TypeInitializationException when targeting .NET Framework - Could not load type 'System.Collections.Concurrent.ConcurrentBag`1'

[viktorgobbi]

Describe the bug
Test execution of an NUnit project fails with 'Zero Tests ran' when running dotnet test --coverlet with the latest nightly packages (10.0.2-preview.30.geeb431030d) for .net Framework projects (net481) and an error is logged: TypeInitializationException with Could not load type 'System.Collections.Concurrent.ConcurrentBag1' from assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.` .

To Reproduce:

1. Create a new .net framework project targeting net481 with the latest Coverlet.MTP and other required packages. My test project file:
   • CoverletIssueRepro.csproj:

     <Project Sdk="Microsoft.NET.Sdk">
         <PropertyGroup>
             <OutputType>Exe</OutputType>
             <TargetFramework>net481</TargetFramework>
             <RootNamespace>CoverletIssueRepro</RootNamespace>
         </PropertyGroup>
         <PropertyGroup>
             <OutputType>Exe</OutputType>
             <EnableNUnitRunner>true</EnableNUnitRunner>
             <TestingPlatformDotnetTestSupport>true</TestingPlatformDotnetTestSupport>
         </PropertyGroup>
         <ItemGroup>
             <PackageReference Include="coverlet.MTP" Version="10.0.2-preview.30.geeb431030d" />
             <PackageReference Include="NUnit" Version="4.6.1" />
             <PackageReference Include="NUnit3TestAdapter" Version="6.2.0" />
             <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.8.1" />
         </ItemGroup>
     </Project>

2. Create a file with one dummy unit test:
   • Repro.cs:

     using NUnit.Framework;
     
     namespace CoverletIssueRepro
     {
        [TestFixture]
        public class Repro
        {
            [Test]
            public void DemoTest()
            {
                Assert.Pass();
            }
        }
     }

3. Enable MTP for dotnet CLI in global.json (not sure if this is still required with NET 10 SDK)
   • global.json:

     {
       "test": {
         "runner": "Microsoft.Testing.Platform"
       }
     }

4. Run dotnet test with coverlet dotnet test --coverlet

Expected behavior
dotnet test --coverlet should show "1 succeeded test" and a coverage file should be present in the TestResrults directory

Actual behavior
dotnet test --coverlet shows "Zero test ran" and "error: 1" as well as a logged error output with a TypeInitializationException with Could not load type 'System.Collections.Concurrent.ConcurrentBag1' from assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.`

Full command output:

