TestFx 2.2.0-preview won't discover or run tests on VS16.8

Description

TestFx won’t discover or run tests on .Net Framework 4.7 when using Visual Studio 16.8.

Steps to reproduce

1. Create a Unit Test Project on .Net Framework 4.7.
2. Verify that tests are getting discovered and run successfully.
3. Update MSTest.TestAdapter and MSTest.TestFramework to version 2.2.0-preview-20201126-03.
4. Run the tests using the Test Explorer Window.

Expected behavior

Tests are discovered and ran.

Actual behavior

---------- Starting test discovery for requested test run ----------
An exception occurred while test discoverer 'MSTestDiscoverer' was loading tests. Exception: Could not load type 'Microsoft.VisualStudio.TestPlatform.ObjectModel.Utilities.TypesToLoadAttribute' from assembly 'Microsoft.VisualStudio.TestPlatform.ObjectModel, Version=15.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.
No test is available in C:\...\UnitTestProject9\UnitTestProject9\bin\Debug\UnitTestProject9.dll. Make sure that test discoverer & executors are registered, and platform & framework version settings are appropriate and try again.
========== Test discovery finished: 0 Tests found in 8.1 sec ==========
---------- Starting test run ----------
An exception occurred while invoking executor 'executor://mstestadapter/v2': Could not load type 'Microsoft.VisualStudio.TestPlatform.ObjectModel.Utilities.TypesToLoadAttribute' from assembly 'Microsoft.VisualStudio.TestPlatform.ObjectModel, Version=15.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.
========== Test run finished: 0 Tests run in 7.4 sec (0 Passed, 0 Failed, 0 Skipped) ==========

Environment

Visual Studio v16.8.0

[Haplois]

This problem is not about one simple change. Since we started updating the TestPlatform and referencing it from TestFx, any change can be breaking. If we want to be able to update TestPlatform without breaking anything, this needs to be addressed. I was chatting with @jfleisher yesterday; he is looking into it from TestWindow’ss perspective; maybe something can be done there and retroactively fixed in older versions.

Also, I like to point out that this doesn’t fail when using NETCoreApp, since we’re referencing Microsoft.NET.Test.Sdk and using the referenced version. Is it possible to do that for NetFramework versions as well, @shyamnamboodiripad?

[Sanan07]

We need think how can we release testfx to avoid this type of exception. Problem occurs when you use VS which does not have TP version which contains specific logic which we use inside testfx. If you use VS 16.8, it does not have our recent changes, and if you update MSTestAdapter inside your project to newer version which has recent changes it will break. Because MSTestAdapter has new types inside it, but TP in VS does not have it, that’s why it breaks. For instance in VS 16.9 Preview 2 everything works fine.