Roslyn and unit tests suck


Title: Imperial Gardens in Tokyo, Source: own resources, Authors: Agnieszka and Michał Komorowscy

I'm working on the project where I have an opportunity to use Roslyn compiler as a service. It is very good :) However yesterday it took me more than 2 hours to write working unit tests (based on MSTest) for my code! Here are some tips that may save your time.

Let's start with the simle thing. When I run unit tests for the first time the following exception was thrown:

System.IO.FileNotFoundException: Could not load file or assembly 'System.Runtime...' or one of its dependencies.

To fix this problem I simply installed the following packages via Nuget:
  • Microsoft.CodeAnalysis.CSharp
  • Microsoft.CodeAnalysis.CSharp.Workspaces
Later it was harder. In my code I use MSBuildWorkspace.OpenSolutionAsync and MSBuildWorkspace.OpenProjectAsync methods to respectively open the entire solution or a single project for further processing.

The next issue was that the first method called from within a unit test was returning an empty solution i.e. without any projects. Whereas the second one was throwing an exception with the message: The language 'C#' is not supported. What was strange these problems occurred only in unit tests! To investigate a problem I opened Exception settings window in Visual Studio and selected a check box next to Common Language Runtime Exceptions. Then, I run the unit tests one more time and Visual Studio quickly reported the exception in the line with MSBuildWorkspace.OpenProjectAsync:

System.IO.FileNotFoundException: Could not load file or assembly 'Microsoft.CodeAnalysis.CSharp.Workspaces...' or one of its dependencies.

It was even more strange because my unit test project was actually referencing Microsoft.CodeAnalysis.CSharp.Workspaces.dll! To double check, I went to the unit tests working directory. It is a folder called TestResults which by default is located in the solution directory. To my surprised this dll was missing!

Fortunately, I reminded myself the similar situation from the past. The problem is that MSTest doesn't copy all assemblies to the output directory by default. As far as I know it tries to figure out which assemblies are really needed by the code being tested. Here, I'm not sure but Microsoft.CodeAnalysis.CSharp.Workspaces.dll may be cumbersome because it is not directly referenced by other Roslyn assemblies. Instead, it is probably loaded dynamically when needed.

To fix a problem you can use the simple hack i.e. use directly any code from Microsoft.CodeAnalysis.CSharp.Workspaces.dll in your unit tests in the following way:

public static void ClassInitialize(TestContext ctx)
   var t = typeof(Microsoft.CodeAnalysis.CSharp.Formatting.LabelPositionOptions);

Why did I use LabelPositionOptions? Because majority of types defined in aforementioned assembly is internal and this one was the first public type I found :)


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