-> % dotnet test --coverlet
Running tests from CoverletIssueRepro\bin\Debug\net481\CoverletIssueRepro.exe (net48|x64)
CoverletIssueRepro\bin\Debug\net481\CoverletIssueRepro.exe (net48|x64) Zero tests ran (3s 220ms)
Exit code: 7
  Standard output: [16:33:52.906] SynchronizedRunTestsAsync: ENTRY
  [16:33:52.907] SynchronizedRunTestsAsync: Creating executor
  Error output:
  Unhandled Exception: System.TypeInitializationException: The type initializer for 'NUnit.VisualStudio.TestAdapter.VsTestFilter' threw an exception. ---> System.TypeInitializationException: The type initializer for 'Microsoft.VisualStudio.TestPlatform.ObjectModel.TestCaseProperties' threw an exception. ---> System.TypeInitializationException: The type initializer for 'Coverlet.Core.Instrumentation.Tracker.Microsoft.TestPlatform.CoreUtilities_288d6f6f-5c15-4bc8-8840-5fe054fdf339' threw an exception. ---> System.TypeLoadException: Could not load type 'System.Collections.Concurrent.ConcurrentBag`1' from assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.
     at Coverlet.Core.Instrumentation.Tracker.Microsoft.TestPlatform.CoreUtilities_288d6f6f-5c15-4bc8-8840-5fe054fdf339.RegisterUnloadEvents()
     at Coverlet.Core.Instrumentation.Tracker.Microsoft.TestPlatform.CoreUtilities_288d6f6f-5c15-4bc8-8840-5fe054fdf339..cctor()
     --- End of inner exception stack trace ---
     at Coverlet.Core.Instrumentation.Tracker.Microsoft.TestPlatform.CoreUtilities_288d6f6f-5c15-4bc8-8840-5fe054fdf339.RecordHit(Int32 )
     at Microsoft.VisualStudio.TestPlatform.ObjectModel.ValidateArg.NotNullOrEmpty(String arg, String parameterName) in /_/src/Microsoft.TestPlatform.CoreUtilities/ValidateArg.cs:line 60
     at Microsoft.VisualStudio.TestPlatform.ObjectModel.TestProperty.Register(String id, String label, Type valueType, TestPropertyAttributes attributes, Type owner) in /_/src/Microsoft.TestPlatform.ObjectModel/TestProperty/TestProperty.cs:line 299
     at NUnit.VisualStudio.TestAdapter.CategoryList..cctor() in D:\a\nunit3-vs-adapter\nunit3-vs-adapter\src\NUnitTestAdapter\CategoryList.cs:line 35
     --- End of inner exception stack trace ---
     at NUnit.VisualStudio.TestAdapter.VsTestFilter..cctor() in D:\a\nunit3-vs-adapter\nunit3-vs-adapter\src\NUnitTestAdapter\VsTestFilter.cs:line 61
     --- End of inner exception stack trace ---
     at NUnit.VisualStudio.TestAdapter.VsTestFilter..ctor(IRunContext runContext)
     at NUnit.VisualStudio.TestAdapter.VsTestFilterFactory.CreateVsTestFilter(IAdapterSettings settings, IRunContext context) in D:\a\nunit3-vs-adapter\nunit3-vs-adapter\src\NUnitTestAdapter\VsTestFilter.cs:line 182
     at NUnit.VisualStudio.TestAdapter.NUnit3TestExecutor.InitializeForExecution(IRunContext runContext, IFrameworkHandle frameworkHandle) in D:\a\nunit3-vs-adapter\nunit3-vs-adapter\src\NUnitTestAdapter\NUnit3TestExecutor.cs:line 551
     at NUnit.VisualStudio.TestAdapter.NUnit3TestExecutor.RunTests(IEnumerable`1 sources, IRunContext runContext, IFrameworkHandle frameworkHandle) in D:\a\nunit3-vs-adapter\nunit3-vs-adapter\src\NUnitTestAdapter\NUnit3TestExecutor.cs:line 184
     at NUnit.VisualStudio.TestAdapter.TestingPlatformAdapter.NUnitBridgedTestFramework.<SynchronizedRunTestsAsync>d__20.MoveNext() in D:\a\nunit3-vs-adapter\nunit3-vs-adapter\src\NUnitTestAdapter\TestingPlatformAdapter\NUnitBridgedTestFramework.cs:line 105
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
     at NUnit.VisualStudio.TestAdapter.TestingPlatformAdapter.NUnitBridgedTestFramework.<SynchronizedRunTestsAsync>d__20.MoveNext() in D:\a\nunit3-vs-adapter\nunit3-vs-adapter\src\NUnitTestAdapter\TestingPlatformAdapter\NUnitBridgedTestFramework.cs:line 113
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
     at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
     at Microsoft.Testing.Extensions.VSTestBridge.SynchronizedSingleSessionVSTestBridgedTestFramework.<>c__DisplayClass22_0.<<ExecuteRequestAsync>b__0>d.MoveNext() in /_/src/Platform/Microsoft.Testing.Extensions.VSTestBridge/SynchronizedSingleSessionVSTestAndTestAnywhereAdapter.cs:line 166
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
     at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
     at Microsoft.Testing.Extensions.VSTestBridge.SynchronizedSingleSessionVSTestBridgedTestFramework.<ExecuteRequestWithRequestCountGuardAsync>d__24.MoveNext() in /_/src/Platform/Microsoft.Testing.Extensions.VSTestBridge/SynchronizedSingleSessionVSTestAndTestAnywhereAdapter.cs:line 188
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
     at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
     at Microsoft.Testing.Extensions.VSTestBridge.VSTestBridgedTestFrameworkBase.<ExecuteRequestAsync>d__24.MoveNext() in /_/src/Platform/Microsoft.Testing.Extensions.VSTestBridge/VSTestBridgedTestFrameworkBase.cs:line 102
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
     at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
     at Microsoft.Testing.Platform.Requests.TestHostTestFrameworkInvoker.<ExecuteRequestAsync>d__16.MoveNext() in /_/src/Platform/Microsoft.Testing.Platform/Requests/TestHostTestFrameworkInvoker.cs:line 112
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
     at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
     at Microsoft.Testing.Platform.Requests.TestHostTestFrameworkInvoker.<ExecuteAsync>d__15.MoveNext() in /_/src/Platform/Microsoft.Testing.Platform/Requests/TestHostTestFrameworkInvoker.cs:line 87
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
     at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
     at Microsoft.Testing.Platform.Hosts.CommonHost.<ExecuteRequestAsync>d__15.MoveNext() in /_/src/Platform/Microsoft.Testing.Platform/Hosts/CommonTestHost.cs:line 212
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
     at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
     at Microsoft.Testing.Platform.Hosts.ConsoleTestHost.<InternalRunAsync>d__10.MoveNext() in /_/src/Platform/Microsoft.Testing.Platform/Hosts/ConsoleTestHost.cs:line 92
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
     at Microsoft.Testing.Platform.Hosts.ConsoleTestHost.<InternalRunAsync>d__10.MoveNext() in /_/src/Platform/Microsoft.Testing.Platform/Hosts/ConsoleTestHost.cs:line 117
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
     at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
     at Microsoft.Testing.Platform.Hosts.CommonHost.<RunTestAppAsync>d__13.MoveNext() in /_/src/Platform/Microsoft.Testing.Platform/Hosts/CommonTestHost.cs:line 175
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
     at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
     at Microsoft.Testing.Platform.Hosts.CommonHost.<RunAsync>d__8.MoveNext() in /_/src/Platform/Microsoft.Testing.Platform/Hosts/CommonTestHost.cs:line 71
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
     at Microsoft.Testing.Platform.Hosts.CommonHost.<RunAsync>d__8.MoveNext() in /_/src/Platform/Microsoft.Testing.Platform/Hosts/CommonTestHost.cs:line 79
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
     at Microsoft.Testing.Platform.Hosts.CommonHost.<RunAsync>d__8.MoveNext() in /_/src/Platform/Microsoft.Testing.Platform/Hosts/CommonTestHost.cs:line 103
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
     at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
     at Microsoft.Testing.Platform.Hosts.TestHostControlledHost.<RunAsync>d__4.MoveNext() in /_/src/Platform/Microsoft.Testing.Platform/Hosts/TestHostControlledHost.cs:line 25
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
     at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
     at Microsoft.Testing.Platform.Builder.TestApplication.<RunAsync>d__10.MoveNext() in /_/src/Platform/Microsoft.Testing.Platform/Builder/TestApplication.cs:line 231
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
     at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
     at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
     at CoverletIssueRepro.MicrosoftTestingPlatformEntryPoint.<Main>d__0.MoveNext() in C:\Users\USERNAME\RiderProjects\CoverletIssueRepro\CoverletIssueRepro\obj\Debug\net481\MicrosoftTestingPlatformEntryPoint.cs:line 18
  --- End of stack trace from previous location where exception was thrown ---
     at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
     at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
     at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
     at CoverletIssueRepro.MicrosoftTestingPlatformEntryPoint.<Main>(String[] args)

