Safer formatting of Xunit assertion messages

src/Akka.sln

@@ -288,6 +288,8 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ClusterToolsExample.Seed",
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ClusterToolsExample.Node", "examples\Cluster\ClusterTools\ClusterToolsExample.Node\ClusterToolsExample.Node.csproj", "{337A85B5-4A7C-4883-8634-46E7E52A765F}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Akka.TestKit.Xunit2.Tests", "contrib\testkits\Akka.TestKit.Xunit2.Tests\Akka.TestKit.Xunit2.Tests.csproj", "{95017C99-E960-44E5-83AD-BF21461DF06F}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -1343,6 +1345,18 @@ Global
 		{337A85B5-4A7C-4883-8634-46E7E52A765F}.Release|x64.Build.0 = Release|Any CPU
 		{337A85B5-4A7C-4883-8634-46E7E52A765F}.Release|x86.ActiveCfg = Release|Any CPU
 		{337A85B5-4A7C-4883-8634-46E7E52A765F}.Release|x86.Build.0 = Release|Any CPU
+		{95017C99-E960-44E5-83AD-BF21461DF06F}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{95017C99-E960-44E5-83AD-BF21461DF06F}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{95017C99-E960-44E5-83AD-BF21461DF06F}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{95017C99-E960-44E5-83AD-BF21461DF06F}.Debug|x64.Build.0 = Debug|Any CPU
+		{95017C99-E960-44E5-83AD-BF21461DF06F}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{95017C99-E960-44E5-83AD-BF21461DF06F}.Debug|x86.Build.0 = Debug|Any CPU
+		{95017C99-E960-44E5-83AD-BF21461DF06F}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{95017C99-E960-44E5-83AD-BF21461DF06F}.Release|Any CPU.Build.0 = Release|Any CPU
+		{95017C99-E960-44E5-83AD-BF21461DF06F}.Release|x64.ActiveCfg = Release|Any CPU
+		{95017C99-E960-44E5-83AD-BF21461DF06F}.Release|x64.Build.0 = Release|Any CPU
+		{95017C99-E960-44E5-83AD-BF21461DF06F}.Release|x86.ActiveCfg = Release|Any CPU
+		{95017C99-E960-44E5-83AD-BF21461DF06F}.Release|x86.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
@@ -1469,6 +1483,7 @@ Global
 		{88D7D845-2F50-4D37-9026-B0A8353D0E8D} = {7735F35A-E7B7-44DE-B6FB-C770B53EB69C}
 		{ED00E6F4-2B5C-4F16-ADE4-45E4A73C17B8} = {7735F35A-E7B7-44DE-B6FB-C770B53EB69C}
 		{337A85B5-4A7C-4883-8634-46E7E52A765F} = {7735F35A-E7B7-44DE-B6FB-C770B53EB69C}
+		{95017C99-E960-44E5-83AD-BF21461DF06F} = {7625FD95-4B2C-4A5B-BDD5-94B1493FAC8E}
 	EndGlobalSection
 	GlobalSection(ExtensibilityGlobals) = postSolution
 		SolutionGuid = {03AD8E21-7507-4E68-A4E9-F4A7E7273164}

src/contrib/testkits/Akka.TestKit.Xunit2.Tests/Akka.TestKit.Xunit2.Tests.csproj

@@ -0,0 +1,16 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <Import Project="../../../xunitSettings.props" />
+
+  <PropertyGroup>
+    <TargetFrameworks>$(NetFrameworkTestVersion);$(NetTestVersion)</TargetFrameworks>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="$(TestSdkVersion)" />
+    <PackageReference Include="xunit" Version="$(XunitVersion)" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="$(XunitVersion)" />
+    <ProjectReference Include="../Akka.TestKit.Xunit2/Akka.TestKit.Xunit2.csproj" />
+  </ItemGroup>
+
+</Project>

src/contrib/testkits/Akka.TestKit.Xunit2.Tests/Internals/AkkaEqualExceptionSpec.cs

