Handling F# assemblies correctly by catching reflection exceptions an…

src/MediatR/Registration/ServiceRegistrar.cs

@@ -76,7 +76,7 @@ public static void AddMediatRClasses(IServiceCollection services, MediatRService
             var arity = multiOpenInterface.GetGenericArguments().Length;
 
             var concretions = assembliesToScan
-                .SelectMany(a => a.DefinedTypes)
+                .SelectMany(a => a.GetLoadableDefinedTypes())
                 .Where(type => type.FindInterfacesThatClose(multiOpenInterface).Any())
                 .Where(type => type.IsConcrete() && type.IsOpenGeneric())
                 .Where(type => type.GetGenericArguments().Length == arity)
@@ -103,7 +103,7 @@ private static void ConnectImplementationsToTypesClosing(Type openRequestInterfa
         var genericInterfaces = new List<Type>();
 
         var types = assembliesToScan
-            .SelectMany(a => a.DefinedTypes)
+            .SelectMany(a => a.GetLoadableDefinedTypes())
             .Where(t => !t.ContainsGenericParameters || configuration.RegisterGenericHandlers)
             .Where(t => t.IsConcrete() && t.FindInterfacesThatClose(openRequestInterface).Any())
             .Where(configuration.TypeEvaluator)
@@ -233,7 +233,7 @@ private static (Type Service, Type Implementation) GetConcreteRegistrationTypes(
 
         var typesThatCanCloseForEachParameter = constraintsForEachParameter
             .Select(constraints => assembliesToScan
-                .SelectMany(assembly => assembly.GetTypes())
+                .SelectMany(assembly => assembly.GetLoadableDefinedTypes())
                 .Where(type => type.IsClass && !type.IsAbstract && constraints.All(constraint => constraint.IsAssignableFrom(type))).ToList()
             ).ToList();
 
@@ -385,6 +385,18 @@ private static void Fill<T>(this IList<T> list, T value)
         list.Add(value);
     }
 
+    private static IEnumerable<Type> GetLoadableDefinedTypes(this Assembly assembly)
+    {
+        try
+        {
+            return assembly.DefinedTypes;
+        }
+        catch (ReflectionTypeLoadException ex)
+        {
+            return ex.Types.OfType<Type>();
+        }
+    }
+
     private static bool HasNestedGenericResponseType(Type openBehaviorType)
     {
         var iface = openBehaviorType.GetInterfaces()
@@ -425,7 +437,7 @@ private static void RegisterClosedBehaviorsFromAssemblies(
         var behaviorParams  = openBehaviorType.GetGenericArguments();
 
         var requests = assemblies
-            .SelectMany(a => a.DefinedTypes)
+            .SelectMany(a => a.GetLoadableDefinedTypes())
             .Where(t => t.IsConcrete() && !t.ContainsGenericParameters)
             .SelectMany(t => t.GetInterfaces()
                 .Where(i => i.IsGenericType && i.GetGenericTypeDefinition() == typeof(IRequest<>))

test/MediatR.Tests/MicrosoftExtensionsDI/AssemblyResolutionTests.cs

@@ -3,7 +3,9 @@
 namespace MediatR.Extensions.Microsoft.DependencyInjection.Tests;
 
 using System;
+using System.Collections.Generic;
 using System.Linq;
+using System.Reflection;
 using Shouldly;
 using Xunit;
 
@@ -87,4 +89,30 @@ public void ShouldResolveVoidGenericPingRequestHandler()
     {
         _provider.GetService<IRequestHandler<VoidGenericPing<Pong>>>().ShouldNotBeNull();
     }
+
+    [Fact]
+    public void ShouldNotThrowWhenAssemblyThrowsReflectionTypeLoadException()
+    {
+        IServiceCollection services = new ServiceCollection();
+        services.AddFakeLogging();
+        services.AddSingleton(new Logger());
+
+        Action registration = () => services.AddMediatR(cfg =>
+        {
+            cfg.RegisterServicesFromAssemblies(typeof(Ping).Assembly, new BrokenAssembly());
+        });
+
+        registration.ShouldNotThrow();
+
+        var provider = services.BuildServiceProvider();
+        provider.GetService<IRequestHandler<Ping, Pong>>().ShouldNotBeNull();
+    }
+
+    private class BrokenAssembly : Assembly
+    {
+        public override IEnumerable<TypeInfo> DefinedTypes =>
+            throw new ReflectionTypeLoadException(
+                new Type?[] { typeof(string), null, typeof(int) },
+                new Exception?[] { null, new Exception("ByRef-like type cannot be loaded"), null });
+    }
 }