SAVEPOINT

Src/FluentAssertions/Common/Services.cs

@@ -2,6 +2,7 @@
 using System.Linq;
 using System.Reflection;
 using FluentAssertions.Execution;
+using FluentAssertions.Extensibility;
 using JetBrains.Annotations;
 
 namespace FluentAssertions.Common;
@@ -50,18 +51,41 @@ internal static void EnsureInitialized()
         {
             if (!isInitialized)
             {
-                var assemblies = AppDomain.CurrentDomain.GetAssemblies();
+                InitializeExtensionAssemblies();
 
                 Reflector = new FullFrameworkReflector();
-    #if NETFRAMEWORK || NETCOREAPP
+#if NETFRAMEWORK || NETCOREAPP
                 ConfigurationStore = new ConfigurationStoreExceptionInterceptor(new AppSettingsConfigurationStore());
-    #else
+#else
                 ConfigurationStore = new NullConfigurationStore();
-    #endif
+#endif
                 ThrowException = new TestFrameworkProvider(Configuration).Throw;
 
                 isInitialized = true;
             }
         }
     }
+
+    private static void InitializeExtensionAssemblies()
+    {
+        var currentAssembly = Assembly.GetExecutingAssembly().GetName();
+
+        var testAssemblies = AppDomain.CurrentDomain.GetAssemblies()
+            .Where(a => a.GetReferencedAssemblies().Any(r => r.FullName == currentAssembly.FullName) && !a.IsDynamic && !IsFramework(a.GetName()));
+
+        foreach (Assembly testAssembly in testAssemblies)
+        {
+            foreach (var attribute in testAssembly.GetCustomAttributes<ExtensionAssemblyAttribute>())
+            {
+                attribute.Initialize();
+            }
+        }
+    }
+
+    private static bool IsFramework(AssemblyName assembly)
+    {
+#pragma warning disable CA1310
+        return assembly.FullName.StartsWith("Microsoft") || assembly.FullName.StartsWith("System");
+#pragma warning restore CA1310
+    }
 }

Src/FluentAssertions/Extensibility/ExtensionAssemblyAttribute.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Reflection;
+
+namespace FluentAssertions.Extensibility;
+
+[AttributeUsage(AttributeTargets.Assembly, AllowMultiple = true)]
+public sealed class ExtensionAssemblyAttribute : Attribute
+{
+    private readonly string methodName;
+    private readonly Type type;
+
+    public ExtensionAssemblyAttribute()
+    {
+    }
+
+#pragma warning disable CA1019
+    public ExtensionAssemblyAttribute(Type type, string methodName)
+#pragma warning restore CA1019
+    {
+        this.type = type;
+        this.methodName = methodName;
+    }
+
+    public void Initialize()
+    {
+        type?.GetMethod(methodName, BindingFlags.Public | BindingFlags.Static)?.Invoke(obj: null, parameters: null);
+    }
+}

Src/FluentAssertions/FluentAssertions.csproj

@@ -11,6 +11,7 @@
     <AllowedOutputExtensionsInPackageBuildOutputFolder>$(AllowedOutputExtensionsInPackageBuildOutputFolder);.pdb</AllowedOutputExtensionsInPackageBuildOutputFolder>
     <EmbedUntrackedSources>true</EmbedUntrackedSources>
     <ProduceReferenceAssembly>true</ProduceReferenceAssembly>
+    <Version>7.0.0</Version>
   </PropertyGroup>
 
   <PropertyGroup Label="Package info">
@@ -53,8 +54,10 @@
     <PackageReference Include="JetBrains.Annotations" Version="2023.2.0" PrivateAssets="all" />
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.1.1" PrivateAssets="All" />
     <PackageReference Include="PolySharp" Version="1.13.2" PrivateAssets="all" />
+    <PackageReference Include="xunit.extensibility.core" Version="2.5.0" />
   </ItemGroup>
 
+
   <!-- Target framework dependent configuration -->
   <Choose>
     <When Condition="'$(TargetFramework)' == 'net6.0'">

Tests/FluentAssertions.Equivalency.Specs/FluentAssertions.Equivalency.Specs.csproj

@@ -58,6 +58,7 @@
   <ItemGroup>
     <PackageReference Include="Chill" Version="4.1.0" />
     <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
+    <PackageReference Include="NSubstitute" Version="5.0.0" />
     <PackageReference Include="xunit" Version="2.5.0" />
     <PackageReference Include="Xunit.StaFact" Version="1.1.11" />
     <PackageReference Include="coverlet.collector" Version="6.0.0" PrivateAssets="all">

Tests/FluentAssertions.Specs/Extensibility/ExtensionAssemblyAttributeSpecs.cs

@@ -0,0 +1,15 @@
+using Xunit;
+
+namespace FluentAssertions.Specs.Extensibility;
+
+public class ExtensionAssemblyAttributeSpecs
+{
+        [Fact]
+        public void Extension_assembly_initialization_code_will_be_called_only_once()
+        {
+            for (int i = 0; i < 10; i++)
+            {
+                FileContainingAssemblyLevelAttributes.ShouldBeCalledOnlyOnce.Should().Be(1);
+            }
+        }
+}

Tests/FluentAssertions.Specs/Extensibility/FileContainingAssemblyLevelAttributes.cs

@@ -0,0 +1,22 @@
+using FluentAssertions.Extensibility;
+using FluentAssertions.Specs.Extensibility;
+
+// With specific initialization code to invoke before the first assertion happens
+[assembly: ExtensionAssembly(
+    typeof(FileContainingAssemblyLevelAttributes),
+    nameof(FileContainingAssemblyLevelAttributes.InitializeBeforeFirstAssertion))]
+
+// Multiple attributes are allowed and no initialization code is required
+[assembly: ExtensionAssembly]
+
+namespace FluentAssertions.Specs.Extensibility;
+
+public static class FileContainingAssemblyLevelAttributes
+{
+    public static int ShouldBeCalledOnlyOnce { get; private set; }
+
+    public static void InitializeBeforeFirstAssertion()
+    {
+        ++ShouldBeCalledOnlyOnce;
+    }
+}

Tests/FluentAssertions.Specs/FluentAssertions.Specs.csproj

@@ -74,4 +74,5 @@
     <ProjectReference Include="..\AssemblyB\AssemblyB.csproj" />
   </ItemGroup>
 
-</Project>
+
+  </Project>

nuget.config

@@ -3,5 +3,6 @@
   <packageSources>
     <clear />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
+    <add key="Local" value="C:\Workspaces\fluentassertions.datasets\Artifacts\" />
   </packageSources>
 </configuration>