@@ -0,0 +1,33 @@
+// -----------------------------------------------------------------------
+//  <copyright file="AkkaEqualException.cs" company="Akka.NET Project">
+//      Copyright (C) 2009-2025 Lightbend Inc. <http://www.lightbend.com>
+//      Copyright (C) 2013-2025 .NET Foundation <https://github.com/akkadotnet/akka.net>
+//  </copyright>
+// -----------------------------------------------------------------------
+
+using Akka.TestKit.Xunit2.Internals;
+using Xunit;
+
+namespace Akka.TestKit.Xunit2.Tests.Internals;
+
+public static class AkkaEqualExceptionSpec
+{
+#if NETFRAMEWORK
+    [Fact]
+    public static void Constructor_deserializes_message()
+    {
+        var originalException = new AkkaEqualException("Test message");
+
+        AkkaEqualException deserializedException;
+        using (var memoryStream = new System.IO.MemoryStream())
+        {
+            var formatter = new System.Runtime.Serialization.Formatters.Binary.BinaryFormatter();
+            formatter.Serialize(memoryStream, originalException);
+            memoryStream.Seek(0, System.IO.SeekOrigin.Begin);
+            deserializedException = (AkkaEqualException)formatter.Deserialize(memoryStream);
+        }
+
+        Assert.Equal(originalException.Message, deserializedException.Message);
+    }
+#endif
+}

src/contrib/testkits/Akka.TestKit.Xunit2.Tests/XunitAssertionsSpec.cs

@@ -0,0 +1,81 @@
+// -----------------------------------------------------------------------
+//  <copyright file="XunitAssertionsTests.cs" company="Akka.NET Project">
+//      Copyright (C) 2009-2025 Lightbend Inc. <http://www.lightbend.com>
+//      Copyright (C) 2013-2025 .NET Foundation <https://github.com/akkadotnet/akka.net>
+//  </copyright>
+// -----------------------------------------------------------------------
+
+using Xunit;
+using Xunit.Sdk;
+
+namespace Akka.TestKit.Xunit2.Tests;
+
+public class XunitAssertionsSpec
+{
+    private readonly XunitAssertions _assertions = new();
+
+    [Fact]
+    public void Assert_does_not_format_message_when_no_arguments_are_specified()
+    {
+        const string testMessage = "{Value} with different format placeholders {0}";
+
+        var exception = Assert.ThrowsAny<XunitException>(() => _assertions.Fail(testMessage));
+        Assert.Contains(testMessage, exception.Message);
+
+        exception = Assert.ThrowsAny<XunitException>(() => _assertions.AssertTrue(false, testMessage));
+        Assert.Contains(testMessage, exception.Message);
+
+        exception = Assert.ThrowsAny<XunitException>(() => _assertions.AssertFalse(true, testMessage));
+        Assert.Contains(testMessage, exception.Message);
+
+        exception = Assert.ThrowsAny<XunitException>(() => _assertions.AssertEqual(4, 2, testMessage));
+        Assert.Contains(testMessage, exception.Message);
+
+        exception = Assert.ThrowsAny<XunitException>(() => _assertions.AssertEqual(4, 2, (_, _) => false, testMessage));
+        Assert.Contains(testMessage, exception.Message);
+    }
+
+    [Fact]
+    public void Assert_formats_message_when_arguments_are_specified()
+    {
+        const string testMessage = "Meaning: {0}";
+        const string expectedMessage = "Meaning: 42";
+
+        var exception = Assert.ThrowsAny<XunitException>(() => _assertions.Fail(testMessage, 42));
+        Assert.Contains(expectedMessage, exception.Message);
+
+        exception = Assert.ThrowsAny<XunitException>(() => _assertions.AssertTrue(false, testMessage, 42));
+        Assert.Contains(expectedMessage, exception.Message);
+
+        exception = Assert.ThrowsAny<XunitException>(() => _assertions.AssertFalse(true, testMessage, 42));
+        Assert.Contains(expectedMessage, exception.Message);
+
+        exception = Assert.ThrowsAny<XunitException>(() => _assertions.AssertEqual(4, 2, testMessage, 42));
+        Assert.Contains(expectedMessage, exception.Message);
+
+        exception = Assert.ThrowsAny<XunitException>(() => _assertions.AssertEqual(4, 2, (_, _) => false, testMessage, 42));
+        Assert.Contains(expectedMessage, exception.Message);
+    }
+
+    [Fact]
+    public void Assert_catches_format_exceptions()
+    {
+        const string testMessage = "Meaning: {0} {1}";
+        const string expectedMessage = "Could not string.Format";
+
+        var exception = Assert.ThrowsAny<XunitException>(() => _assertions.Fail(testMessage, 42));
+        Assert.Contains(expectedMessage, exception.Message);
+
+        exception = Assert.ThrowsAny<XunitException>(() => _assertions.AssertTrue(false, testMessage, 42));
+        Assert.Contains(expectedMessage, exception.Message);
+
+        exception = Assert.ThrowsAny<XunitException>(() => _assertions.AssertFalse(true, testMessage, 42));
+        Assert.Contains(expectedMessage, exception.Message);
+
+        exception = Assert.ThrowsAny<XunitException>(() => _assertions.AssertEqual(4, 2, testMessage, 42));
+        Assert.Contains(expectedMessage, exception.Message);
+
+        exception = Assert.ThrowsAny<XunitException>(() => _assertions.AssertEqual(4, 2, (_, _) => false, testMessage, 42));
+        Assert.Contains(expectedMessage, exception.Message);
+    }
+}

