.NET Core 7 Minimal API MediatR IRequest Handler mapping error

I am currently getting the following error when executing a command via .NET Core 7 Minimal API:

 Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[1]
      An unhandled exception has occurred while executing the request.
      System.InvalidOperationException: Error constructing handler for request of type MediatR.IRequestHandler`2[mNet.FileServer.Commands.Core.Application.Features.
StoredFileTypes.NewStoredFileType.NewStoredFileTypeCommand,mNet.Common.Core.Application.Commands.BaseCommandResponse`1[mNet.FileServer.Shared.Core.Domain.StoredFile
Types.ValueObjects.StoredFileTypeId]]. Register your handlers with the container. See the samples in GitHub for examples.
       ---> System.InvalidOperationException: No service for type 'MediatR.IRequestHandler`2[mNet.FileServer.Commands.Core.Application.Features.StoredFileTypes.NewS
toredFileType.NewStoredFileTypeCommand,mNet.Common.Core.Application.Commands.BaseCommandResponse`1[mNet.FileServer.Shared.Core.Domain.StoredFileTypes.ValueObjects.S
toredFileTypeId]]' has been registered.
         at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService(IServiceProvider provider, Type serviceType)
         at MediatR.ServiceFactoryExtensions.GetInstance[T](ServiceFactory factory)
         at MediatR.Wrappers.HandlerBase.GetHandler[THandler](ServiceFactory factory)
         --- End of inner exception stack trace ---
         at MediatR.Wrappers.HandlerBase.GetHandler[THandler](ServiceFactory factory)
         at MediatR.Wrappers.RequestHandlerWrapperImpl`2.<>c__DisplayClass1_0.<Handle>g__Handler|0()
         at MediatR.Pipeline.RequestExceptionProcessorBehavior`2.Handle(TRequest request, RequestHandlerDelegate`1 next, CancellationToken cancellationToken)
         at MediatR.Pipeline.RequestExceptionProcessorBehavior`2.Handle(TRequest request, RequestHandlerDelegate`1 next, CancellationToken cancellationToken)
         at MediatR.Pipeline.RequestExceptionActionProcessorBehavior`2.Handle(TRequest request, RequestHandlerDelegate`1 next, CancellationToken cancellationToken)
         at MediatR.Pipeline.RequestExceptionActionProcessorBehavior`2.Handle(TRequest request, RequestHandlerDelegate`1 next, CancellationToken cancellationToken)
         at MediatR.Pipeline.RequestPostProcessorBehavior`2.Handle(TRequest request, RequestHandlerDelegate`1 next, CancellationToken cancellationToken)
         at MediatR.Pipeline.RequestPreProcessorBehavior`2.Handle(TRequest request, RequestHandlerDelegate`1 next, CancellationToken cancellationToken)
         at mNet.FileServer.Commands.Ui.MinimalApi.Endpoints.StoredFileTypeEndpoint.NewStoredFileTypeAsync(NewStoredFileTypeDto request, IMediator mediator) in C:\U
sers\dcmea\OneDrive\mNet Microservices\mNet.FileServer\mNet.FileServer.Commands.Ui.MinimalApi\Endpoints\StoredFileTypeEndpoint.cs:line 47
         at Microsoft.AspNetCore.Http.RequestDelegateFactory.<ExecuteTaskOfT>g__ExecuteAwaited|111_0[T](Task`1 task, HttpContext httpContext)
         at Microsoft.AspNetCore.Http.RequestDelegateFactory.<>c__DisplayClass89_2.<<HandleRequestBodyAndCompileRequestDelegateForJson>b__2>d.MoveNext()
      --- End of stack trace from previous location ---
         at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
         at Swashbuckle.AspNetCore.SwaggerUI.SwaggerUIMiddleware.Invoke(HttpContext httpContext)
         at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
         at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)

The key parts of the code are in the following projects and are as follows:

• mNet.Common.Core.Application: Contains the BaseCommand and BaseCommandResponse classes
• mNet.FileServer.Shared.Core.Domain: Contains the StoredFileTypeId value object
• mNet.FileServer.Commands.Core.Domain: Contains the StoredFileType aggregate
• mNet.FileServer.Commands.Core.Application: Contains the NewStoredFileTypeCommand and NewStoredFileTypeCommandHandler classes
• mNet.FileServer.Commands.Ui.MinimalApi: Contains the main Program.cs and minimal api endpoints

The key classes are:

Program.cs (extract)

builder.Services.AddMediatR(AppDomain.CurrentDomain.GetAssemblies());

Minimal Api Endpoint

private static async Task<BaseCommandResponse<StoredFileTypeId>> NewStoredFileTypeAsync(NewStoredFileTypeDto request, IMediator mediator)
    {
        var messageId = Guid.NewGuid();
        var command = new NewStoredFileTypeCommand
        {
            Id = new MessageId(messageId),
            CorrelationId = new CorrelationId(messageId),
            CausationId = new CausationId(messageId),
            CommandDto = request
        };

        var response = await mediator.Send(command); //Error happens after this MediatR call and is line 47 as referenced in the error
        return response;

    }

NewStoredFileTypeCommand

public class NewStoredFileTypeCommand : BaseCommand, IRequest<BaseCommandResponse<StoredFileTypeId>>
{
    public NewStoredFileTypeDto CommandDto { get; init; } = default!;
}

NewStoredFileTypeCommandHandler

public class NewStoredFileTypeCommandHandler : IRequestHandler<NewStoredFileTypeCommand, BaseCommandResponse<StoredFileTypeId>>
{
    private readonly IMapper _mapper;
    private readonly IEventSourcingHandler<StoredFileType, StoredFileTypeId> _eventSourcingHandler;

    public NewStoredFileTypeCommandHandler(IMapper mapper,
        IEventSourcingHandler<StoredFileType, StoredFileTypeId> eventSourcingHandler)
    {
        _mapper = mapper;
        _eventSourcingHandler = eventSourcingHandler;
    }

    public async Task<BaseCommandResponse<StoredFileTypeId>> Handle(NewStoredFileTypeCommand request, CancellationToken cancellationToken)
    {
       var response = new BaseCommandResponse<StoredFileTypeId>();

        var aggregate = new StoredFileType(
            new StoredFileTypeId(Guid.NewGuid()), 
            request.CorrelationId, 
            new CausationId(request.Id.Id),
            request.CommandDto.Name, 
            request.CommandDto.IsImageFileType,
            _mapper.Map<BootstrapIconCode>(request.CommandDto.BootstrapIconCode),
            _mapper.Map<MimeType>(request.CommandDto.MimeType)
            );
        
        await _eventSourcingHandler.SaveAsync(aggregate);

        response.Id = aggregate.Id;
        response.Message = $"Created new {nameof(StoredFileType)} aggregate!";
        response.IsSuccessful = true;

        return response;


    }

From doing a few debug traces, it seems like MediatR isn't mapping the NewStoredFileTypeCommandHandler to the NewStoredFileTypeCommand and I cannot work out why.

Also, as I have declared AppDomain.CurrentDomain.GetAssemblies() in Program.cs, it shouldn't be the scope.

Any ideas would be massively appreciated as it's driving me mad!

My guess relies on the following quote from the docs for AppDomain.GetAssemblies:

> Gets the assemblies that have been loaded into the execution context of this application domain.

There is a chance that the corresponding assembly has not yet been loaded into the app domain. Try using the AddMediatr and providing assemblies/types from the assemblies storing mediatr parts:

// use a type per assembly containing the MediatR components
builder.Services.AddMediatR(typeof(NewStoredFileTypeCommandHandler), typeof(BaseCommand), ...);