the file 'D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.wxs' is invalid. [D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.Installer.csproj]
D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.Installer.csproj(30,5): error MSB3073: The command "cd .\
D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.Installer.csproj(30,5): error MSB3073: set ide=true
D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.Installer.csproj(30,5): error MSB3073: "D:\a\ILSpy\ILSpy\ILSpy.Installer\bin\Release\net472\ILSpy.Installer.exe"" exited with code -1.
Done Building Project "D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.Installer.csproj" (default targets) -- FAILED.
Done Building Project "D:\a\ILSpy\ILSpy\ILSpy.Installer.sln" (default targets) -- FAILED.

Build FAILED.

"D:\a\ILSpy\ILSpy\ILSpy.Installer.sln" (default target) (1) ->
"D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.Installer.csproj" (default target) (2) ->
(PostBuild target) -> 
  WixToolset.UI.wixext : warning WIX6101: Could not find expected package root folder wixext6. Ensure WixToolset.UI.wixext/7.0.0-rc.1 is compatible with WiX v6. [D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.Installer.csproj]


"D:\a\ILSpy\ILSpy\ILSpy.Installer.sln" (default target) (1) ->
"D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.Installer.csproj" (default target) (2) ->
(PostBuild target) -> 
  EXEC : error : Cannot find WiX extension 'WixToolset.UI.wixext'. WixSharp attempted to install the extension but did not succeed. Please install the extension manually with: `wix.exe extension add -g WixToolset.UI.wixext` [D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.Installer.csproj]
  wix.exe : error WIX0144: The extension 'D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.wxs' could not be loaded because of the following reason: Bad IL format. The format of the file 'D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.wxs' is invalid. [D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.Installer.csproj]
  D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.Installer.csproj(30,5): error MSB3073: The command "cd .\
D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.Installer.csproj(30,5): error MSB3073: set ide=true
D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.Installer.csproj(30,5): error MSB3073: "D:\a\ILSpy\ILSpy\ILSpy.Installer\bin\Release\net472\ILSpy.Installer.exe"" exited with code -1.

https://github.com/icsharpcode/ILSpy/actions/runs/21996917625/job/63559229683?pr=3663

We are locking to Wix 6 for ages (see https://github.com/icsharpcode/ILSpy/blob/d8c619d2ad6f851df0534ff9b963e01ce7ba7404/.github/workflows/build-ilspy.yml#L53) but funnily WixSharp seems to try to pick up "stuff" from the just released (seven days at time of writing) Wix 7 RC1 https://www.nuget.org/packages/wix/7.0.0-rc.1#versions-body-tab. Given that release happened since the last successful build, this seems a likely culprit.

The PR itself was intended to be a simple NuGet update icsharpcode/ILSpy#3663 (initially didn't touch CI oder installer generation)

Tried:

• going from 2022 build agent to 2025
• going to latest point release of WixSharp
• going to 6.0.2 global tool of Wix.

Any ideas / tips what else to try?

[christophwille]

I tried v7 as well, that adds yet another message

[3](https://github.com/icsharpcode/ILSpy/actions/runs/21999191807/job/63567123720?pr=3663#step:21:254)
wix.exe : error WIX7015: You must accept the Open Source Maintenance Fee (OSMF) EULA to use WiX Toolset v7. For instructions, see https://wixtoolset.org/osmf/ [D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.Installer.csproj]

https://github.com/icsharpcode/ILSpy/actions/runs/21999191807/job/63567123720?pr=3663

[christophwille]

On the fifth try I happened on a winner by modifying the build pipeline like this:

    - name: Install wix extension (locked version)
      run: wix.exe extension add -g WixToolset.UI.wixext/6.0.2

icsharpcode/ILSpy@2cd2832

To me it seems as if wix extension add always grabs latest, and now with 7rc1 that behavior backfires? (if as is the default no version is specified)

[oleg-shilo]

Hi Christoph, I will need to dig into it to give you the accurate answer.
Sorry, I was completely consumed by CS-Script work.
Will come back to you in a day or so. But from my memory, installing the prerelease versions of the extensions will trigger the use of the latest installed package.
What I do not remember if it is a WixSharp's or a WiX's constraint. That's why I need to have a look.

[christophwille]

Note that I only installed 7rc1 as a later test, it had nothing to do with the initial failure. Current working setup: install Wix global tool and manually install extension to avoid WixSharp failing to install the extension.

[oleg-shilo]

OK, let's dissect the log file.

You are using WiX v6. however the latest Ensure WixToolset.UI.wixext extension found is a pre-release of v7, which is internally (via dir structure of the extension folder) marked as incompatible with the WiX6:

Could not find expected package root folder wixext6.

I have checked, and indeed WiX always installs the latest extension even if it is a prerelease or even incompatible with the current wix.exe. To overcome the problem, WixSharp allows targeting a specific version of the extension:

WixExtension.UI.PreferredVersion = "5.0.1"; // it will be installed if not found

I have also added the workaround/suggestion part to the error message to assist users in the future:

The second problem is more peculiar:

wix.exe : error WIX0144: The extension 'D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.wxs' could not be loaded because of the following reason: Bad IL format. The format of the file 'D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.wxs' is invalid. [D:\a\ILSpy\ILSpy\ILSpy.Installer\ILSpy.Installer.csproj]

It is a direct outcome of the first problem that leads to the WiX CLI build arguments to skip one position since the not-found extension file is empty, and taking the next argument as an extension dll file, which is definitely wrong. When you fix the first problem, the second one will also be fixed.

Though I have added a dummy but informative argument replacement, so there is no confusion anymore:

The new release will be available within a week. But you are good to go anyway.
......

Unrelated. Love ILSpy. 😄
Great work.

[christophwille]

Thanks for such a detailed analysis!