Emit diagnostic message when unable to load config

* Adds proxy for ConfigReader to enable mock injection
* Modifies xunit to emit diagnostic message per assembly

Fixes xunit#1655

src/xunit.v3.runner.msbuild.tests/xunitTests.cs

@@ -81,6 +81,37 @@ public static void ReturnsFalseWhenExitCodeIsNonZero()
 
 			Assert.False(result);
 		}
+
+		[Fact]
+		public static void ReportsWhenUnableToReadConfig()
+		{
+			var mockedReporter = Substitute.For<IRunnerReporter>();
+			mockedReporter.IsEnvironmentallyEnabled.Returns(true);
+
+			var mockedMessageSink = Substitute.For<_IMessageSink>();
+			mockedReporter.CreateMessageHandler(Arg.Any<IRunnerLogger>(), Arg.Any<_IMessageSink>())
+				.Returns(new System.Threading.Tasks.ValueTask<_IMessageSink>(mockedMessageSink));
+
+			var mockedProxy = Substitute.For<_ConfigReaderProxy>();
+			mockedProxy.Load(Arg.Any<TestAssemblyConfiguration>(), Arg.Any<string>(), Arg.Any<string>())
+				.Returns(false);
+
+			var stubAssembly = Substitute.For<ITaskItem>();
+			stubAssembly.GetMetadata("ConfigFile").Returns("DummyPath");
+
+			var assemblies = new ITaskItem[] { stubAssembly };
+
+			var xunit = new Testable_xunit(mockedProxy, mockedReporter)
+			{
+				Assemblies = assemblies,
+				NoLogo = true
+			};
+
+			xunit.Cancel();
+			xunit.Execute();
+
+			mockedMessageSink.Received().OnMessage(Arg.Any<_DiagnosticMessage>());
+		}
 	}
 
 	public class GetReporter
@@ -186,14 +217,22 @@ public void BadChosenReporter_WithAvailableReporters()
 
 	public class Testable_xunit : xunit
 	{
-		public readonly List<IRunnerReporter> AvailableReporters = new();
+		public readonly List<IRunnerReporter> AvailableReporters;
+		public IRunnerReporter? MockedReporter { get; init; }
 
 		public Testable_xunit()
 			: this(0)
 		{ }
 
+		public Testable_xunit(_ConfigReaderProxy proxy, IRunnerReporter reporter)
+			: base(proxy)
+		{
+			AvailableReporters = new() { reporter };
+		}
+
 		public Testable_xunit(int exitCode)
 		{
+			AvailableReporters = new();
 			BuildEngine = Substitute.For<IBuildEngine>();
 			Assemblies = new ITaskItem[0];
 			ExitCode = exitCode;
@@ -202,7 +241,6 @@ public Testable_xunit(int exitCode)
 		protected override List<IRunnerReporter> GetAvailableRunnerReporters() =>
 			AvailableReporters;
 
-		public new IRunnerReporter? GetReporter() =>
-			base.GetReporter();
+		public new IRunnerReporter? GetReporter() => base.GetReporter();
 	}
 }

src/xunit.v3.runner.msbuild/xunit.cs

@@ -22,6 +22,8 @@ public class xunit : MSBuildTask, ICancelableTask
 	{
 		volatile bool cancel;
 		readonly ConcurrentDictionary<string, ExecutionSummary> completionMessages = new();
+
+		readonly _ConfigReaderProxy configReader;
 		bool? diagnosticMessages;
 		bool? failSkips;
 		XunitFilters? filters;
@@ -106,6 +108,15 @@ protected XunitFilters Filters
 
 		public ITaskItem? XmlV1 { get; set; }
 
+		public xunit()
+			: this(new _ConfigReaderProxy())
+		{ }
+
+		public xunit(_ConfigReaderProxy proxy)
+		{
+			configReader = proxy;
+		}
+
 		public void Cancel()
 		{
 			cancel = true;
@@ -205,7 +216,10 @@ public override bool Execute()
 						TargetFramework = targetFramework
 					};
 
-					ConfigReader.Load(projectAssembly.Configuration, assemblyFileName, configFileName);
+					if (!configReader.Load(projectAssembly.Configuration, assemblyFileName, configFileName) && configFileName != null)
+					{
+						reporterMessageHandler.OnMessage(new _DiagnosticMessage() { Message = $"Unable to read '{configFileName}'; falling back to default values." });
+					}
 
 					if (Culture != null)
 						projectAssembly.Configuration.Culture = Culture switch
@@ -241,7 +255,7 @@ public override bool Execute()
 					foreach (var assembly in project.Assemblies)
 					{
 						var assemblyElement = ExecuteAssembly(assembly, appDomains);
-						if (assemblyElement != null)
+						if (assemblyElement != null && assemblyElement.Result != null)
 							assembliesElement!.Add(assemblyElement);
 					}
 				}

src/xunit.v3.runner.utility/Configuration/ConfigReaderProxy.cs

@@ -0,0 +1,19 @@
+namespace Xunit.Runner.Common
+{
+	/// <summary>
+	/// This class is a simple proxy for ConfigReader allowing for dependency injection.
+	/// </summary>
+	public class _ConfigReaderProxy
+	{
+		/// <summary>
+		/// Proxies calls to ConfigReader.Load
+		/// </summary>
+		public virtual bool Load(
+			TestAssemblyConfiguration configuration,
+			string? assemblyFileName,
+			string? configFileName = null)
+		{
+			return ConfigReader.Load(configuration, assemblyFileName, configFileName);
+		}
+	}
+}