Why is the 64-bit MSBuild loading 32-bit extension

Question

I'm attempting to build a project using MSBuild (v4.0) on a 64-bit machine. For some reason, MSBuild is attempting to load a 32-bit extension, and I cannot figure out why. I've reduced the problem to the smallest set in order to demonstrate the issue.

Using the following MSBuild project file:

<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="4.0">
    <Target Name="test">
        <Message Text="bin path: $(MSBuildBinPath)" />
        <Message Text="extensions path: $(MSBuildExtensionsPath)" />
        <Message Text="extensions path (x86): $(MSBuildExtensionsPath32)" />
        <Message Text="extensions path (x64): $(MSBuildExtensionsPath64)" />
    </Target>
</Project>

I get this output:

Microsoft (R) Build Engine Version 4.0.30319.1
[Microsoft .NET Framework, Version 4.0.30319.1]
Copyright (C) Microsoft Corporation 2007. All rights reserved.

Build started 8/27/2010 9:56:35 AM.
Project "D:\5\test.proj" on node 1 (default targets).
test:
  bin path: C:\Windows\Microsoft.NET\Framework64\v4.0.30319
  extensions path: C:\Program Files (x86)\MSBuild
  extensions path (x86): C:\Program Files (x86)\MSBuild
  extensions path (x64): C:\Program Files\MSBuild
Done Building Project "D:\5\test.proj" (default targets).


Build succeeded.
    0 Warning(s)
    0 Error(s)

Time Elapsed 00:00:00.03

MSBuild obviously knows about the 32bit and 64bit extensions path, and from the binary path it seems clear that I'm running the 64-bit MSBuild.exe, but for some reason it believes that extensions should be loaded from Program Files (x86) instead of Program Files. This is causing me trouble, as I have an extension that I need loaded, that MUST be loaded correctly in a 32bit/64bit process, and it will not load (MSBuild is attempting to load the 32bit version in a 64bit process).

Why?

Answer 1

I filed a bug on Microsoft Connect, and it was closed as "By Design", with this explanation:

You're exactly right -- this has changed, and strictly speaking, it's wrong now. However, this was a conscious decision. The reason it was changed was that very many extensions (such as .targets files) installed by other products are only installed in the 32 bit program files location. They did not anticipate 64 bit scenarios, but generally would work just fine in 64 bit MSBuild. When a user runs 64 bit MSBuild, which is quite common now because it's the default for Team Build 2010, MSBuildExtensionsPath would have in the past resolved to the 64 bit Program Files as you expect. However this meant that all those .targets files were not longer found and the build failed. It was not practical to get all those products to fix their setup authoring, especially since it had already shipped to customers. So we made the change to make MSBuildExetnsionsPath always point to the 32 bit location. Almost nobody seems to really want the 64 bit location, and those people can change to MSBuildExtensionsPath64. It was really a question of the least bad option here.

I accept the evidence, but I disagree with the conclusion. I believe that authors of broken installers deserve to have their extensions not work on 64-bit machines.