MSTest v4 on .net6 fails test discovery with `MissingMethodException` - `DoesReferencesAssembly`

[ehanover]

Describe the bug

I understand MSTest v4 drops official support for .NET 6, but I would like to try to work around this limitation because I must continue to support .NET 6. My solution builds but Test Discovery fails with MissingMethodException:

========== Starting test discovery ==========
Process: C:\...\TestProject\bin\x64\Debug\net6.0\TestProject1.dll, Exception: StreamJsonRpc.RemoteInvocationException: System.MissingMethodException: Method not found: 'System.Nullable`1<Boolean> Microsoft.VisualStudio.TestPlatform.ObjectModel.Utilities.AssemblyHelper.DoesReferencesAssembly(System.String, System.Reflection.AssemblyName)'.
   at Microsoft.VisualStudio.TestPlatform.MSTestAdapter.PlatformServices.TestSourceHandler.IsAssemblyReferenced(AssemblyName assemblyName, String source)
   at Microsoft.VisualStudio.TestPlatform.MSTest.TestAdapter.Discovery.AssemblyEnumeratorWrapper.GetTests(String assemblyFileName, IRunSettings runSettings, ITestSourceHandler testSourceHandler, List`1& warnings) in /_/src/Adapter/MSTestAdapter.PlatformServices/Discovery/AssemblyEnumeratorWrapper.cs:line 42
   at Microsoft.VisualStudio.TestPlatform.MSTest.TestAdapter.UnitTestDiscoverer.DiscoverTestsInSource(String source, IMessageLogger logger, ITestCaseDiscoverySink discoverySink, IDiscoveryContext discoveryContext) in /_/src/Adapter/MSTestAdapter.PlatformServices/Discovery/UnitTestDiscoverer.cs:line 58
   at Microsoft.VisualStudio.TestPlatform.MSTest.TestAdapter.UnitTestDiscoverer.DiscoverTests(IEnumerable`1 sources, IMessageLogger logger, ITestCaseDiscoverySink discoverySink, IDiscoveryContext discoveryContext) in /_/src/Adapter/MSTestAdapter.PlatformServices/Discovery/UnitTestDiscoverer.cs:line 41
   at Microsoft.VisualStudio.TestPlatform.MSTest.TestAdapter.MSTestDiscoverer.DiscoverTests(IEnumerable`1 sources, IDiscoveryContext discoveryContext, IMessageLogger logger, ITestCaseDiscoverySink discoverySink, IConfiguration configuration) in /_/src/Adapter/MSTest.TestAdapter/VSTestAdapter/MSTestDiscoverer.cs:line 52
   at Microsoft.VisualStudio.TestTools.UnitTesting.MSTestBridgedTestFramework.SynchronizedDiscoverTestsAsync(VSTestDiscoverTestExecutionRequest request, IMessageBus messageBus, CancellationToken cancellationToken) in /_/src/Adapter/MSTest.TestAdapter/TestingPlatformAdapter/MSTestBridgedTestFramework.cs:line 40
   at Microsoft.Testing.Extensions.VSTestBridge.SynchronizedSingleSessionVSTestBridgedTestFramework.<>c__DisplayClass22_0.<<ExecuteRequestAsync>b__0>d.MoveNext() in /_/src/Platform/Microsoft.Testing.Extensions.VSTestBridge/SynchronizedSingleSessionVSTestAndTestAnywhereAdapter.cs:line 160
--- End of stack trace from previous location ---
...
========== Test discovery aborted: 0 Tests found in 418.8 ms ==========

Is there a way to resolve this issue so that the tests in .NET 6 are able to run? Or get closer to running? Thank you.

Steps To Reproduce

1. Tested in Visual Studio 2022
2. Project with <TargetFrameworks>net6.0;net8.0</TargetFrameworks> and <PackageReference Include="MSTest" Version="4.0.1" />
3. Run tests
4. Observe tests on net8.0 pass, while the test run on net6.0 fails during test discovery with the error above (in Tests output window) before executing

[Youssef1313]

All you can do is either stay on MSTest 3, or update your test project to net8.0 (that should work even if your product is still net6.0). I'm closing as by-design.

[Evangelink]

@ehanover Is there anything preventing you to have the test app targeting net8.0 even if your main code still targets net6.0?

[SimonKimDev]

@Evangelink its an unit test suite for our Client project which we distribute to our customers and some of them would rather not move out of .NET6 unfortunately. Having said that, there isn't anything preventing us from making test app targeting .NET8, the angle is that we might miss bugs that'd only appear on .NET 6 runtime and that is the risk we didn't wanted to introduce.

@abatishchev fyi