.NET Core - calculating code coverage with OpenCover (on Windows)

1. Introduction.#

I’ve been struggling for some time to make OpenCover work with .NET Core projects, so I’ve decided to document a working example. This is basically an aggregation of hints and answers I found on the Web (especially in here)

2. Working example.#

If you run an OpenCover with the same arguments you were running it for a classic framework, you will end up with something like this

As you can see the coverage was not calculated and OpenCover is complaining about a missing PDBs. In order to fix that we have to do two things. First of all, we have to build the project with a full PDBs

dotnet build "PathToYourSolution.sln" /p:DebugType=Full

And then run OpenCover with and -oldStyle switch

OpenCover.Console.exe -target:"C:/Program Files/dotnet/dotnet.exe" -targetargs:"test \"PathToTestProject.csproj\" --configuration Debug --no-build" -filter:"+[*]* -[*.Tests*]*" -oldStyle -register:user -output:"OpenCover.xml"

From now on, OpenCover recognizes PDB files and the code coverage is calculated correctly

3. Cake script.#

As we use a Cake as our build automation tool I’ve also prepared a sample script for generating a code coverage. The core part of the script looks as follows

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// rest of the code omitted for brevity
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Task("Build")
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    .IsDependentOn("Restore-NuGet-Packages")
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    .Does(() =>
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{
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    DotNetCoreBuild(paths.Files.Solution.ToString(), new DotNetCoreBuildSettings
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    {
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        Configuration = parameters.Configuration,
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        ArgumentCustomization = arg => arg.AppendSwitch("/p:DebugType","=","Full")
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    });
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});
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Task("Run-Tests")
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    .IsDependentOn("Build")
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    .Does(() =>
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{
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    var success = true;
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    var openCoverSettings = new OpenCoverSettings
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    {
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        OldStyle = true,
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        MergeOutput = true
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    }
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    .WithFilter("+[*]* -[*.Tests*]*");
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    foreach(var project in paths.Files.TestProjects)
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    {
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        try
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        {
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            var projectFile = MakeAbsolute(project).ToString();
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            var dotNetTestSettings = new DotNetCoreTestSettings
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            {
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                Configuration = parameters.Configuration,
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                NoBuild = true
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            };
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            OpenCover(context => context.DotNetCoreTest(projectFile, dotNetTestSettings), paths.Files.TestCoverageOutput, openCoverSettings);
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        }
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        catch(Exception ex)
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        {
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            success = false;
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            Error("There was an error while running the tests", ex);
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        }
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    }
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    ReportGenerator(paths.Files.TestCoverageOutput, paths.Directories.TestResults);
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    if(success == false)
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    {
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        throw new CakeException("There was an error while running the tests");
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    }
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});

Notice that script is able to merge code coverage results from multiple test projects thanks to MergeOutput flag passed to OpenCover.

Full sources of that script (along with a dotnet vstest alternative approach) can be found here.