Add support for xUnit.net v3 (fluentassertions#2718)

Author: 0xced

Build/Build.cs

@@ -244,7 +244,9 @@ class Build : NukeBuild
                 Solution.TestFrameworks.MSTestV2_Specs,
                 Solution.TestFrameworks.NUnit3_Specs,
                 Solution.TestFrameworks.NUnit4_Specs,
-                Solution.TestFrameworks.XUnit2_Specs
+                Solution.TestFrameworks.XUnit2_Specs,
+                Solution.TestFrameworks.XUnit3_Specs,
+                Solution.TestFrameworks.XUnit3Core_Specs,
             ];
 
             var testCombinations =

FluentAssertions.sln

@@ -59,6 +59,10 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "FluentAssertions.Extensibil
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "TUnit.Specs", "Tests\TestFrameworks\TUnit.Specs\TUnit.Specs.csproj", "{6540564E-9B6E-4E1E-8881-6F0DD0F35576}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "XUnit3.Specs", "Tests\TestFrameworks\XUnit3.Specs\XUnit3.Specs.csproj", "{AF1479D8-519F-4C6B-9E8C-5A84A6D5549A}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "XUnit3Core.Specs", "Tests\TestFrameworks\XUnit3Core.Specs\XUnit3Core.Specs.csproj", "{77BB9496-169D-43DA-BCED-7DB3ACD9179A}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		CI|Any CPU = CI|Any CPU
@@ -173,6 +177,18 @@ Global
 		{6540564E-9B6E-4E1E-8881-6F0DD0F35576}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{6540564E-9B6E-4E1E-8881-6F0DD0F35576}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{6540564E-9B6E-4E1E-8881-6F0DD0F35576}.Release|Any CPU.Build.0 = Release|Any CPU
+		{AF1479D8-519F-4C6B-9E8C-5A84A6D5549A}.CI|Any CPU.ActiveCfg = Debug|Any CPU
+		{AF1479D8-519F-4C6B-9E8C-5A84A6D5549A}.CI|Any CPU.Build.0 = Debug|Any CPU
+		{AF1479D8-519F-4C6B-9E8C-5A84A6D5549A}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{AF1479D8-519F-4C6B-9E8C-5A84A6D5549A}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{AF1479D8-519F-4C6B-9E8C-5A84A6D5549A}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{AF1479D8-519F-4C6B-9E8C-5A84A6D5549A}.Release|Any CPU.Build.0 = Release|Any CPU
+		{77BB9496-169D-43DA-BCED-7DB3ACD9179A}.CI|Any CPU.ActiveCfg = Debug|Any CPU
+		{77BB9496-169D-43DA-BCED-7DB3ACD9179A}.CI|Any CPU.Build.0 = Debug|Any CPU
+		{77BB9496-169D-43DA-BCED-7DB3ACD9179A}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{77BB9496-169D-43DA-BCED-7DB3ACD9179A}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{77BB9496-169D-43DA-BCED-7DB3ACD9179A}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{77BB9496-169D-43DA-BCED-7DB3ACD9179A}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
@@ -196,6 +212,8 @@ Global
 		{8DF4A6FE-AAD0-41E5-B2F4-34166D1B139C} = {963262D0-9FD5-4741-8C0E-E2F34F110EF3}
 		{450FC408-A4E2-4483-B064-2007024D6CF1} = {963262D0-9FD5-4741-8C0E-E2F34F110EF3}
 		{6540564E-9B6E-4E1E-8881-6F0DD0F35576} = {4D8FA213-8724-4C43-B68A-F018148D238C}
+		{AF1479D8-519F-4C6B-9E8C-5A84A6D5549A} = {4D8FA213-8724-4C43-B68A-F018148D238C}
+		{77BB9496-169D-43DA-BCED-7DB3ACD9179A} = {4D8FA213-8724-4C43-B68A-F018148D238C}
 	EndGlobalSection
 	GlobalSection(ExtensibilityGlobals) = postSolution
 		SolutionGuid = {75DDA3D8-9D6F-4865-93F4-DDE11DEE8290}

Src/FluentAssertions/Execution/AssertionFailedException.cs

@@ -7,7 +7,7 @@ namespace FluentAssertions.Execution;
 /// </summary>
 /// <param name="message">The <em>mandatory</em> exception message</param>
 #pragma warning disable CA1032, RCS1194 // AssertionFailedException should never be constructed with an empty message
-public class AssertionFailedException(string message) : Exception(message)
+public class AssertionFailedException(string message) : Exception(message), IAssertionException
 #pragma warning restore CA1032, RCS1194
 {
 }

