Write warnings outside of appdomain

src/Adapter/MSTest.TestAdapter/Discovery/AssemblyEnumerationResult.cs

@@ -0,0 +1,14 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using Microsoft.VisualStudio.TestPlatform.MSTest.TestAdapter.ObjectModel;
+
+namespace Microsoft.VisualStudio.TestPlatform.MSTest.TestAdapter.Discovery;
+
+/// <summary>
+/// Helps us communicate results that were created inside of AppDomain, when AppDomains are available and enabled.
+/// </summary>
+/// <param name="TestElements">The test elements that were discovered.</param>
+/// <param name="Warnings">Warnings that happened during discovery.</param>
+[Serializable]
+internal sealed record AssemblyEnumerationResult(List<UnitTestElement> TestElements, List<string> Warnings);

src/Adapter/MSTest.TestAdapter/Discovery/AssemblyEnumerator.cs

@@ -62,12 +62,11 @@
     /// Enumerates through all types in the assembly in search of valid test methods.
     /// </summary>
     /// <param name="assemblyFileName">The assembly file name.</param>
     /// <param name="warnings">Contains warnings if any, that need to be passed back to the caller.</param>
     /// <returns>A collection of Test Elements.</returns>
-    internal ICollection<UnitTestElement> EnumerateAssembly(
-        string assemblyFileName,
-        List<string> warnings)
+    internal AssemblyEnumerationResult EnumerateAssembly(string assemblyFileName)
     {
+        List<string> warnings = new();
         DebugEx.Assert(!StringEx.IsNullOrWhiteSpace(assemblyFileName), "Invalid assembly file name.");
         var tests = new List<UnitTestElement>();
         // Contains list of assembly/class names for which we have already added fixture tests.
@@ -117,7 +116,7 @@
             tests.AddRange(testsInType);
         }
 
-        return tests;
+        return new AssemblyEnumerationResult(tests, warnings);
     }
 
     /// <summary>

src/Adapter/MSTest.TestAdapter/Discovery/AssemblyEnumeratorWrapper.cs

@@ -114,6 +114,10 @@ private static ICollection<UnitTestElement> GetTestsInIsolation(string fullFileP
             PlatformServiceProvider.Instance.AdapterTraceLogger.LogWarning(Resource.OlderTFMVersionFound);
         }
 
-        return assemblyEnumerator.EnumerateAssembly(fullFilePath, warnings);
+        // This method runs inside of appdomain, when appdomains are available and enabled.
+        // Don't move the 'warning' parameter to be written into by reference, it will not return the warnings from appdomain.
+        AssemblyEnumerationResult result = assemblyEnumerator.EnumerateAssembly(fullFilePath);
+        warnings.AddRange(result.Warnings);
+        return result.TestElements;
     }
 }