Improve coverlet.MTP code coverage (coverlet-coverage#1813)

* Update coverlet package question in bug report template

Clarified the question about the coverlet package in the bug report template.

* revert update

* Simplify GetTestAssemblyPath and add uncovered path tests

Refactored CoverageConfiguration.GetTestAssemblyPath to use only the entry assembly and throw if unavailable, removing all fallback logic. Added CollectorExtensionMoreTests.cs to cover previously untested code paths in CollectorExtension, including output directory creation and report generation scenarios, using Moq and xUnit.

* Refactor coverage report generation for testability

Extract report file generation and display logic into separate methods. Introduce IReporterFactory for dependency injection and mocking. Add CreateDirectory to IFileSystem for better test support. Enable Moq internal type mocking via InternalsVisibleTo. Add unit tests for report file generation with multiple formats.

* Add cross-platform report method tests and path helpers

Added CollectorExtensionReportMethodsTests for comprehensive coverage of report generation methods, including file output, output device interaction, and path handling. Refactored CollectorExtensionUncoveredPathsTests to use a platform-agnostic CreatePlatformPath helper, replacing hardcoded paths for improved cross-platform reliability. Updated all simulated file paths to use platform-specific construction.

* Remove unused helper methods from test class

Removed CreateMockServiceProvider and CreateMockReporterFactory from CollectorExtensionReportMethodsTests. These methods were responsible for setting up mock dependencies for report generation but are no longer used. No other changes were made to the test class.

* Make FileSystem.CreateDirectory virtual for better testability

Refactored FileSystem to allow mocking CreateDirectory in tests.
Updated unit tests to mock directory creation and removed tests
that relied on unmockable static methods. Improved mocking of
output device in report generation tests and removed redundant
multi-format report tests. Enhances test isolation and coverage.

Author: Bertk

src/coverlet.MTP/Collector/CollectorExtension.cs

@@ -8,7 +8,6 @@
 using Coverlet.Core;
 using Coverlet.Core.Abstractions;
 using Coverlet.Core.Helpers;
-using Coverlet.Core.Reporters;
 using Coverlet.Core.Symbols;
 using Coverlet.MTP.CommandLine;
 using Coverlet.MTP.Configuration;
@@ -45,6 +44,7 @@ internal sealed class CollectorExtension : ITestHostProcessLifetimeHandler, ITes
   private string? _coverageIdentifier;
 
   private readonly CoverletExtension _extension = new();
+  private readonly IReporterFactory _reporterFactory;
 
   string IExtension.Uid => _extension.Uid;
   string IExtension.Version => _extension.Version;
@@ -56,13 +56,15 @@ public CollectorExtension(
     Microsoft.Testing.Platform.CommandLine.ICommandLineOptions commandLineOptions,
     Microsoft.Testing.Platform.OutputDevice.IOutputDevice? outputDevice,
     Microsoft.Testing.Platform.Configurations.IConfiguration? configuration,
-    IFileSystem? fileSystem = null)  // Add optional parameter for backward compatibility
+    IFileSystem? fileSystem = null,
+    IReporterFactory? reporterFactory = null)
   {
     _loggerFactory = loggerFactory ?? throw new ArgumentNullException(nameof(loggerFactory));
     _commandLineOptions = commandLineOptions ?? throw new ArgumentNullException(nameof(commandLineOptions));
     _platformConfiguration = configuration ?? throw new ArgumentNullException(nameof(configuration));
     _outputDisplay = outputDevice ?? throw new ArgumentNullException(nameof(outputDevice));
     _fileSystem = fileSystem ?? new FileSystem();  // Use provided or create default
+    _reporterFactory = reporterFactory ?? new DefaultReporterFactory();
     _configuration = new CoverletExtensionConfiguration();
     _logger = new CoverletLoggerAdapter(_loggerFactory);
 
@@ -324,27 +326,26 @@ private void InitializeCoverage()
       cecilSymbolHelper);
   }
 
-  private async Task GenerateReportsAsync(CoverageResult result, CancellationToken cancellation)
-  {
-    string outputDirectory = _platformConfiguration!.GetTestResultDirectory() ??
-      Path.GetDirectoryName(_testModulePath) + Path.DirectorySeparatorChar;
-
-    _logger.LogVerbose($"Coverage output directory: {outputDirectory}");
-
-    // Ensure directory exists
-    if (!_fileSystem.Exists(outputDirectory))
-    {
-      Directory.CreateDirectory(outputDirectory);
-    }
-
-    ISourceRootTranslator sourceRootTranslator = _serviceProvider!.GetRequiredService<ISourceRootTranslator>();
-    IFileSystem fileSystem = _serviceProvider!.GetRequiredService<IFileSystem>();
+  // Add internal property for testing
+  internal IReporterFactory? ReporterFactoryOverride { get; set; }
 
