Adding a typed HTTP client twice, throws exception

EDIT by @rynowak - see below for shiproom template.

Calling

services.AddHttpClient<SomeType>()

more than once throws an exception in 3.0 preview 7 - this used to work in 2.x.

Is that the intended behavior?

[rynowak]

Description

In 3.0 we added validation to HttpClient Factory’s registration code path to prevent some common mistakes. This new validation code blocks some valid use cases that we didn’t know about.

Customer Impact

Attempting to call AddHttpClient<T> twice with the same T throws an exception. This pattern is used when both library code and user code want to collaborate on configuration of a client type.

Regression?

Yes, this is a regression from 2.2

Risk

Low. This add special casing to our validation code path to allow more patterns. The impact of this is that cases that are explicitly blocked by an exception in 3.0 (but were allowed in 2.2) will be allowed again.

[photomoose]

We’ve just stumbled into this issue after upgrading our projects from 2.2 to 3.1. In 2.2, we were relying on the ability to call AddHttpClient<T>() subsequent times, e.g. in .ConfigureTestServices() to override an HttpClient’s message handler in tests.

           var hostBuilder = new HostBuilder()
                .ConfigureWebHost(builder =>
                {
                    builder.UseTestServer()
                           .UseStartup<Startup>()
                           .ConfigureTestServices(services => 
                           {
                                 services.AddHttpClient<IIdentityServiceClient, IdentityServiceClient>()
                 .ConfigurePrimaryHttpMessageHandler(s => _testIdentityServer.CreateHandler());
                           });
                 });