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BuildShadowTask task failed unexpectedly after merging Test Projects

I merged two branches of a solution which each had new MSUnit test projects in them. After the merge, this build error was raised: Error 91 The "BuildShadowTask" task failed unexpectedly.

System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.VisualStudio.TestTools.BuildShadowReferences.BuildShadowTask.Execute()
   at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()
   at Microsoft.Build.BackEnd.TaskBuilder.<ExecuteInstantiatedTask>d__26.MoveNext() MyProject.OrderProcessor.Tests

Take a note of which project is causing the problem - here, it's MyProject.OrderProcessor.Tests. Now open that particular .csproj (or .vbproj) in a text editor other than Visual Studio (I use Notepad++) and search for 'Shadow'...

    <Shadow Include="Test References\MyProject.OrderProcessor.accessor" />

You can simple delete the Shadow Include line. When you switch back to VS, it will want to reload the project. Allow this, then Rebuild the project or solution.

If you needed the Private Accessor

In my case, I didn't need the Private Accessor we just deleted. But if you need yours, you can regenerate it by following the MS guide for Unit Tests for Private, Internal, and Friend Methods.

Hope it helps!

P.s. This is my 100th post on my upblog!

It has been 1068 days since I started this blog (on 7th August 2014) which means that on average, I post here every 10-and-a-half days :) I think that's quite good...