Test run summary: Zero tests ran
  error: 1

  total: 0
  failed: 0
  succeeded: 0
  skipped: 0
  duration: 5s 831ms
Test run completed with non-success exit code: 7 (see: https://aka.ms/testingplatform/exitcodes)

Configuration (please complete the following information):
Please provide more information on your .NET configuration:

• Which coverlet package (collector, console, msbuild, ...) and version was used?
  • Reproducible with:
    • 10.0.2-preview.30.geeb431030d
    • 10.0.2-preview.28.g4032f53415
    • 10.0.2-preview.21.gb734a326e7
    • 10.0.2-preview.17.g856637d9d2
    • 10.0.2-preview.16.g68e4db09d3
  • Not reproducible with 10.0.2-preview.15.g09fd57f9a9
• Which version of .NET is the code running on?
  • I'd assume net481 but I'm not sure what this exactly refers to
• What OS and version, and what distro if applicable?
  • Windows 11 25H2 (ver output Microsoft Windows [Version 10.0.26200.8893])
• What is the architecture (x64, x86, ARM, ARM64)?
  • x64
• Do you know whether it is specific to that configuration?
• dotnet --info output:

  •   dotnet.exe --info
      .NET SDK:
       Version:           10.0.301
       Commit:            96856fd726
       Workload version:  10.0.300-manifests.8c7d7c03
       MSBuild version:   18.6.4+96856fd72
    
      Runtime Environment:
       OS Name:     Windows
       OS Version:  10.0.26200
       OS Platform: Windows
       RID:         win-x64
       Base Path:   C:\Program Files\dotnet\sdk\10.0.301\
      
      .NET workloads installed:
      There are no installed workloads to display.
      Configured to use workload sets when installing new manifests.
      No workload sets are installed. Run "dotnet workload restore" to install a workload set.
      
