How to generate a coverage report for multiple test-projects?

[FroggieFrog]

Question

How can I generate a (combined) coverage report for multiple test-projects using cake?

At the moment there are no reports generated from coverlet. (coverageDirectory is empty)

General information

• project (net5.0-windows) with multiple test-projects in different locations
• all test-projects have a reference to xunit and Microsoft.NET.Test.Sdk and coverlet.collector
• build script was created using the .NET Tool (https://cakebuild.net/docs/getting-started/setting-up-a-new-project)
• build and test inside of Visual Studio 2019 is working (using xunit.runner.visualstudio)
• build via cake script is also working

Current script

#addin nuget:?package=Cake.Coverlet&version=2.5.1
#tool nuget:?package=ReportGenerator&version=4.6.1

...

// Define directories.
var rootAbsoluteDir = MakeAbsolute(Directory("./")).FullPath;
var logDirectory = MakeAbsolute(Directory($"./logs/{configuration}")).FullPath;
var outDirectory = MakeAbsolute(Directory($"./out/{configuration}")).FullPath;
var reportDirectory = MakeAbsolute(Directory($"./reports/{configuration}")).FullPath;
var coverageDirectory = MakeAbsolute(Directory($"{reportDirectory}/coverage")).FullPath;
var reportGeneratorDirectory = MakeAbsolute(Directory($"{reportDirectory}/ReportGenerator")).FullPath;
var reportGeneratorHistoryDirectory = MakeAbsolute(Directory($"{reportDirectory}/History")).FullPath;

...

var runUnitTestsTask = Task("Run-Unit-Tests")
.IsDependentOn(buildTask)
    .Does(() =>
{
        var testProjects = GetFiles($"./../**/*.Tests.csproj");
        Information($"found {testProjects.Count} test projects");
        Information("");

        foreach (var curTestProject in testProjects)
        {
            var testSettings = new DotNetCoreTestSettings
            {
                NoLogo = true,
                Configuration = configuration,
                NoRestore = true,
                NoBuild = true
            };

            var coverletSettings = new CoverletSettings
            {
                CollectCoverage = true,
                CoverletOutputFormat = CoverletOutputFormat.opencover,
                CoverletOutputDirectory = coverageDirectory,
                //separate name for every test-project?
                //CoverletOutputName = new FilePath($"{System.IO.Path.GetFileNameWithoutExtension(curTestProject)}.opencover.xml").FullPath
            };

            DotNetCoreTest(MakeAbsolute(curTestProject).FullPath, testSettings, coverletSettings);
        }
});

var reportTask = Task("Report")
    .IsDependentOn(runUnitTestsTask)
    .Does(() =>
{
    Information("");
    Information($"{DateTime.Now:hh:mm:ss.ff} starting ReportGenerator");

    //How to get the combined filePath?
    var coverageResultsFilePath = new FilePath($"{coverageDirectory}/OpenCover_results.xml");

    var reportSettings = new ReportGeneratorSettings
    {
        ReportTypes = new[] { ReportGeneratorReportType.Html },
    };
    ReportGenerator(coverageResultsFilePath, reportGeneratorDirectory, reportSettings);
});