LuckyPennySoftware/MediatR · error · InvalidOperationException

Could not find method {nameof(IRequestExceptionHandler<TRequ

Error message

Could not find method {nameof(IRequestExceptionHandler<TRequest, TResponse, Exception>.Handle)} on type {exceptionHandlerInterfaceType}

What it means

Thrown by GetMethodInfoForHandler if reflection cannot locate the Handle method on the synthesized IRequestExceptionHandler<TRequest,TResponse,TException> interface built via MakeGenericType. This is a defensive invariant: Handle is declared on the interface and should always resolve; firing it points to an abnormal runtime/build/proxying condition, not normal usage.

Source

Thrown at src/MediatR/Pipeline/RequestExceptionProcessorBehavior.cs:102

    }

    private IEnumerable<(Type ExceptionType, object Handler)> GetHandlersForException(Type exceptionType, TRequest request)
    {
        var exceptionHandlerInterfaceType = typeof(IRequestExceptionHandler<,,>).MakeGenericType(typeof(TRequest), typeof(TResponse), exceptionType);
        var enumerableExceptionHandlerInterfaceType = typeof(IEnumerable<>).MakeGenericType(exceptionHandlerInterfaceType);

        var exceptionHandlers = (IEnumerable<object>) _serviceProvider.GetRequiredService(enumerableExceptionHandlerInterfaceType);

        return HandlersOrderer.Prioritize(exceptionHandlers.ToList(), request)
            .Select(handler => (exceptionType, action: handler));
    }

    private static MethodInfo GetMethodInfoForHandler(Type exceptionType)
    {
        var exceptionHandlerInterfaceType = typeof(IRequestExceptionHandler<,,>).MakeGenericType(typeof(TRequest), typeof(TResponse), exceptionType);
        
        var handleMethodInfo = exceptionHandlerInterfaceType.GetMethod(nameof(IRequestExceptionHandler<TRequest, TResponse, Exception>.Handle))
                           ?? throw new InvalidOperationException($"Could not find method {nameof(IRequestExceptionHandler<TRequest, TResponse, Exception>.Handle)} on type {exceptionHandlerInterfaceType}");

        return handleMethodInfo;
    }
}

View on GitHub (pinned to 916ef1b3d6)

Solutions

1. Align MediatR package versions across all projects/assemblies in the solution.
2. Ensure any proxy/weaver retains the Handle method on the IRequestExceptionHandler<,,> interface.
3. Clean and rebuild to discard stale compiled artifacts.
4. Reproduce in a minimal console app to rule out environment-specific reflection limits.

var method = typeof(IRequestExceptionHandler<MyReq, MyResp, MyException>).GetMethod("Handle");
Debug.Assert(method is not null, "Handle must exist on IRequestExceptionHandler<,,>");

When it happens

Trigger: Should not occur under standard .NET. Possible with dynamic proxies that drop the interface method, IL weavers that alter the interface, partial-trust reflection restrictions, or a corrupt/version-mismatched MediatR assembly.

Common situations: Proxy frameworks (Castle, Moq) mocking exception handlers without preserving Handle; AOP weavers; mismatched MediatR versions in the dependency graph.

Related errors

• Could not find method {nameof(IRequestExceptionAction<TReque
• Did not return a Task from the exception handler.
• Could not create task for action method {actionForException.
• Open behavior type can not be null.
• The type "{openBehaviorType.Name}" must implement IPipelineB

AI-assisted analysis of LuckyPennySoftware/MediatR@916ef1b3d6 (2026-08-13). Data as JSON: /api/errors/cd8561d74470a6f5. Report an issue: GitHub.