+  /// <summary>
+  /// Generates coverage report files. Separated for testability.
+  /// </summary>
+  internal List<string> GenerateCoverageReportFiles(
+    CoverageResult result,
+    ISourceRootTranslator sourceRootTranslator,
+    IFileSystem fileSystem,
+    string outputDirectory,
+    string[] formats)
+  {
     var generatedReports = new List<string>();
 
-    foreach (string format in _configuration.formats)
+    foreach (string format in formats)
     {
-      IReporter reporter = new ReporterFactory(format).CreateReporter() ??
+      // Use override if set (for testing), otherwise use injected factory
+      IReporterFactory factory = ReporterFactoryOverride ?? _reporterFactory;
+      IReporter reporter = factory.CreateReporter(format) ??
         throw new InvalidOperationException($"Specified output format '{format}' is not supported");
 
       if (reporter.OutputType == ReporterOutputType.Console)
@@ -355,34 +356,71 @@ private async Task GenerateReportsAsync(CoverageResult result, CancellationToken
       {
         string filename = $"coverage.{reporter.Extension}";
         string reportPath = Path.Combine(outputDirectory, filename);
-        await Task.Run(() => fileSystem.WriteAllText(reportPath, reporter.Report(result, sourceRootTranslator)), cancellation);
+        fileSystem.WriteAllText(reportPath, reporter.Report(result, sourceRootTranslator));
         generatedReports.Add(reportPath);
       }
     }
 
-    // Output successfully generated reports to console
-    if (generatedReports.Count > 0)
+    return generatedReports;
+  }
+
+  /// <summary>
+  /// Displays generated report paths to output device.
+  /// </summary>
+  private async Task DisplayGeneratedReportsAsync(List<string> generatedReports, CancellationToken cancellation)
+  {
+    if (generatedReports.Count == 0)
     {
-      _logger.LogInformation("Coverage reports generated:", important: true);
-      foreach (string reportPath in generatedReports)
-      {
-        _logger.LogInformation($"  {reportPath}", important: true);
-      }
-      // Output successfully generated reports to console (like TRX report extension does)
-      // Use FormattedTextOutputDeviceData for proper display like TRX extension
-      var outputBuilder = new StringBuilder();
-      outputBuilder.AppendLine();
-      outputBuilder.AppendLine("  Out of process file artifacts produced:");
-      foreach (string reportPath in generatedReports)
-      {
-        outputBuilder.AppendLine($"    - {reportPath}");
-      }
+      return;
+    }
+
+    _logger.LogInformation("Coverage reports generated:", important: true);
+    foreach (string reportPath in generatedReports)
+    {
+      _logger.LogInformation($"  {reportPath}", important: true);
+    }
+
+    var outputBuilder = new StringBuilder();
+    outputBuilder.AppendLine();
+    outputBuilder.AppendLine("  Out of process file artifacts produced:");
+    foreach (string reportPath in generatedReports)
+    {
+      outputBuilder.AppendLine($"    - {reportPath}");
+    }
+
+    await _outputDisplay.DisplayAsync(
+      this,
+      new TextOutputDeviceData(outputBuilder.ToString()),
+      cancellation).ConfigureAwait(false);
+  }
+
+  // Refactor GenerateReportsAsync to use extracted methods
+  private async Task GenerateReportsAsync(CoverageResult result, CancellationToken cancellation)
+  {
+    string outputDirectory = _platformConfiguration!.GetTestResultDirectory() ??
+      Path.GetDirectoryName(_testModulePath) + Path.DirectorySeparatorChar;
+
+    _logger.LogVerbose($"Coverage output directory: {outputDirectory}");
 
-      await _outputDisplay.DisplayAsync(
-        this,
-        new TextOutputDeviceData(outputBuilder.ToString()),
-        cancellation).ConfigureAwait(false);
+    // Ensure directory exists
+    if (!_fileSystem.Exists(outputDirectory))
+    {
+      _fileSystem.CreateDirectory(outputDirectory);
     }
+
+    ISourceRootTranslator sourceRootTranslator = _serviceProvider!.GetRequiredService<ISourceRootTranslator>();
+    IFileSystem fileSystem = _serviceProvider!.GetRequiredService<IFileSystem>();
+
+    // Generate reports (now testable separately)
+    List<string> generatedReports = GenerateCoverageReportFiles(
+      result,
+      sourceRootTranslator,
+      fileSystem,
+      outputDirectory,
+      _configuration.formats);
+
+    // Display results
+    await DisplayGeneratedReportsAsync(generatedReports, cancellation);
   }
 
   private string GetHitsFilePath()

