How is HttpClient injected into a ctor?

Question

I found this ctor:

public class MobileOtpService : IMobileOptService
{
    private readonly HttpClient _client;

    public MobileOtpService(HttpClient client) // How is this injected?
    {
        _client = client;
    }
}

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddScoped<IMobileOptService, MobileOtpService>();
        ...
        services.AddHttpClient();
    }
}

I can't find any hard-coded instantiations of MobileOtpService, so it must just be using DI.

How is the HttpClient injected?

Does this happen automatically because of services.AddHttpClient()?

I can't find any code that sets the BaseAddress etc.

Answer 1

There are many ways to register and use HttpClients. Let me list here the options:

Default client

This option can be used to get an HttpClient with default settings.

Registration

services.AddHttpClient();

Usage

private readonly HttpClient uniqueClient;
public XYZService(IHttpClientFactory clientFactory)
  => uniqueClient = clientFactory.CreateClient();

Named client

This option can be used to get a preset HttpClient.

Registration

services.AddHttpClient("UniqueName", client => client.BaseAdress = ...);

Usage

private readonly HttpClient uniqueClient;
public XYZService(IHttpClientFactory clientFactory)
  => uniqueClient = clientFactory.CreateClient("UniqueName");

Typed client

This option can be used to wrap a default/preset HttpClient into a higher-level abstraction.

Registration

services.AddHttpClient<UniqueClient>();
services.AddHttpClient<IUniqueClient, UniqueClient>(client => client.BaseAdress = ...);

Usage

private readonly HttpClient httpClient;
public UniqueClient(HttpClient client)
  => httpClient = client;

private readonly IUniqueClient uniqueClient;
public XYZService(IUniqueClient client)
  => uniqueClient = client;

Named and Typed client

This option can be used to register different preset HttpClients and wrap them into a higher-level abstraction.

Registration

services.AddHttpClient<IUniqueClient, UniqueClient>("primary", client => client.BaseAdress = ...);
services.AddHttpClient<IUniqueClient, UniqueClient>("secondary", client => client.BaseAdress = ...);

Usage

private readonly HttpClient httpClient;
public UniqueClient(HttpClient client)
  => httpClient = client;

public XYZService(
   IHttpClientFactory namedClientFactory, 
   ITypedHttpClientFactory<UniqueClient> namedTypedClientFactory)
{
    var namedClient = namedClientFactory.CreateClient("primary");
    var namedTypedClient = namedTypedClientFactory.CreateClient(namedClient);
}

If you are interested about their lifecycle management then please check out this SO topic.