LuckyPennySoftware/MediatR · critical · InvalidOperationException

Could not create wrapper type for {requestType}

Error message

Could not create wrapper type for {requestType}

What it means

In the strongly-typed Send<TResponse>(IRequest<TResponse>) path, MediatR builds and caches a RequestHandlerWrapperImpl<TRequest,TResponse> via reflection. MakeGenericType constructs the closed wrapper type and Activator.CreateInstance instantiates it; the null-coalescing throw fires only if Activator returns null, which for these internal parameterless wrapper types should never happen under normal runtime conditions.

Source

Thrown at src/MediatR/Mediator.cs:60

        _serviceProvider.CheckLicense();
    }

    /// <summary>
    /// Gets or sets the license key. You can find your license key in your <a href="https://luckypennysoftware.com/account">account</a>.
    /// </summary>
    public static string? LicenseKey { get; set; }

    public Task<TResponse> Send<TResponse>(IRequest<TResponse> request, CancellationToken cancellationToken = default)
    {
        if (request == null)
        {
            throw new ArgumentNullException(nameof(request));
        }

        var handler = (RequestHandlerWrapper<TResponse>)_requestHandlers.GetOrAdd(request.GetType(), static requestType =>
        {
            var wrapperType = typeof(RequestHandlerWrapperImpl<,>).MakeGenericType(requestType, typeof(TResponse));
            var wrapper = Activator.CreateInstance(wrapperType) ?? throw new InvalidOperationException($"Could not create wrapper type for {requestType}");
            return (RequestHandlerBase)wrapper;
        });

        return handler.Handle(request, _serviceProvider, cancellationToken);
    }

    public Task Send<TRequest>(TRequest request, CancellationToken cancellationToken = default)
        where TRequest : IRequest
    {
        if (request == null)
        {
            throw new ArgumentNullException(nameof(request));
        }

        var handler = (RequestHandlerWrapper)_requestHandlers.GetOrAdd(request.GetType(), static requestType =>
        {
            var wrapperType = typeof(RequestHandlerWrapperImpl<>).MakeGenericType(requestType);
            var wrapper = Activator.CreateInstance(wrapperType) ?? throw new InvalidOperationException($"Could not create wrapper type for {requestType}");

View on GitHub (pinned to 916ef1b3d6)

Solutions

  1. Confirm you are using an unmodified, released MediatR assembly from NuGet.
  2. If running under trimming/AOT, ensure MediatR's wrapper types are preserved (dynamic registration requires reflection support).
  3. If running in partial trust, grant ReflectionPermission / use a full-trust host.
Defensive patterns

Strategy: try-catch

Try / catch

try { return await mediator.Send<TResponse>(request, ct); }
catch (InvalidOperationException ex) when (ex.Message.Contains("Could not create wrapper type"))
{ /* internal-invariant: report assembly/trimming/AOT health, do not retry unchanged */ throw; }

Prevention

When it happens

Trigger: Reached only if Activator.CreateInstance returns null for the closed RequestHandlerWrapperImpl<,> type. Practically this implies a type-loading or reflection-permission failure, or a corrupted MediatR build, since the wrapper types are internal MediatR types with public parameterless constructors.

Common situations: Essentially an internal-invariant failure: partial-trust / reflection-restricted environments, a custom MediatR build where wrapper constructors were removed, or AOT/trimming scenarios where the wrapper type was trimmed away.

Related errors

AI-assisted analysis of LuckyPennySoftware/MediatR@916ef1b3d6 (2026-08-13). Data as JSON: /api/errors/4c494ea2dc6377be. Report an issue: GitHub.