src/coverlet.MTP/Collector/IReporterFactory.cs

@@ -0,0 +1,26 @@
+// Copyright (c) Toni Solarin-Sodara
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using Coverlet.Core.Abstractions;
+using Coverlet.Core.Reporters;
+
+namespace Coverlet.MTP.Collector;
+
+/// <summary>
+/// Factory abstraction for creating reporters. Enables testing.
+/// </summary>
+internal interface IReporterFactory
+{
+  IReporter? CreateReporter(string format);
+}
+
+/// <summary>
+/// Default implementation that delegates to Coverlet's ReporterFactory.
+/// </summary>
+internal sealed class DefaultReporterFactory : IReporterFactory
+{
+  public IReporter? CreateReporter(string format)
+  {
+    return new ReporterFactory(format).CreateReporter();
+  }
+}

src/coverlet.MTP/Configuration/CoverageConfiguration.cs

@@ -158,38 +158,17 @@ public string[] GetDoesNotReturnAttributes()
   }
 
   /// <summary>
-  /// Gets the test assembly path using multiple fallback strategies.
+  /// Gets the test assembly path.
   /// </summary>
   public static string GetTestAssemblyPath()
   {
-    // Try multiple methods to get the test assembly path
-    // 1. Entry assembly (most reliable for test scenarios)
+    // In test scenarios, the entry assembly is always the test assembly
     string? path = System.Reflection.Assembly.GetEntryAssembly()?.Location;
-    if (!string.IsNullOrEmpty(path) && File.Exists(path))
-    {
-      return path!;
-    }
-
-    // 2. Current process main module
-    path = System.Diagnostics.Process.GetCurrentProcess().MainModule?.FileName;
-    if (!string.IsNullOrEmpty(path) && File.Exists(path))
-    {
-      return path!;
-    }
-
-    // 3. Base directory + first command line argument
-    string[] args = Environment.GetCommandLineArgs();
-    if (args.Length > 0)
-    {
-      string fullPath = Path.GetFullPath(args[0]);
-      if (File.Exists(fullPath))
-      {
-        return fullPath;
-      }
-    }
 
-    // 4. Fallback to base directory
-    return AppContext.BaseDirectory;
+    return string.IsNullOrEmpty(path)
+      ? throw new InvalidOperationException(
+        "Unable to determine test assembly path. Entry assembly location is not available.")
+      : path!;
   }
 
   /// <summary>

src/coverlet.MTP/Properties/AssemblyInfo.cs

@@ -8,3 +8,5 @@
 
 [assembly: InternalsVisibleTo("coverlet.MTP.tests, PublicKey=0024000004800000940000000602000000240000525341310004000001000100cd2d5ae566fc3eb2d1837ec8706f78606b38d3bef1cdd80fc0c5f82cf9f2f221c60424274cbc70850c71e67f8695a74d26aad15395fa7473ecaa07c3048f32f5b83003eb9a927d9b0b688ebc423954dd883800b63a51091e355d1758d6360f7085a34606a18236a85f085cad80ec388ac340bb22770c3a01f39fcd7fa4d53cc6")]
 [assembly: InternalsVisibleTo("coverlet.MTP.validation.tests, PublicKey=00240000048000009400000006020000002400005253413100040000010001001575e172562f7b3ca4e105ef1539802ddf92de5f3d3a76937f99c73b1c64f46ec6ed1c5de46f2d39bc8916050376f749507bb5082958890e6e3ba9c68b4c6e4a56f16c1401f21f908c437a2b0b4dc263ef3bc1bc15d6a02a8e6cbf26a077f1590f91e248cf5ccd4642b7493b31cfa2bd9f921b662cd2cb8bcc66fc2cb533e2a8")]
+// Needed to mock internal type https://github.com/Moq/moq4/wiki/Quickstart#advanced-features
+[assembly: InternalsVisibleTo("DynamicProxyGenAssembly2, PublicKey=0024000004800000940000000602000000240000525341310004000001000100c547cac37abd99c8db225ef2f6c8a3602f3b3606cc9891605d02baa56104f4cfc0734aa39b93bf7852f7d9266654753cc297e7d2edfe0bac1cdcf9f717241550e0a7b191195b7667bb4f64bcb8e2121380fd1d9d46ad2d92d2d15605093924cceaf74c4861eff62abf69b9291ed0a340e113be11e6a7d3113e92484cf7045cc7")]

src/coverlet.core/Abstractions/IFileSystem.cs

@@ -7,6 +7,7 @@ namespace Coverlet.Core.Abstractions
 {
   internal interface IFileSystem
   {
+    void CreateDirectory(string path); // Add this method
     bool Exists(string path);
 
     void WriteAllText(string path, string contents);

src/coverlet.core/Helpers/FileSystem.cs

@@ -8,6 +8,13 @@ namespace Coverlet.Core.Helpers
 {
   internal class FileSystem : IFileSystem
   {
+
+    // We need to partial mock this method on tests
+    public virtual void CreateDirectory(string path)
+    {
+      Directory.CreateDirectory(path);
+    }
+
     // We need to partial mock this method on tests
     public virtual bool Exists(string path)
     {