dotnet test (.NET runner / dotCover): net48 tests not discovered when paired with net10.0-windows (MSTest.TestAdapter load fails from wrong TFM output folder)

[Perka1000]

Summary

For a test project multi-targeting net48 and net10.0-windows, TeamCity’s .NET dotnet test build step (with JetBrains dotCover in our case) fails to discover/execute tests on net48, while the same solution run via a Command Line step (dotnet test without dotCover) discovers and runs tests on both target frameworks.

Changing the modern TFM to net8.0-windows (i.e. net48;net8.0-windows) restores correct behavior for both TFMs with the .NET runner.

Environment

• TeamCity 2025.11.2 (build 208045)
• .NET SDK 10.0.202 on the build agent (Windows)
• Test project: se.Trimble.PT.ScriptEditor.Test with <TargetFrameworks>net48;net10.0-windows</TargetFrameworks>
• Solution: VS DotNet PT ScriptEditor.sln

Steps to reproduce

1. Create (or use) a test project with <TargetFrameworks>net48;net10.0-windows</TargetFrameworks> and MSTest-based tests.
2. Add two build steps against the same solution, same arguments in spirit:
   • Command Line: dotnet test "VS DotNet PT ScriptEditor.sln" --configuration Release --no-restore --no-build (plus any filters you use)
   • .NET → dotnet test runner (optionally with dotCover coverage enabled — our failing case uses dotCover wrapping dotnet test)
3. Run the build on an agent with .NET 10 SDK installed.

Expected

Both TFMs are discovered and executed; TeamCity reports tests for net48 and net10.0-windows (same as plain dotnet test).

Actual

• CLI dotnet test: runs net48 and net10.0-windows successfully.
• .NET runner (dotnet test via TeamCity / dotCover): net48 discovery fails; net10.0-windows runs.

Evidence (TeamCity build log excerpts)

From the .NET runner step (dotCover + dotnet test), during the net48 test host session:

Build_VS_DotNet_PT_ScriptEditor_1.2.4-alpha-STA-13748-update-se.113.log

[boris-yakhno]

Thank you for the report. I’ve filed a corresponding YouTrack issue: https://youtrack.jetbrains.com/issue/TW-100332