Overwrite xunit's app domain handling to not call AppDomain.Unload

Roslyn.sln

@@ -471,6 +471,8 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Microsoft.CodeAnalysis.Edit
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Microsoft.CodeAnalysis.Test.Utilities", "src\Compilers\Test\Core\Microsoft.CodeAnalysis.Test.Utilities.csproj", "{5D94ED65-EFA3-44EB-A5DA-62E85BAC9DD6}"
 EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Microsoft.CodeAnalysis.XunitHook", "src\EditorFeatures\XunitHook\Microsoft.CodeAnalysis.XunitHook.csproj", "{967A8F5E-7D18-436C-97ED-1DB303FE5DE0}"
+EndProject
 Global
 	GlobalSection(SharedMSBuildProjectFiles) = preSolution
 		src\Analyzers\VisualBasic\CodeFixes\VisualBasicCodeFixes.projitems*{0141285d-8f6c-42c7-baf3-3c0ccd61c716}*SharedItemsImports = 5
@@ -1230,6 +1232,10 @@ Global
 		{5D94ED65-EFA3-44EB-A5DA-62E85BAC9DD6}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{5D94ED65-EFA3-44EB-A5DA-62E85BAC9DD6}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{5D94ED65-EFA3-44EB-A5DA-62E85BAC9DD6}.Release|Any CPU.Build.0 = Release|Any CPU
+		{967A8F5E-7D18-436C-97ED-1DB303FE5DE0}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{967A8F5E-7D18-436C-97ED-1DB303FE5DE0}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{967A8F5E-7D18-436C-97ED-1DB303FE5DE0}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{967A8F5E-7D18-436C-97ED-1DB303FE5DE0}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
@@ -1447,6 +1453,7 @@ Global
 		{9B25E472-DF94-4E24-9F5D-E487CE5A91FB} = {FD0FAF5F-1DED-485C-99FA-84B97F3A8EEC}
 		{67F44564-759B-4643-BD86-407B010B0B74} = {EE97CB90-33BB-4F3A-9B3D-69375DEC6AC6}
 		{5D94ED65-EFA3-44EB-A5DA-62E85BAC9DD6} = {A41D1B99-F489-4C43-BBDF-96D61B19A6B9}
+		{967A8F5E-7D18-436C-97ED-1DB303FE5DE0} = {EE97CB90-33BB-4F3A-9B3D-69375DEC6AC6}
 	EndGlobalSection
 	GlobalSection(ExtensibilityGlobals) = postSolution
 		SolutionGuid = {604E6B91-7BC0-4126-AE07-D4D2FEFC3D29}

src/EditorFeatures/TestUtilities/Microsoft.CodeAnalysis.EditorFeatures.Test.Utilities.csproj

@@ -23,6 +23,7 @@
     <ProjectReference Include="..\CSharp\Microsoft.CodeAnalysis.CSharp.EditorFeatures.csproj" />
     <ProjectReference Include="..\Text\Microsoft.CodeAnalysis.EditorFeatures.Text.csproj" />
     <ProjectReference Include="..\VisualBasic\Microsoft.CodeAnalysis.VisualBasic.EditorFeatures.vbproj" />
+    <ProjectReference Include="..\XunitHook\Microsoft.CodeAnalysis.XunitHook.csproj" />
   </ItemGroup>
   <ItemGroup>
     <PackageReference Include="Moq" Version="$(MoqVersion)" />

src/EditorFeatures/TestUtilities/Threading/StaTaskScheduler.cs

@@ -5,8 +5,10 @@
 #nullable disable
 
 using System;
+using System.Reflection;
 using System.Threading;
 using System.Windows.Threading;
+using Microsoft.CodeAnalysis.Test.Utilities;
 
 namespace Roslyn.Test.Utilities
 {
@@ -24,6 +26,12 @@ public sealed class StaTaskScheduler : IDisposable
 
