I’m trying to set up a template for myself using .NET 8, PostgreSQL as database and CQRS pattern (with MediatR library).

I’ve done all the configuration, but when I try a simple QueryHandler, this is the error I get:

System.InvalidOperationException: The exception handler configured on ExceptionHandlerOptions produced a 404 status response. This InvalidOperationException containing the original exception was thrown since this is often due to a misconfigured ExceptionHandlingPath. If the exception handler is expected to return 404 status responses then set AllowStatusCode404Response to true.
 ---> System.InvalidOperationException: Cannot resolve 'MediatR.IRequestHandler`2[Application.Features.Todo.Queries.GetAllTodos.GetAllTodosQuery,System.Collections.Generic.List`1[Application.Dtos.Todo.TodoDto]]' from root provider because it requires scoped service 'Infrastructure.Data.ApplicationDbContext'.
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteValidator.ValidateResolution(ServiceCallSite callSite, IServiceScope scope, IServiceScope rootScope)
   at Microsoft.Extensions.DependencyInjection.ServiceProvider.GetService(ServiceIdentifier serviceIdentifier, ServiceProviderEngineScope serviceProviderEngineScope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.GetService(Type serviceType)
   at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService(IServiceProvider provider, Type serviceType)
   at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService[T](IServiceProvider provider)
   at MediatR.Wrappers.RequestHandlerWrapperImpl`2.<>c__DisplayClass1_0.<Handle>g__Handler|0()
   at MediatR.Wrappers.RequestHandlerWrapperImpl`2.Handle(IRequest`1 request, IServiceProvider serviceProvider, CancellationToken cancellationToken)
   at MediatR.Mediator.Send[TResponse](IRequest`1 request, CancellationToken cancellationToken)
   at Presentation.Endpoints.Todo.TodoEndpoints.<AddRoutes>b__3_0() in /Users/manuelraso/Documents/repo/wanderlust-nest/backend/src/Presentation/Endpoints/Todo/TodoEndpoints.cs:line 23
   at Microsoft.AspNetCore.Http.RequestDelegateFactory.<ExecuteTaskOfT>g__ExecuteAwaited|133_0[T](Task`1 task, HttpContext httpContext, JsonTypeInfo`1 jsonTypeInfo)
   at Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddlewareImpl.<Invoke>g__Awaited|10_0(ExceptionHandlerMiddlewareImpl middleware, HttpContext context, Task task)
   --- End of inner exception stack trace ---
   at Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddlewareImpl.HandleException(HttpContext context, ExceptionDispatchInfo edi)
   at Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddlewareImpl.<Invoke>g__Awaited|10_0(ExceptionHandlerMiddlewareImpl middleware, HttpContext context, Task task)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)

This is how I configured the DbContext (I tried with ServiceLifetime.Transient too):

var connectionString = configuration["ConnectionStrings:DefaultConnection"];
services.AddDbContext<ApplicationDbContext>(
    options => options.UseNpgsql(connectionString),
    ServiceLifetime.Scoped);

This is how I register MediatR:

    services.AddMediatR(cfg =>
    {
        cfg.RegisterServicesFromAssemblies(
        [
            Assembly.GetExecutingAssembly(), // Assembly Application
        ]);
    });

This is the QueryHandler:

internal sealed class GetAllTodosQueryHandler: IRequestHandler<GetAllTodosQuery, List<TodoDto>>
{
    private readonly ApplicationDbContext _context;

public GetAllTodosQueryHandler(ApplicationDbContext context)
{
    _context = context;
}

async Task<List<TodoDto>> IRequestHandler<GetAllTodosQuery, List<TodoDto>>.Handle(GetAllTodosQuery request, CancellationToken cancellationToken)
{
    var test = await _context.Todos.ToListAsync(cancellationToken);

    return await Task.FromResult(test.Adapt<List<TodoDto>>());
}
}

This is how I send the query (I tried both injecting ISender and creating a scope of MediatR, but same error):

public class TodoEndpoints : CarterModule
{
    private readonly ISender _sender;
    private readonly IServiceScopeFactory _serviceScopeFactory;
    public TodoEndpoints(ISender sender, IServiceScopeFactory serviceScopeFactory)
        : base("/todo")
    {
        _sender = sender;
        _serviceScopeFactory = serviceScopeFactory;
    }

    public override void AddRoutes(IEndpointRouteBuilder app)
    {
        app.MapGet("/", async () => await _sender.Send(new GetAllTodosQuery()));
        app.MapGet("/test", async () =>
        {
            using var scope = _serviceScopeFactory.CreateScope();
            var mediator = scope.ServiceProvider.GetRequiredService<IMediator>();
            return await mediator.Send(new GetAllTodosQuery());
        });
    }
}