Fix O(n²) output accumulation in data-driven tests causing large TRX files

src/Adapter/MSTestAdapter.PlatformServices/Execution/TestMethodInfo.cs

@@ -148,9 +148,9 @@ public virtual async Task<TestResult> InvokeAsync(object?[]? arguments)
             if (result != null)
             {
                 var testContextImpl = TestContext as TestContextImplementation;
-                result.LogOutput = testContextImpl?.GetOut();
-                result.LogError = testContextImpl?.GetErr();
-                result.DebugTrace = testContextImpl?.GetTrace();
+                result.LogOutput = testContextImpl?.GetAndClearOut();
+                result.LogError = testContextImpl?.GetAndClearErr();
+                result.DebugTrace = testContextImpl?.GetAndClearTrace();
                 result.TestContextMessages = TestContext?.GetAndClearDiagnosticMessages();
                 result.ResultFiles = TestContext?.GetResultFiles();
                 result.Duration = watch.Elapsed;

src/Adapter/MSTestAdapter.PlatformServices/Services/TestContextImplementation.cs

@@ -383,9 +383,30 @@ private SynchronizedStringBuilder GetTestContextMessagesStringBuilder()
     internal string? GetOut()
         => _stdOutStringBuilder?.ToString();
 
+    internal string? GetAndClearOut()
+    {
+        string? result = _stdOutStringBuilder?.ToString();
+        _stdOutStringBuilder?.Clear();
+        return result;
+    }
+
     internal string? GetErr()
         => _stdErrStringBuilder?.ToString();
 
+    internal string? GetAndClearErr()
+    {
+        string? result = _stdErrStringBuilder?.ToString();
+        _stdErrStringBuilder?.Clear();
+        return result;
+    }
+
     internal string? GetTrace()
         => _traceStringBuilder?.ToString();
+
+    internal string? GetAndClearTrace()
+    {
+        string? result = _traceStringBuilder?.ToString();
+        _traceStringBuilder?.Clear();
+        return result;
+    }
 }

test/IntegrationTests/MSTest.Acceptance.IntegrationTests/TrxReportTests.cs

@@ -1,6 +1,8 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
+using System.Xml.Linq;
+
 using Microsoft.Testing.Platform.Acceptance.IntegrationTests;
 using Microsoft.Testing.Platform.Acceptance.IntegrationTests.Helpers;
 using Microsoft.Testing.Platform.Helpers;
@@ -89,3 +91,117 @@ public void FailingTest()
 
     public TestContext TestContext { get; set; } = null!;
 }
+
+[TestClass]
+public sealed class TrxReportDataDrivenOutputTests : AcceptanceTestBase<TrxReportDataDrivenOutputTests.TestAssetFixture>
+{
+    /// <summary>
+    /// Regression test for https://github.com/microsoft/testfx/issues/7908.
+    /// Verifies that data-driven test output does not accumulate across data rows in the TRX file.
+    /// Each data row's StdOut in the TRX should contain only that row's output, not output from previous rows.
+    /// </summary>
+    [TestMethod]
+    [DynamicData(nameof(TargetFrameworks.AllForDynamicData), typeof(TargetFrameworks))]
+    public async Task TrxReport_DataDrivenTestOutput_DoesNotAccumulateAcrossRows(string tfm)
+    {
+        string fileName = Guid.NewGuid().ToString("N");
+        var testHost = TestHost.LocateFrom(AssetFixture.TargetAssetPath, TestAssetFixture.ProjectName, tfm);
+        TestHostResult testHostResult = await testHost.ExecuteAsync($"--report-trx --report-trx-filename {fileName}.trx", cancellationToken: TestContext.CancellationToken);
+
+        testHostResult.AssertExitCodeIs(ExitCodes.Success);
+
+        string trxFile = Directory.GetFiles(testHost.DirectoryName, $"{fileName}.trx", SearchOption.AllDirectories).Single();
+        string trxContent = File.ReadAllText(trxFile);
+
+        XNamespace ns = "http://microsoft.com/schemas/VisualStudio/TeamTest/2010";
+        var trxDoc = XDocument.Parse(trxContent);
+        var results = trxDoc.Descendants(ns + "UnitTestResult").ToList();
+
+        // We have 3 data rows, each writing a unique marker like "UNIQUE_ROW_0_MARKER", "UNIQUE_ROW_1_MARKER", "UNIQUE_ROW_2_MARKER"
+        Assert.IsGreaterThanOrEqualTo(3, results.Count, $"Expected at least 3 test results but found {results.Count}. TRX content:\n{trxContent}");
+
+        int resultsWithOutput = 0;
+        foreach (XElement result in results)
+        {
+            string? stdOut = result.Descendants(ns + "StdOut").FirstOrDefault()?.Value;
+            if (stdOut is null)
+            {
+                continue;
+            }
+
+            resultsWithOutput++;
+
+            // Count how many unique row markers appear in this single result's StdOut.
+            // Each result should contain exactly ONE marker (its own row's output).
+            int markerCount = 0;
+            for (int i = 0; i < 3; i++)
+            {
+                if (stdOut.Contains($"UNIQUE_ROW_{i}_MARKER"))
+                {
+                    markerCount++;
+                }
+            }
+
+            Assert.AreEqual(
+                1,
+                markerCount,
+                $"Test result '{result.Attribute("testName")?.Value}' contains output from {markerCount} data rows. " +
+                $"Each row should only contain its own output. StdOut:\n{stdOut}");
+        }
+
+        Assert.IsGreaterThanOrEqualTo(3, resultsWithOutput, $"Expected at least 3 test results to have StdOut output but only {resultsWithOutput} did. TRX content:\n{trxContent}");
+    }
+
+    public sealed class TestAssetFixture() : TestAssetFixtureBase()
+    {
+        public const string ProjectName = "MSTestTrxDataDriven";
+
+        public string TargetAssetPath => GetAssetPath(ProjectName);
+
+        public override (string ID, string Name, string Code) GetAssetsToGenerate() => (ProjectName, ProjectName,
+                SourceCode
+                .PatchTargetFrameworks(TargetFrameworks.All)
+                .PatchCodeWithReplace("$MicrosoftTestingPlatformVersion$", MicrosoftTestingPlatformVersion)
+                .PatchCodeWithReplace("$MSTestVersion$", MSTestVersion));
+
+        private const string SourceCode = """
+#file MSTestTrxDataDriven.csproj
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <EnableMSTestRunner>true</EnableMSTestRunner>
+    <TargetFrameworks>$TargetFrameworks$</TargetFrameworks>
+    <LangVersion>latest</LangVersion>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.Testing.Platform" Version="$MicrosoftTestingPlatformVersion$" />
+    <PackageReference Include="MSTest" Version="$MSTestVersion$" />
+  </ItemGroup>
+
+</Project>
+
+#file UnitTest1.cs
+using System;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace MSTestTrxDataDriven;
+
+[TestClass]
+public class UnitTest1
+{
+    [TestMethod]
+    [DataRow(0)]
+    [DataRow(1)]
+    [DataRow(2)]
+    public void DataDrivenTestWithOutput(int row)
+    {
+        Console.WriteLine($"UNIQUE_ROW_{row}_MARKER");
+    }
+}
+""";
+    }
+
+    public TestContext TestContext { get; set; } = null!;
+}

