testconfig.json with comments is silently ignored

[FlukeFan]

Describe the bug

If I have a testconfig.json, it is automatically consumed.

If it is malformed JSON, it (correctly) fails the build.

If the JSON includes comments, it is silently ignored.

Version used

dotnet 10.0.302
Microsoft.Testing.Extensions.CodeCoverage 18.10.0

Steps To Reproduce

dotnet new install xunit.v3.templates
dotnet new xunit3
dotnet add package Microsoft.Testing.Extensions.CodeCoverage

Add a testconfig.json with:

{
  "codeCoverage": {
    "Configuration": {
      "IncludeTestAssembly": true // this is a comment
    }
  }
}

Then run:

dotnet run -- --coverage --coverage-output-format cobertura

Expected behavior

Either the build should fail (because comments are not allowed), or the config should
be respected (preferred).

Actual behavior

The project runs the tests successfully, but the generated coverage file does not contain the test project.

Note, if you remove the comment, then the coverage of the test project is included.

Additional context

I originally found this using TUnit, but I reproduced here using xUnit.

testconfig_bug.zip

[FlukeFan]

Could be related to #10137 maybe?

[Evangelink]

I investigated this, and it is not the same issue as #10137 / #10144.

#10144 added comment support to Microsoft.Testing.Platform's .NET Framework/Jsonite parser. This reproduction targets net8.0 and resolves Microsoft.Testing.Platform 2.3.3, which already contains that fix. MTP itself successfully parses the commented testconfig.json.

Microsoft.Testing.Extensions.CodeCoverage 18.10.0 independently discovers and reopens testconfig.json rather than consuming MTP's parsed configuration. Its JSON reader calls JsonDocument.ParseAsync with the default options, which reject comments. The parse failure is then caught and treated as if no coverage configuration existed, so coverage silently falls back to its defaults (IncludeTestAssembly is false).

I reproduced the reported behavior with .NET SDK 10.0.303:

• With the comment, the Cobertura report contains <packages />.
• Removing only the comment includes the testconfig assembly and its covered lines.

The fix therefore belongs in the Code Coverage implementation: parse JSON with JsonCommentHandling.Skip (and preferably AllowTrailingCommas = true for parity with MTP), and avoid silently falling back when an existing configuration file cannot be parsed. The same limitation was also observed in microsoft/codecoverage#163.