Test for task ALC behavior

src/MSBuild.UnitTests/ValidateAssemblyLoadContext.cs

@@ -0,0 +1,41 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+#if FEATURE_ASSEMBLYLOADCONTEXT
+
+using Microsoft.Build.Framework;
+using Microsoft.Build.Shared;
+using Microsoft.Build.Utilities;
+using System.Runtime.Loader;
+
+namespace Microsoft.Build.UnitTests
+{
+    public class ValidateAssemblyLoadContext : Task
+    {
+        public override bool Execute()
+        {
+            var thisLoadContext = AssemblyLoadContext.GetLoadContext(typeof(ValidateAssemblyLoadContext).Assembly);
+
+            // Check by name because this always reports false, presumably due to ALC shenanigans:
+            // if (thisLoadContext is MSBuildLoadContext context)
+            if (thisLoadContext.GetType().FullName == typeof(MSBuildLoadContext).FullName)
+            {
+                if (!thisLoadContext.Name.EndsWith(typeof(ValidateAssemblyLoadContext).Assembly.GetName().Name + ".dll"))
+                {
+                    Log.LogError($"Unexpected AssemblyLoadContext name: \"{thisLoadContext.Name}\", but the current executing assembly was {typeof(ValidateAssemblyLoadContext).Assembly.GetName().Name}");
+                }
+                else
+                {
+                    Log.LogMessage(MessageImportance.High, $"Task {nameof(ValidateAssemblyLoadContext)} loaded in AssemblyLoadContext named {thisLoadContext.Name}");
+                }
+            }
+            else
+            {
+                Log.LogError($"Load context was a {thisLoadContext.GetType().AssemblyQualifiedName} instead of an {typeof(MSBuildLoadContext).AssemblyQualifiedName}");
+            }
+
+            return !Log.HasLoggedErrors;
+        }
+    }
+}
+#endif

src/MSBuild.UnitTests/XMake_Tests.cs

@@ -18,6 +18,7 @@
 using Xunit.Abstractions;
 using Shouldly;
 using System.IO.Compression;
+using System.Reflection;
 
 namespace Microsoft.Build.UnitTests
 {
@@ -2119,6 +2120,34 @@ public void BinaryLogContainsImportedFiles()
             }
         }
 
+#if FEATURE_ASSEMBLYLOADCONTEXT
+        /// <summary>
+        /// Ensure that tasks get loaded into their own <see cref="System.Runtime.Loader.AssemblyLoadContext"/>.
+        /// </summary>
+        /// <remarks>
+        /// When loading a task from a test assembly in a test within that assembly, the assembly is already loaded
+        /// into the default context. So put the test here and isolate the task into an MSBuild that runs in its
+        /// own process, causing it to newly load the task (test) assembly in a new ALC.
+        /// </remarks>
+        [Fact]
+        public void TasksGetAssemblyLoadContexts()
+        {
+            string customTaskPath = Assembly.GetExecutingAssembly().Location;
+
+            string projectContents = $@"<Project ToolsVersion=`msbuilddefaulttoolsversion` xmlns=`msbuildnamespace`>
+  <UsingTask TaskName=`ValidateAssemblyLoadContext` AssemblyFile=`{customTaskPath}` />
+
+  <Target Name=`Build`>
+    <ValidateAssemblyLoadContext />
+  </Target>
+</Project>";
+
+            ExecuteMSBuildExeExpectSuccess(projectContents);
+        }
+
+#endif
+
+
         private string CopyMSBuild()
         {
             string dest = null;