         static StaTaskScheduler()
         {
+            // Overwrite xunit's app domain handling to not call AppDomain.Unload
+            var getDefaultDomain = typeof(AppDomain).GetMethod("GetDefaultDomain", BindingFlags.NonPublic | BindingFlags.Static);
+            var defaultDomain = (AppDomain)getDefaultDomain.Invoke(null, null);
+            var hook = (XunitDisposeHook)defaultDomain.CreateInstanceFromAndUnwrap(typeof(XunitDisposeHook).Assembly.CodeBase, typeof(XunitDisposeHook).FullName, ignoreCase: false, BindingFlags.CreateInstance, binder: null, args: null, culture: null, activationAttributes: null);
+            hook.Execute();
+
             // We've created an STA thread, which has some extra requirements for COM Runtime
             // Callable Wrappers (RCWs). If any COM object is created on the STA thread, calls to that
             // object must be made from that thread; when the RCW is no longer being used by any

src/EditorFeatures/XunitHook/Microsoft.CodeAnalysis.XunitHook.csproj

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Licensed to the .NET Foundation under one or more agreements. The .NET Foundation licenses this file to you under the MIT license. See the LICENSE file in the project root for more information. -->
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Library</OutputType>
+    <RootNamespace>Microsoft.CodeAnalysis.Test.Utilities</RootNamespace>
+    <TargetFramework>net472</TargetFramework>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+    <IsShipping>false</IsShipping>
+  </PropertyGroup>
+  <ItemGroup>
+    <InternalsVisibleTo Include="Microsoft.CodeAnalysis.EditorFeatures.Test.Utilities" />
+  </ItemGroup>
+</Project>

src/EditorFeatures/XunitHook/XunitDisposeHook.cs

@@ -0,0 +1,65 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Diagnostics.CodeAnalysis;
+using System.Linq;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+
+namespace Microsoft.CodeAnalysis.Test.Utilities
+{
+    internal sealed class XunitDisposeHook : MarshalByRefObject
+    {
+        [SuppressMessage("Performance", "CA1822:Mark members as static", Justification = "Invoked across app domains")]
+        public void Execute()
+        {
+            if (!AppDomain.CurrentDomain.IsDefaultAppDomain())
+                throw new InvalidOperationException();
+
+            var xunitUtilities = AppDomain.CurrentDomain.GetAssemblies().Where(static assembly => assembly.GetName().Name.StartsWith("xunit.runner.utility")).ToArray();
+            foreach (var xunitUtility in xunitUtilities)
+            {
+                var appDomainManagerType = xunitUtility.GetType("Xunit.AppDomainManager_AppDomain");
+                if (appDomainManagerType is null)
+                    continue;
+
+                var method = appDomainManagerType.GetMethod("Dispose");
+                RuntimeHelpers.PrepareMethod(method.MethodHandle);
+                var functionPointer = method.MethodHandle.GetFunctionPointer();
+                if (IntPtr.Size == 4)
+                {
+                    // Overwrite the compiled method to just return
+
+                    var offset = 0;
+
+                    // push ebp
+                    Marshal.WriteByte(functionPointer, offset++, 0x55);
+
+                    // mov ebp,esp
+                    Marshal.WriteByte(functionPointer, offset++, 0x8B);
+                    Marshal.WriteByte(functionPointer, offset++, 0xEC);
+
+                    // lea esp,[ebp-8]
+                    Marshal.WriteByte(functionPointer, offset++, 0x8D);
+                    Marshal.WriteByte(functionPointer, offset++, 0x65);
+                    Marshal.WriteByte(functionPointer, offset++, 0xF8);
+
+                    // pop ebp
+                    Marshal.WriteByte(functionPointer, offset++, 0x5D);
+
+                    // ret
+                    Marshal.WriteByte(functionPointer, offset++, 0xC3);
+                }
+                else
+                {
+                    // Overwrite the compiled method to just return
+
+                    // ret
+                    Marshal.WriteByte(functionPointer, 0xC3);
+                }
+            }
+        }
+    }
+}