test/UnitTests/MSTestAdapter.PlatformServices.UnitTests/Execution/TestMethodInfoTests.cs

@@ -314,6 +314,72 @@ public async Task TestMethodInfoInvokeShouldClearTestContextMessagesAfterReporti
         result.TestContextMessages!.Contains("SeaShore").Should().BeTrue();
     }
 
+    public async Task TestMethodInfoInvokeShouldClearStdOutAfterReporting()
+    {
+        DummyTestClass.TestMethodBody = o => _testContextImplementation.WriteConsoleOut("output1");
+
+        var method = new TestMethodInfo(
+            _methodInfo,
+            _testClassInfo)
+        {
+            TimeoutInfo = TimeoutInfo.FromTimeout(3600 * 1000),
+            Executor = _testMethodAttribute,
+        };
+
+        TestResult result1 = await method.InvokeAsync(null);
+        result1.LogOutput.Should().Contain("output1");
+
+        DummyTestClass.TestMethodBody = o => _testContextImplementation.WriteConsoleOut("output2");
+        TestResult result2 = await method.InvokeAsync(null);
+
+        result2.LogOutput.Should().Contain("output2");
+        result2.LogOutput.Should().NotContain("output1", "StdOut should be cleared between invocations to prevent O(n^2) output accumulation in data-driven tests");
+    }
+
+    public async Task TestMethodInfoInvokeShouldClearStdErrAfterReporting()
+    {
+        DummyTestClass.TestMethodBody = o => _testContextImplementation.WriteConsoleErr("error1");
+
+        var method = new TestMethodInfo(
+            _methodInfo,
+            _testClassInfo)
+        {
+            TimeoutInfo = TimeoutInfo.FromTimeout(3600 * 1000),
+            Executor = _testMethodAttribute,
+        };
+
+        TestResult result1 = await method.InvokeAsync(null);
+        result1.LogError.Should().Contain("error1");
+
+        DummyTestClass.TestMethodBody = o => _testContextImplementation.WriteConsoleErr("error2");
+        TestResult result2 = await method.InvokeAsync(null);
+
+        result2.LogError.Should().Contain("error2");
+        result2.LogError.Should().NotContain("error1", "StdErr should be cleared between invocations to prevent O(n^2) output accumulation in data-driven tests");
+    }
+
+    public async Task TestMethodInfoInvokeShouldClearDebugTraceAfterReporting()
+    {
+        DummyTestClass.TestMethodBody = o => _testContextImplementation.WriteTrace("trace1");
+
+        var method = new TestMethodInfo(
+            _methodInfo,
+            _testClassInfo)
+        {
+            TimeoutInfo = TimeoutInfo.FromTimeout(3600 * 1000),
+            Executor = _testMethodAttribute,
+        };
+
+        TestResult result1 = await method.InvokeAsync(null);
+        result1.DebugTrace.Should().Contain("trace1");
+
+        DummyTestClass.TestMethodBody = o => _testContextImplementation.WriteTrace("trace2");
+        TestResult result2 = await method.InvokeAsync(null);
+
+        result2.DebugTrace.Should().Contain("trace2");
+        result2.DebugTrace.Should().NotContain("trace1", "DebugTrace should be cleared between invocations to prevent O(n^2) output accumulation in data-driven tests");
+    }
+
     public async Task Invoke_WhenTestMethodThrowsMissingMethodException_TestOutcomeIsFailedAndExceptionIsPreserved()
     {
         DummyTestClass.TestMethodBody = _ =>