Handling F# assemblies correctly by catching reflection exceptions an…

[jbogard]

…d falling back to GetTypeInfo

[jbogard]

Fix: Assembly Scanner Crashes on F# inref Parameters (Issue #1140)

Context

When an assembly being scanned by MediatR contains F# code with inref parameters (e.g. member inline _.Yield (span: inref<Span<char>>)), the assembly scanner throws:

System.Reflection.ReflectionTypeLoadException: Unable to load one or more of the requested types. A ByRef or ByRef-like type cannot be used as the type for an instance field in a non-ByRef-like type.

F# inref<T> and byref<T> are ByRef-like types. .NET reflection throws ReflectionTypeLoadException when trying to enumerate all types in an assembly containing such constructs because it cannot represent them as standard CLR instance fields during type metadata inspection.

Currently, all four type-loading sites in ServiceRegistrar.cs call a.DefinedTypes or assembly.GetTypes() directly, with no exception handling.

Fix: Add a single private helper GetLoadableDefinedTypes(this Assembly) that catches ReflectionTypeLoadException and returns only the successfully-loaded types from e.Types. Replace all four call sites with this helper.

File to Modify

src/MediatR/Registration/ServiceRegistrar.cs

Four Call Sites to Fix

• Line 79 — a.DefinedTypes in AddMediatRClasses (multi-open interfaces scan)
• Line 106 — a.DefinedTypes in ConnectImplementationsToTypesClosing
• Line 236 — assembly.GetTypes() in GetConcreteRequestTypes
• Line 428 — a.DefinedTypes in RegisterClosedBehaviorsFromAssemblies

Implementation

Note on DefinedTypes vs GetTypes()

Per the .NET docs, both return the same set of types (public, internal, private, and nested). The only difference is the return type: DefinedTypes → IEnumerable<TypeInfo>, GetTypes() → Type[]. Since TypeInfo inherits from Type, every property and method used in ServiceRegistrar.cs (IsClass, IsAbstract, IsAssignableFrom, GetInterfaces, etc.) is identical. Replacing the GetTypes() call at line 236 with GetLoadableDefinedTypes() has no behavior change.

1. Add helper method (after the existing Fill extension, before HasNestedGenericResponseType)

private static IEnumerable<TypeInfo> GetLoadableDefinedTypes(this Assembly assembly)
{
    try
    {
        return assembly.DefinedTypes;
    }
    catch (ReflectionTypeLoadException ex)
    {
        return ex.Types.Where(t => t != null).Select(t => t!.GetTypeInfo());
    }
}

TypeInfo inherits from Type, so this return type is compatible with all four call sites.

2. Replace the four call sites

Line 79 — multi-open interfaces scan:

// before
.SelectMany(a => a.DefinedTypes)
// after
.SelectMany(a => a.GetLoadableDefinedTypes())

Line 106 — ConnectImplementationsToTypesClosing:

// before
.SelectMany(a => a.DefinedTypes)
// after
.SelectMany(a => a.GetLoadableDefinedTypes())

Line 236 — GetConcreteRequestTypes (note: currently uses GetTypes(), not DefinedTypes):

// before
.SelectMany(assembly => assembly.GetTypes())
// after
.SelectMany(assembly => assembly.GetLoadableDefinedTypes())

Line 428 — RegisterClosedBehaviorsFromAssemblies:

// before
.SelectMany(a => a.DefinedTypes)
// after
.SelectMany(a => a.GetLoadableDefinedTypes())

Verification

dotnet test

All existing tests should continue to pass. The fix is purely defensive — it only changes behavior when ReflectionTypeLoadException is thrown during type enumeration, which does not happen with normal C# assemblies.

[Copilot]

Pull request overview

This pull request adds robust handling for F# assemblies and other assemblies that may throw ReflectionTypeLoadException when accessing their defined types. The changes introduce a new extension method GetLoadableDefinedTypes that gracefully catches these exceptions and returns only the successfully loaded types.

Changes:

• Added GetLoadableDefinedTypes extension method to safely access assembly types with exception handling
• Updated all assembly type scanning operations to use the new safe method
• Added comprehensive test coverage for assemblies that throw ReflectionTypeLoadException

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/MediatR/Registration/ServiceRegistrar.cs	Added GetLoadableDefinedTypes extension method that catches ReflectionTypeLoadException and returns loadable types; replaced all direct DefinedTypes accesses with calls to this new method (lines 79, 106, 236, 440)
test/MediatR.Tests/MicrosoftExtensionsDI/AssemblyResolutionTests.cs	Added test ShouldNotThrowWhenAssemblyThrowsReflectionTypeLoadException with a mock BrokenAssembly class that simulates F# assembly behavior by throwing ReflectionTypeLoadException

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