One of the nice things about the new HttpClient in System.Net.Http is the support for mocking responses or handling requests in a http server hosted in-memory. While the first option is useful for scenarios in which we want to test our client code in isolation (unit tests for example), the second one enables more complete integration testing scenarios that could include some more components in the stack such as model binders or message handlers for example.  

The HttpClient can receive a HttpMessageHandler as argument in one of its constructors.

public class HttpClient : HttpMessageInvoker
{
   public HttpClient();
   public HttpClient(HttpMessageHandler handler);
   public HttpClient(HttpMessageHandler handler, bool disposeHandler);

For the first scenario, you can create a new HttpMessageHandler that fakes the response, which you can use in your unit test. The only requirement is that you somehow inject an HttpClient with this custom handler in the client code.

public class FakeHttpMessageHandler : HttpMessageHandler
{
  HttpResponseMessage response;

  public FakeHttpMessageHandler(HttpResponseMessage response)
  {
       this.response = response;
   }

   protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, 
System.Threading.CancellationToken cancellationToken)
   {
      var tcs = new TaskCompletionSource<HttpResponseMessage>();
            
      tcs.SetResult(response);

      return tcs.Task;
    }
}

In an unit test, you can do something like this.

var fakeResponse = new HttpResponse();
var fakeHandler = new FakeHttpMessageHandler(fakeResponse);
var httpClient = new HttpClient(fakeHandler);

var customerService = new CustomerService(httpClient);

// Do something

// Asserts

CustomerService in this case is the class under test, and the one that receives an HttpClient initialized with our fake handler.

For the second scenario in integration tests, there is a In-Memory host “System.Web.Http.HttpServer” that also derives from HttpMessageHandler and you can use with a HttpClient instance in your test. This has been discussed already in these two great posts from Pedro and Filip.