      Host:
        Version:      10.0.9
        Architecture: x64
        Commit:       901ca94124
      
      .NET SDKs installed:
        8.0.422 [C:\Program Files\dotnet\sdk]
        10.0.301 [C:\Program Files\dotnet\sdk]
      
      .NET runtimes installed:
        Microsoft.AspNetCore.App 8.0.28 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
        Microsoft.AspNetCore.App 10.0.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
        Microsoft.NETCore.App 8.0.28 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
        Microsoft.NETCore.App 10.0.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
        Microsoft.WindowsDesktop.App 8.0.28 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
        Microsoft.WindowsDesktop.App 10.0.9 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
      
      Other architectures found:
        x86   [C:\Program Files (x86)\dotnet]
          registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]
      
      Environment variables:
        Not set
      
      global.json file:
        C:\Users\USERNAME\RiderProjects\CoverletIssueRepro\global.json
      
      Learn more:
        https://aka.ms/dotnet/info
      
      Download .NET:
        https://aka.ms/dotnet/download
    

Additional context
I'm in the process of switching our testrunner over to MTP and originally ran into the issue described in #1984 which affects the latest released version as of writing (10.0.1) but I saw that there was a fix available so I wanted to try the nightly build where I encountered this issue. Due to the specific build it is failing at and the the affected types I assume it is related to #1987 with 62021a9 which recently introduced the type that fails to load.
Without Coverlet enabled the tests run fine:

-> dotnet test
Running tests from CoverletIssueRepro\bin\Debug\net481\CoverletIssueRepro.exe (net48|x64)
CoverletIssueRepro\bin\Debug\net481\CoverletIssueRepro.exe (net48|x64) passed (947ms)

Test run summary: Passed!
  total: 1
  failed: 0
  succeeded: 1
  skipped: 0
  duration: 1s 278ms

I also noticed that the issue disappeared when I ran dotnet test --coverlet --coverlet-exclude "[Microsoft.*]*" but as soon as I add a class library project (again net481) and use that in the unit tests, I get the same error again:

dotnet test --coverlet  --coverlet-exclude "[Microsoft.*]*"
Running tests from CoverletIssueRepro\bin\Debug\net481\CoverletIssueRepro.exe (net48|x64)
failed DemoTest (118ms)
  from CoverletIssueRepro\bin\Debug\net481\CoverletIssueRepro.exe (net48|x64)
  System.TypeInitializationException : The type initializer for 'Coverlet.Core.Instrumentation.Tracker.ClassLibrary1_b7058d04-60e1-4767-9e88-b09d2adec467' threw an exception.
    TypeName: Coverlet.Core.Instrumentation.Tracker.ClassLibrary1_b7058d04-60e1-4767-9e88-b09d2adec467
    ----> System.TypeLoadException : Could not load type 'System.Collections.Concurrent.ConcurrentBag`1' from assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.
    TypeName: System.Collections.Concurrent.ConcurrentBag`1
    at Coverlet.Core.Instrumentation.Tracker.ClassLibrary1_b7058d04-60e1-4767-9e88-b09d2adec467.RecordHit(Int32 )
    at LibraryUnderTest.ClassUnderTest.ReturnSomething() in LibraryUnderTest\ClassUnderTest.cs:5
    at CoverletIssueRepro.Repro.DemoTest() in CoverletIssueRepro\Repro.cs:12
    --TypeLoadException
    at Coverlet.Core.Instrumentation.Tracker.ClassLibrary1_b7058d04-60e1-4767-9e88-b09d2adec467.RegisterUnloadEvents()
    at Coverlet.Core.Instrumentation.Tracker.ClassLibrary1_b7058d04-60e1-4767-9e88-b09d2adec467..cctor()
    1)    at Coverlet.Core.Instrumentation.Tracker.ClassLibrary1_b7058d04-60e1-4767-9e88-b09d2adec467.RecordHit(Int32 )
    at LibraryUnderTest.ClassUnderTest.ReturnSomething() in LibraryUnderTest\ClassUnderTest.cs:5
    at CoverletIssueRepro.Repro.DemoTest() in CoverletIssueRepro\Repro.cs:12
    --TypeLoadException
    at Coverlet.Core.Instrumentation.Tracker.ClassLibrary1_b7058d04-60e1-4767-9e88-b09d2adec467.RegisterUnloadEvents()
    at Coverlet.Core.Instrumentation.Tracker.ClassLibrary1_b7058d04-60e1-4767-9e88-b09d2adec467..cctor()