src/contrib/testkits/Akka.TestKit.Xunit2/Internals/AkkaEqualException.cs

@@ -15,14 +15,12 @@ namespace Akka.TestKit.Xunit2.Internals
     /// <summary>
     /// TBD
     /// </summary>
+    [Serializable]
     public class AkkaEqualException : XunitException
     {
         // Length of "Expected: " and "Actual:   "
         private static readonly string NewLineAndIndent = Environment.NewLine + new string(' ', 10);
 
-        private readonly string? _format;
-        private readonly object[] _args = Array.Empty<object>();
-
         public static AkkaEqualException ForMismatchedValues(
             object? expected,
             object? actual,
@@ -46,48 +44,46 @@ public static AkkaEqualException ForMismatchedValues(
                 args
             );
         }
-        
+
         /// <summary>
         /// Initializes a new instance of the <see cref="AkkaEqualException"/> class.
         /// </summary>
         /// <param name="format">A template string that describes the error.</param>
         /// <param name="args">An optional object array that contains zero or more objects to format.</param>
-        public AkkaEqualException(string format = "", params object[] args): base(null)
-        {
-            _format = format;
-            _args = args;
-        }
+        public AkkaEqualException(string format = "", params object[] args)
+            : base(BuildAssertionMessage(format, args)) { }
 
         /// <summary>
         /// Initializes a new instance of the <see cref="AkkaEqualException"/> class.
         /// </summary>
         /// <param name="info">The <see cref="SerializationInfo"/> that holds the serialized object data about the exception being thrown.</param>
         /// <param name="context">The <see cref="StreamingContext"/> that contains contextual information about the source or destination.</param>
-        protected AkkaEqualException(SerializationInfo info, StreamingContext context): base(null)
-        {
-        }
+        protected AkkaEqualException(SerializationInfo info, StreamingContext context)
+            : base(info.GetString("Message")) { }
 
         /// <summary>
-        /// The message that describes the error.
+        /// Builds assertion message by applying specified arguments to the format string.
+        /// When no arguments are specified, format string is returned as-is.
         /// </summary>
-        public override string Message
+        internal static string? BuildAssertionMessage(string format, object[] args)
         {
-            get
+            if (string.IsNullOrEmpty(format))
             {
-                if(string.IsNullOrEmpty(_format))
-                    return base.Message;
+                return null;
+            }
 
-                string message;
-                try
-                {
-                    message = string.Format(_format!, _args);
-                }
-                catch(Exception)
-                {
-                    message = $@"[Could not string.Format(""{_format}"", {string.Join(", ", _args)})]";
-                }
+            if (args is not { Length: > 0 })
+            {
+                return format;
+            }
 
