Fix TaskHost crash when task returns string[] with null elements

src/Build.UnitTests/BackEnd/StringArrayWithNullsTask.cs

@@ -0,0 +1,32 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Diagnostics;
+using Microsoft.Build.Framework;
+using Microsoft.Build.Utilities;
+
+#nullable disable
+
+namespace Microsoft.Build.UnitTests.BackEnd
+{
+    /// <summary>
+    /// A task that returns a string array containing null elements.
+    /// Used to test the fix for https://github.com/dotnet/msbuild/issues/13174
+    /// </summary>
+    public class StringArrayWithNullsTask : Task
+    {
+        [Output]
+        public string[] OutputArray { get; set; }
+
+        [Output]
+        public int Pid { get; set; }
+
+        public override bool Execute()
+        {
+            // Return an array with nulls - this pattern occurs in real tasks like GenerateGlobalUsings
+            OutputArray = new string[] { "first", null, "third", null, "fifth" };
+            Pid = Process.GetCurrentProcess().Id;
+            return true;
+        }
+    }
+}

src/Build.UnitTests/BackEnd/TaskHostFactory_Tests.cs

@@ -363,5 +363,45 @@ public void VariousParameterTypesCanBeTransmittedToAndReceivedFromTaskHost()
             projectInstance.GetPropertyValue("CustomStructOutput").ShouldBe(TaskBuilderTestTask.s_customStruct.ToString(CultureInfo.InvariantCulture));
             projectInstance.GetPropertyValue("EnumOutput").ShouldBe(TargetBuiltReason.BeforeTargets.ToString());
         }
+
+        /// <summary>
+        /// Verifies that a task returning a string[] with null elements does not crash
+        /// when executed via TaskHostFactory. This is a regression test for
+        /// https://github.com/dotnet/msbuild/issues/13174
+        /// </summary>
+        [Fact]
+        public void StringArrayWithNullsDoesNotCrashTaskHost()
+        {
+            using TestEnvironment env = TestEnvironment.Create();
+
+            string projectContents = $@"
+<Project>
+    <UsingTask TaskName=""{typeof(StringArrayWithNullsTask).FullName}"" AssemblyFile=""{AssemblyLocation}"" TaskFactory=""TaskHostFactory"" />
+    <Target Name=""TestTarget"">
+        <{typeof(StringArrayWithNullsTask).Name}>
+            <Output ItemName=""OutputItems"" TaskParameter=""OutputArray"" />
+            <Output PropertyName=""TaskPid"" TaskParameter=""Pid"" />
+        </{typeof(StringArrayWithNullsTask).Name}>
+    </Target>
+</Project>";
+
+            TransientTestFile project = env.CreateFile("testProject.csproj", projectContents);
+            ProjectInstance projectInstance = new(project.Path);
+            BuildManager buildManager = BuildManager.DefaultBuildManager;
+            BuildResult buildResult = buildManager.Build(new BuildParameters(), new BuildRequestData(projectInstance, targetsToBuild: new[] { "TestTarget" }));
+
+            // The build should succeed - nulls should be filtered, not cause a crash
+            buildResult.OverallResult.ShouldBe(BuildResultCode.Success);
+
+            // Verify task ran out-of-process (TaskHostFactory should force this)
+            string taskPidStr = projectInstance.GetPropertyValue("TaskPid");
+            taskPidStr.ShouldNotBeNullOrEmpty();
+            int.TryParse(taskPidStr, out int taskPid).ShouldBeTrue();
+            Process.GetCurrentProcess().Id.ShouldNotBe(taskPid, "Task should have run in a separate TaskHost process");
+
+            // Verify output items - nulls should be filtered out, leaving 3 items
+            var outputItems = projectInstance.GetItems("OutputItems");
+            outputItems.Count.ShouldBe(3, "Null elements should be filtered from the string array");
+        }
     }
 }

src/MSBuild/OutOfProcTaskAppDomainWrapperBase.cs

@@ -419,7 +419,16 @@ private OutOfProcTaskHostTaskResult InstantiateAndExecuteTask(
                 {
                     try
                     {
-                        finalParameterValues[value.Name] = value.GetValue(wrappedTask, null);
+                        object outputValue = value.GetValue(wrappedTask, null);
+
+                        // Filter null elements from string[] outputs to avoid crash.
+                        // See https://github.com/dotnet/msbuild/issues/13174
+                        if (outputValue is string[] stringArray)
+                        {
+                            outputValue = FilterNullsFromStringArray(stringArray, value.Name);
+                        }
+
+                        finalParameterValues[value.Name] = outputValue;
                     }
                     catch (Exception e) when (!ExceptionHandling.IsCriticalException(e))
                     {
@@ -451,5 +460,46 @@ private void LogErrorDelegate(string taskLocation, int taskLine, int taskColumn,
                 null,
                 taskName));
         }
+
+        /// <summary>
+        /// Filters null elements from a string[] task output.
+        /// See https://github.com/dotnet/msbuild/issues/13174
+        /// </summary>
+        private string[] FilterNullsFromStringArray(string[] stringArray, string parameterName)
+        {
+            // Count nulls
+            int nullCount = 0;
+            foreach (string s in stringArray)
+            {
+                if (s == null)
+                {
+                    nullCount++;
+                }
+            }
+
+            if (nullCount == 0)
+            {
+                return stringArray;
+            }
+
+            // Filter nulls and log
+            string[] filtered = new string[stringArray.Length - nullCount];
+            int j = 0;
+            foreach (string s in stringArray)
+            {
+                if (s != null)
+                {
+                    filtered[j++] = s;
+                }
+            }
+
+            buildEngine.LogMessageEvent(new BuildMessageEventArgs(
+                message: ResourceUtilities.FormatResourceStringIgnoreCodeAndKeyword("TaskHostAcquired_NullsFiltered", parameterName, nullCount),
+                helpKeyword: null,
+                senderName: taskName,
+                importance: MessageImportance.Normal));
+
+            return filtered;
+        }
     }
 }

src/MSBuild/Resources/Strings.resx

@@ -1719,6 +1719,13 @@
   </data>
   <!-- **** MSBUILD_LOGGING_ARGS strings end **** -->
 
+  <!-- **** TaskHost strings begin **** -->
+  <data name="TaskHostAcquired_NullsFiltered" xml:space="preserve">
+    <value>Task output parameter "{0}" contained {1} null element(s) which have been filtered out.</value>
+    <comment>LOCALIZATION: {0} is the name of the task output parameter, {1} is the count of null elements filtered.</comment>
+  </data>
+  <!-- **** TaskHost strings end **** -->
+
     <!--
         The command line message bucket is: MSB1001 - MSB1999