VS2017 Could not load file or assembly Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll or one of its dependencies

I had a similar issue (with the additional message The "BuildShadowTask" task failed unexpectedly) with a project originally developed with VS2010, and got to spend the last few hours learning about yet another legacy facet of the build process.

There is a good chance that you are dealing with private accessor files (.accessor), which were deprecated in VS2012 (original source). This was foreshadowed in an announcement from the VS2010 team that they were no longer working on these features.

There is also a chance you're just dealing with erroneous refs to the wrong version of UnitTestFramework, but a NuGet restore should fix this. If not, see this GitHub thread for a possible fix (manually change the ref to the public folder), or move to the new MSTest.TestAdapter and MSTest.TestFramework packages (see MSDN support thread).

Solutions

A. Edit the unit test .csproj and change the item Include references from Shadow => None: <Shadow Include="Test References\namespace.accessor" /> to
<None Include="Test References\namespace.accessor" />

B. Better yet, simply delete all the .accessor files from the unit test project's Test References folder.

Ideally, you would also rewrite your unit tests to remove references to private methods, either by re-architecting to separate concerns or by changing properties to internal and using "friend" with the InternalsVisibleToAttribute.

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For those who need to continue supporting testing of private methods for some reason, the same post provides the following suggestions to the logical question "What is available for me then?":

For those who wish to continue testing internal APIs, you have three options:

1. Use the Microsoft.VisualStudio.TestTools.UnitTesting.PrivateObject class to assist in accessing internal and private APIs in your code. This is found in the Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll assembly.
2. Create a reflection framework that would be able to reflect off your code to access internal or private APIs.
3. If the code you are trying to access is internal, you may be able to access your APIs using the InternalsVisibleToAttribute so your test code can have access to the internal APIs.

However, there is not any good replacement for Code Generation for the new features added by the lanugage teams. You may create the TestMethod stubs and then remove the internal code. You only need to keep the stub itself.

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Further reading / sources that helped me piece this together:

• VS 2005 ASP.NET explanation of accessors
• 2008 blog article explaining how to work around this for build servers
• MSDN forum thread with discussion on accessor purposes, implementations, and workarounds. Start about 1/3 down.
• MSDN BaseShadow docs
• MSDN PrivateObject class

Right click the project references folder. Add reference > Assemblies > extensions. Check Microsoft.VisualStudio.QualityTools.UnitTestFramework 10.1, and uncheck any older version.