Src/FluentAssertions/Execution/IAssertionException.cs

@@ -0,0 +1,7 @@
+namespace FluentAssertions.Execution;
+
+/// <summary>
+/// This is a marker interface for xUnit.net v3 to set the test failure cause as an assertion failure.
+/// See <a href="https://xunit.net/docs/getting-started/v3/whats-new#third-party-assertion-library-extension-points">What’s New in xUnit.net v3 - Third party assertion library extension points</a>.
+/// </summary>
+internal interface IAssertionException;

Src/FluentAssertions/Execution/LateBoundTestFramework.cs

@@ -1,40 +1,58 @@
 using System;
 using System.Diagnostics.CodeAnalysis;
+using System.IO;
 using System.Reflection;
 
 namespace FluentAssertions.Execution;
 
 internal abstract class LateBoundTestFramework : ITestFramework
 {
-    private Assembly assembly;
+    private readonly bool loadAssembly;
+    private Func<string, Exception> exceptionFactory;
 
-    [DoesNotReturn]
-    public void Throw(string message)
+    protected LateBoundTestFramework(bool loadAssembly = false)
     {
-        Type exceptionType = assembly.GetType(ExceptionFullName);
-
-        if (exceptionType is null)
-        {
-            throw new NotSupportedException(
-                $"Failed to create the assertion exception for the current test framework: \"{ExceptionFullName}, {assembly.FullName}\"");
-        }
-
-        throw (Exception)Activator.CreateInstance(exceptionType, message);
+        this.loadAssembly = loadAssembly;
+        exceptionFactory = _ => throw new InvalidOperationException($"{nameof(IsAvailable)} must be called first.");
     }
 
+    [DoesNotReturn]
+    public void Throw(string message) => throw exceptionFactory(message);
+
     public bool IsAvailable
     {
         get
         {
-            string prefix = AssemblyName + ",";
+            var assembly = FindExceptionAssembly();
+            var exceptionType = assembly?.GetType(ExceptionFullName);
 
-            assembly = Array.Find(AppDomain.CurrentDomain
-                .GetAssemblies(), a => a.FullName.StartsWith(prefix, StringComparison.OrdinalIgnoreCase));
+            exceptionFactory = exceptionType != null
+                ? message => (Exception)Activator.CreateInstance(exceptionType, message)
+                : _ => throw new InvalidOperationException($"{GetType().Name} is not available");
 
-            return assembly is not null;
+            return exceptionType is not null;
         }
     }
 
+    private Assembly FindExceptionAssembly()
+    {
+        var assembly = Array.Find(AppDomain.CurrentDomain.GetAssemblies(), a => a.GetName().Name == AssemblyName);
+
+        if (assembly is null && loadAssembly)
+        {
+            try
+            {
+                return Assembly.Load(new AssemblyName(AssemblyName));
+            }
+            catch (FileNotFoundException)
+            {
+                return null;
+            }
+        }
+
+        return assembly;
+    }
+
     protected internal abstract string AssemblyName { get; }
 
     protected abstract string ExceptionFullName { get; }

Src/FluentAssertions/Execution/LoadableTestFramework.cs

@@ -1,6 +1,6 @@
 namespace FluentAssertions.Execution;
 
-internal class TUnitFramework : LoadableTestFramework
+internal class TUnitFramework() : LateBoundTestFramework(loadAssembly: true)
 {
     protected override string ExceptionFullName => "TUnit.Assertions.Exceptions.AssertionException";
 

Src/FluentAssertions/Execution/TUnitFramework.cs

@@ -18,8 +18,11 @@ internal class TestFrameworkProvider
         ["mspec"] = new MSpecFramework(),
         ["nunit"] = new NUnitTestFramework(),
         ["mstestv2"] = new MSTestFrameworkV2(),
+
+        // Keep TUnitFramework and XUnitTestFramework last as they use a try/catch approach
         ["tunit"] = new TUnitFramework(),
-        ["xunit2"] = new XUnit2TestFramework() // Keep this the last one as it uses a try/catch approach
+        ["xunit2"] = new XUnitTestFramework("xunit.assert"),
+        ["xunit3"] = new XUnitTestFramework("xunit.v3.assert"),
     };
 
     private readonly Configuration configuration;

Src/FluentAssertions/Execution/TestFrameworkProvider.cs

@@ -0,0 +1,11 @@
+namespace FluentAssertions.Execution;
+
+/// <summary>
+/// Implements the xUnit (version 2 and 3) test framework adapter.
+/// </summary>
+internal class XUnitTestFramework(string assemblyName) : LateBoundTestFramework(loadAssembly: true)
+{
+    protected internal override string AssemblyName => assemblyName;
+
+    protected override string ExceptionFullName => "Xunit.Sdk.XunitException";
+}