Example of generic IRequestHandler

Are there any examples to show how to make a generic IRequestHandler?

I can obviously add lots of interfaces such as

public class MainWindowViewModel : BindableBase
,IRequestHandler<MainViewModelPing1, string>
,IRequestHandler<MainViewModelPing2, string>
,IRequestHandler<MainViewModelPing3, SomeOtherClass>

but cannot find any examples of a generic handler to handle different requests and responses in the same handler. Is something like the following possible?

public class MainViewModelPing : IRequest<T>
    {
        public string Message { get; set; }
	public T Data { get; set; }
    }

which would handle something like the following

    public class MainViewModelPing : IRequest<string>
    {
        public string Message { get; set; }
    }

    public class MainViewModelPing : IRequest<string>
    {
        public string Message { get; set; }
	public SomeClass Data { get; set; }
    }

    public class MainViewModelPing : IRequest<SomeOtherClass>
    {
        public string Message { get; set; }
	public SomeClass Data { get; set; }
    }

[zachpainter77]

I have a similar issue… I was able to resolve it by using AutoFac container. But wished I could have solved it using the default container.

Here is what I did…

Request…

 public class AddNoteCommand<TEntity> : IRequest
        where TEntity : class, INoteEntity
{
    public int ParentId { get; set; }
    public int CurrentUserProfileId { get; set; }
    public string NoteContent { get; set; }
}

Handler

public class AddNoteCommandHandler<TEntity> : IRequestHandler<AddNoteCommand<TEntity>>
        where TEntity : class, INoteEntity, new()
{
    private readonly INoteRepository<TEntity> _repo;
    private readonly ITime _time;

    public AddNoteCommandHandler(INoteRepository<TEntity> repo, ITime time)
    {
        _repo = repo;
        _time = time;
    }
    public async Task<Unit> Handle(AddNoteCommand<TEntity> request, CancellationToken cancellationToken)
    {
        await _repo.Insert(new TEntity
        {
            CreatedByUserId = request.CurrentUserProfileId,
            CreatedDate = _time.Current,
            NoteContent = request.NoteContent,
            ParentId = request.ParentId                
        });

        await _repo.SaveChanges();

        return Unit.Value;
    }
}

Registration with AutoFac

public static ContainerBuilder AddGenericHandlers(this ContainerBuilder builder)
{
    builder.RegisterSource(new ContravariantRegistrationSource());    
    builder.RegisterGeneric(typeof(AddNoteCommandHandler<>)).AsImplementedInterfaces();
    return builder;
}

If anybody knows of a way to do this in the default asp.net core DI container then please show me!

[zachpainter77]

It is my understanding (and I tested this) that explicitly registering the GenericHandlerBase isn’t necessary… If you are going to create concrete handlers for each generic type for that handler then MediatR will find the correct handler without issue…

Here is an example of this:

Here is the registration using Default DI Container:

This works just fine… Now imagine if you had multiple entities, and further more, imagine if you had multiple service dependencies for your handler. While even though you would not have to implement the handle method for each derived handler you still need to create a concrete class that closes the generic type and pass all dependencies to the base constructor… In my opinion that is very tedious to have to do that for every entity and every service dependency… In my opinion it is much simpler to simply register the generic handler via Autofac like I showed above. But if not using a third party DI container then by all means create the concrete handlers… If I couldn’t use a third party di container for whatever reason then I would do it this way…

Cheers.