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Mapping generic handler to generic query in MediatR

I am using [MediatR](https://github.com/jbogard/MediatR) in an ASP.NET Core 3.1 application and I want use a generic query and a generic request that deals with getting lists of some standard items I am using in drop-downs and similar:

    public class GetStandardListItemMapQuery<TEnt> : IRequest<IDictionary<int, StandardListItem>>
        where TEnt : class, IStandardListItem
    {
        public GetStandardListItemMapQuery()
        {
            
        }
    }

    public class GetStandardListItemMapQueryHandler<TEnt> : 
        IRequestHandler<GetStandardListItemMapQuery<TEnt>, IDictionary<int, StandardListItem>>
        where TEnt : class, IStandardListItem
    {
        private readonly IApplicationDbContext _context;

        public GetStandardListItemMapQueryHandler(IApplicationDbContext context)
        {
            _context = context;
        }

        public async Task<IDictionary<int, StandardListItem>> Handle(GetStandardListItemMapQuery<TEnt> request, CancellationToken cancellationToken)
        {
            var dbItems = _context.Set<TEnt>();
            var map = await dbItems
                .Select(e => new {e.Key, e.Value, e.IsEnabled})
                .ToDictionaryAsync(
                    e => e.Key, 
                    e => new StandardListItem { Key = e.Key, Value = e.Value, IsEnabled = e.IsEnabled }, cancellationToken);
            return map;
        }
    }

Registering the mapping between the query and handler like the following works, but it requires explicit call for any used type:

    private static void RegisterGenericHandler<TEnt>(IServiceCollection services) where TEnt : class, IStandardListItem
    {
        services.AddScoped(typeof(IRequestHandler<GetStandardListItemMapQuery<TEnt>, IDictionary<int, StandardListItem>>),
                typeof(GetStandardListItemMapQueryHandler<TEnt>));
    }


    private static void RegisterMediatR(IServiceCollection services)
    {
        // this does not handle the association for generic queries / handlers
        // (all code is in the same assembly)
        services.AddMediatR(Assembly.GetExecutingAssembly());

        RegisterGenericHandler<Foo>(services);
        RegisterGenericHandler<Bar>(services);
    }

I have tried the following, but it does not work as expected:

    services.AddScoped(typeof(IRequestHandler<GetStandardListItemMapQuery<IStandardListItem>, IDictionary<int, string>>), 
        typeof(GetStandardListItemMapQueryHandler<IStandardListItem>));


 > InvalidOperationException: Handler was not found for request of type MediatR.IRequestHandler`2 [PtbNext.Application.ApplicationUser.Queries.GetReferentials.GetStandardListItemMapQuery`1 [PtbNext.Domain.Entities.AppUser.Foo],System.Collections.Generic.IDictionary`2 [System.Int32,System.String]]. Register your handlers with the container. See the samples in GitHub for examples.