CoverletIssueRepro\bin\Debug\net481\CoverletIssueRepro.exe (net48|x64) failed with 1 error(s) (1s 418ms)
Exit code: 2
  Standard output: [17:51:18.046] SynchronizedRunTestsAsync: ENTRY
  [17:51:18.046] SynchronizedRunTestsAsync: Creating executor

Test run summary: Failed!
  total: 1
  failed: 1
  succeeded: 0
  skipped: 0
  duration: 2s 506ms
Test run completed with non-success exit code: 2 (see: https://aka.ms/testingplatform/exitcodes)

Let me know if there's anything else I can provide. Happy to help however I can.

[Bertk]

Thank you for reporting this. I followed your reproduction steps and created an attached ZIP file containing a sample repository.
Instead of using dotnet test --coverlet, I created a small run-test.bat script. It completed without errors in my environment. Could you please try it in yours?

dotnet restore -s c:\local_source
dotnet build
.\artifacts\bin\CoverletIssueRepro\debug\CoverletIssueRepro.exe  --diagnostic --coverlet --results-directory artifacts/results --coverlet-include [ClassLibrary]* --diagnostic-output-directory artifacts/logs --diagnostic-verbosity trace

coverlet-issue-2009.zip

[viktorgobbi]

Hi @Bertk, thanks for the prompt reply!

I've downloaded and ran your sample. I had to modify the global.json as my sdk is on version 10.0.301 not 10.0.302 and the restore in the script used a local feed which I replaced with dotnet restore -s https://pkgs.dev.azure.com/tonerdo/coverlet/_packaging/coverlet-nightly/nuget/v3/index.json but after that it worked and shows similar output to your screenshot:

I've noticed that the "assembly-under-test" in your sample is a netstandard2.0 project and in my second test it was a net481 project (forgot to write that down).
As soon as I change the ClassLibrary.csproj to net481 : <TargetFramework>net481</TargetFramework> I get a similar behavior and error again:

[viktorgobbi]

@Bertk I've been digging in the Instrumenter code and think I've found something interesting. I'm not exactly an expert on the IL stuff, but when debugging the TestInstrument_NetFrameworkAssembly_TrackerUsesCorlibNotSystemRuntime unit test, when AddCustomModuleTrackerToModule

coverlet/src/coverlet.core/Instrumentation/Instrumenter.cs

Line 456 in f9e6c86

private void AddCustomModuleTrackerToModule(ModuleDefinition module)

processes the RegisterUnloadEvents

coverlet/src/coverlet.core/Instrumentation/ModuleTrackerTemplate.cs

Line 49 in f9e6c86

public static void RegisterUnloadEvents()

it iterates over all instructions in the method body and checks all Operands and in case it is a TypeReference it calls ImportToCoreLibrary

coverlet/src/coverlet.core/Instrumentation/Instrumenter.cs

Line 621 in f9e6c86

private static TypeReference ImportToCoreLibrary(ModuleDefinition module, TypeReference type)

but for the castclass instruction (which is the one that uses the type in the exception message) the TypeReference is actully a GenericInstanceType and it looks to me as if the ImportToCoreLibrary would not treat its GenericArgument (the System.EventHandler) correctly as the Scope of the GenericArgument after ImportMethodToCoreLibrary is still {netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51} but for the ConcurrentBag`1 ref it was changed to {mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089}.

I've tried setting the scope for the GenericArguments too in ImportToCoreLibrary but it did not seem to work with my setup so maybe this is unrelated and not really an issue...

[Bertk]

Yes, this is a regression and shall be fixed.