-                return base.Message is not null ? $"{base.Message} {message}" : message;
+            try
+            {
+                return string.Format(format, args);
+            }
+            catch (Exception)
+            {
+                return $"""[Could not string.Format("{format}", {string.Join(", ", args)})]""";
             }
         }
     }

src/contrib/testkits/Akka.TestKit.Xunit2/XunitAssertions.cs

@@ -23,7 +23,7 @@ public class XunitAssertions : ITestKitAssertions
         /// <param name="args">An optional object array that contains zero or more objects to format.</param>
         public void Fail(string format = "", params object[] args)
         {
-            Assert.Fail(string.Format(format, args));
+            Assert.Fail(AkkaEqualException.BuildAssertionMessage(format, args));
         }
 
         /// <summary>
@@ -34,7 +34,7 @@ public void Fail(string format = "", params object[] args)
         /// <param name="args">An optional object array that contains zero or more objects to format.</param>
         public void AssertTrue(bool condition, string format = "", params object[] args)
         {
-            Assert.True(condition, string.Format(format, args));
+            Assert.True(condition, AkkaEqualException.BuildAssertionMessage(format, args));
         }
 
         /// <summary>
@@ -45,7 +45,7 @@ public void AssertTrue(bool condition, string format = "", params object[] args)
         /// <param name="args">An optional object array that contains zero or more objects to format.</param>
         public void AssertFalse(bool condition, string format = "", params object[] args)
         {
-            Assert.False(condition, string.Format(format, args));
+            Assert.False(condition, AkkaEqualException.BuildAssertionMessage(format, args));
         }
 
         /// <summary>

src/core/Akka.API.Tests/verify/CoreAPISpec.ApproveTestKitXunit2.DotNet.verified.txt

@@ -38,7 +38,6 @@ namespace Akka.TestKit.Xunit2.Internals
     {
         public AkkaEqualException(string format = "", params object[] args) { }
         protected AkkaEqualException(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
-        public override string Message { get; }
         [return: System.Runtime.CompilerServices.NullableAttribute(1)]
         public static Akka.TestKit.Xunit2.Internals.AkkaEqualException ForMismatchedValues(object expected, object actual, string format = null, [System.Runtime.CompilerServices.NullableAttribute(1)] params object[] args) { }
     }

src/core/Akka.API.Tests/verify/CoreAPISpec.ApproveTestKitXunit2.Net.verified.txt

@@ -38,7 +38,6 @@ namespace Akka.TestKit.Xunit2.Internals
     {
         public AkkaEqualException(string format = "", params object[] args) { }
         protected AkkaEqualException(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
-        public override string Message { get; }
         [return: System.Runtime.CompilerServices.NullableAttribute(1)]
         public static Akka.TestKit.Xunit2.Internals.AkkaEqualException ForMismatchedValues(object expected, object actual, string format = null, [System.Runtime.CompilerServices.NullableAttribute(1)] params object[] args) { }
     }

src/core/Akka.Persistence.TestKit.Tests/JournalInterceptorsSpecs.cs

@@ -73,7 +73,7 @@ public async Task cancelable_delay_must_call_next_interceptor_immediately_after_
 
             var startedAt = DateTime.Now;
             var task = delay.InterceptAsync(null);
-            await Task.Delay(delayDuration - epsilon);
+            await Task.Delay(delayDuration);
 
             probe.WasCalled.Should().BeFalse();
             cts.Cancel();

src/core/Akka.Persistence.TestKit.Tests/SnapshotStoreInterceptorsSpec.cs

@@ -63,7 +63,7 @@ public async Task cancelable_delay_must_call_next_interceptor_immediately_after_
 
             var startedAt = DateTime.Now;
             var task = delay.InterceptAsync(null, null);
-            await Task.Delay(delayDuration - epsilon);
+            await Task.Delay(delayDuration);
 
             probe.WasCalled.Should().BeFalse();
             cts.Cancel();