Hi,
This is actually a restriction of Visual Studio.
There are two possibilities
:arrow: 1. Use an Environment Variable as you suggested.
You will have to define the environment variable on all machines
<Reference Include="TypeMock...">
<HintPath>$(TypeMockInstallPath)TypeMock.dll</HintPath>
</Reference>
:arrow: 2. With the Enterprise Edition, you can use the AutoDeploy feature.
This will enable multiple versions and enable you to copy the TypeMock files to your source tree.
Once the dll's are in your source tree you can use the relative path.
<Reference Include="TypeMock...">
<HintPath>..BuildArtifactsTypeMockTypeMock.dll</HintPath>
</Reference>
The second method will also work better with version upgrades, where the version used will be the one in the source tree.
This makes the upgrade much simpler, via checking in the new version into the source tree