Cannot inject HttpClient in typed client while using with IMediatR library

Question

According to examples provided by ASP.NET Core 2.2 documentation in MSDN, it is possible to inject HttpClient to a typed clients (service-classes) by adding the following line to Startup.cs:

// Startup.cs
services.AddHttpClient<GitHubService>();

From controller class it will look like (from now I will use GitHub as a simplification for a domain model):

// GitHubController.cs
public class GitHubController : Controller
{
    private readonly GitHubService _service;
    public GitHubController(GitHubService service)
    {
        _service = service;
    }
}

However, I use MediatR library in my project, so my project structure looks a bit different. I have 2 projects - GitHubFun.Api, GitHubFun.Core - ASP.NET Core 2.2 API project and .NET Core 2.2 class library respectively.

My controller:

// GitHubController.cs
public class GitHubController : Controller
{
    private readonly IMediator _mediator;
    public GitHubController(IMediator mediator)
    {
        _mediator= mediator;
    }

    public async Task<IActionResult> GetGitHubRepositoryInfo(
        GetGitHubRepositoryCommand command)
    {
         _mediator.Send(command);
    }
}

And my handler class:

// GetGitHubRepositoryHandler.cs
public class GetGitHubRepositoryHandler : 
    IRequestHandler<GetGitHubRepositoryCommand , GetGitHubRepositoryCommandResult>
{
    private HttpClient _httpClient;

    public GetGitHubRepositoryHandler(HttpClient httpClient)
    {
        _httpClient = httpClient;
    }
}

When I make HTTP request and call an API method, it successfully injects IMediator, but throws an exception on _mediator.Send(command) line.

Exception body:

System.InvalidOperationException: Error constructing handler for request of type MediatR.IRequestHandler`2[IDocs.CryptoServer.Core.Commands.ExtractX509Command,IDocs.CryptoServer.Core.Commands.ExtractX509CommandResult]. Register your handlers with the container. See the samples in GitHub for examples. ---> System.InvalidOperationException: Unable to resolve service for type 'System.Net.Http.HttpClient' while attempting to activate 'IDocs.CryptoServer.Core.Handlers.ExtractX509CommandHandler'

(ExtractX509CommandHandler - is just a real domain model, instead of GetGitHubRepositoryHandler).

It seems that ASP.NET Core DI cannot resolve DI and inject HttpClient to handler.

My Startup.cs has the following lines:

services.AddHttpClient<ExtractX509CommandHandler>();
services.AddMediatR(
       typeof(Startup).Assembly, 
       typeof(ExtractX509CommandHandler).Assembly);

Sounds like it wants you to register the handler itself (i.e. `GetGitHubRepositoryHandler`). Have you done that? — Chris Pratt, Mar 18 '19 at 13:31
Or actually, it may be an issue with `ExtractX509CommandHander`. Does that take `HttpClient` as dependency? If so, did you register a typed client for that? — Chris Pratt, Mar 18 '19 at 13:33
@ChrisPratt, yes, I've already registered HttpClient as dependency. And services.AddMediatR( typeof(Startup).Assembly, typeof(ExtractX509CommandHandler).Assembly) encapsulates registration for ExtractX509CommandHandler too. — Iskander Raimbaev, Mar 18 '19 at 13:36
No. You misunderstand. If `ExtractX509CommandHandler` takes `HttpClient` as dependency, then you need `services.AddHttpClient()` as well. — Chris Pratt, Mar 18 '19 at 13:37
@ChrisPratt, yeah, I've already added services.AddHttpClient(). The line services.AddHttpClient() in my question is just a simplification of this class I made for easy understanding. — Iskander Raimbaev, Mar 18 '19 at 13:41

score 5 · Accepted Answer · answered Mar 18 '19 at 13:34

5

I found a solution. For some reasons, in this case we need to pass IHttpClientFactory from Microsoft.Extensions.Http.dll instead of HttpClient to handler class. I just changed one line, it was:

public GetGitHubRepositoryHandler(HttpClient httpClient)

and now:

public GetGitHubRepositoryHandler(IHttpClientFactory httpClientFactory)

and now it works as it should. I don't know why it works, so it will be perfect, if someone could explain what is the difference between injecting IHttpClientFactory and HttpClient to the class.

answered Mar 18 '19 at 13:34

Iskander Raimbaev

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Did you find a reason for this and what the difference is? – Diederik Jan 15 '20 at 13:18
AddHttpClient registers your service T at the same time. In this case, AddMediatR also does that, so now you get registrations mixed up. – ESG Oct 30 '20 at 15:54

pdouelle · Answer 2 · 2021-04-14T10:16:04.540

I found a solution.

I'm in ASP.NET Core 3.1.

My handler class :

public class GetProductsListQueryHandler : IRequestHandler<GetProductsListQueryModel, IEnumerable<Product>>
{
    private readonly HttpClient _httpClient;

    public GetProductsListQueryHandler(HttpClient httpClient)
    {
        _httpClient = httpClient;
    }

You need to implement your HttpClient in your Startup like this :

services.AddHttpClient<IRequestHandler<GetProductsListQueryModel,IEnumerable<Product>>, GetProductsListQueryHandler>();

And it works! ;)

Cannot inject HttpClient in typed client while using with IMediatR library

2 Answers2