Easy HttpClient Testing with Goldlight.HttpClientTestSupport

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Scenario

We have a series of unit tests and, in them, we want to test a request from HttpClient but, in the spirit of true isolated tests, we don't want to actually perform the HTTP requests. We need some means of replacing the real call to the end point with a fake endpoint in its place.

The problem in this scenario is that there is no apparently simple way to mock HttpClient. There is no IHttpClient interface available to us, that will allow us to set up a mock and the calls we are interested aren't virtual so mocking them is difficult in most mock frameworks. With this apparent limitation in mind, how can we easily mock calls such as PostAsync?

To answer this question, we need to understand how HttpClient actually manages HTTP requests. If we look at the source code for GetAsync in the reference source, we see that the method calls out to SendAsync.

public Task<HttpResponseMessage> GetAsync
       (Uri requestUri, HttpCompletionOption completionOption,
  CancellationToken cancellationToken)
{
  return SendAsync(new HttpRequestMessage(HttpMethod.Get, requestUri), 
                   completionOption, cancellationToken);
}

HttpClient inherits from a class called HttpMessageInvoker which has SendAsync as a virtual method inside it. The HttpMessageInvoker class requires posting an HttpMessageHandler instance it via a constructor. This is important to because SendAsync calls out to the HttpMessageHandler.SendAsync method and this is the "touch point" that we want to interact with to mock our REST call.

Faking the Message Handler

An issue that we have to cope with, with regards to testing the message handler is the problem that the virtual SendAsync method is protected and most mocking frameworks are unable to mock virtual methods. While Moq provides the ability to mock protected properties, if we are using a framework like FakeItEasy, then we can't directly interact with the protected method.

All of this detective work has told us that, in order to replace our calls, we need to inherit from HttpMessageHandler and provide our own implementation that satisfies the ability to simulate web requests.

Rather than making a mockable object, we are going to provide a FakeHttpMessageHandler that allows us to control what the response we receive back contains.

Installing Goldlight.HttpClientTestSupport

Goldlight.HttpClientTestSupport is available as a .NET Standard 2.0 and .NET Standard 2.1 package on NuGet, and can be installed using:

Install-Package Goldlight.HttpClientTestSupport

Using the dotnet command line:

dotnet add package Goldlight.HttpClientTestSupport

Basic usage

The FakeHttpMessageHandler implementation makes certain assumptions when it is instantiated. Instantiation sets the expectation that our response message will be set to version 1.0 and the status code defaults to 200. We have the ability to override these values if we need to using a fluent API.

We are going to start with a simple xUnit test. In all of the examples we are going to see here, we are using xUnit but any testing framework will do. Our examples will also avoid using any mocking framework to show how we can easily test our code.

[Fact]
public async Task EnsureOkStatusIsReturned()
{
  HttpClient httpClient = new HttpClient(new FakeHttpMessageHandler());
  HttpResponseMessage response = await httpClient.GetAsync("https://mydummy.url/");
  Assert.Equals(HttpStatusCode.OK, response.StatusCode);
}

Dealing With Response Content

Obviously, when we are dealing with a GET call, we really want to see some content coming back. Let's see how we can set a test up to deal with this.

First, we want to create a model that we are going to return.

public sealed class SampleModel
{
  public string FirstName => "Stan";
  public string LastName => "Lee";
  public string FullName => FirstName + " " + LastName;
}

With this class in place, what might our test look like? In this test, we see our fluent API in action setting up our fake handler with our model as the expected response content.

[Fact]
public async Task 
  GivenValidRequestWithModelContentExpected_WhenGetIsCalled_ThenContentIsSetToModel()
{
  FakeHttpMessageHandler fake = 
      new FakeHttpMessageHandler().WithExpectedContent(new SampleModel());
  HttpClient httpClient = new HttpClient(fake);
  HttpResponseMessage responseMessage = 
      await httpClient.GetAsync("https://dummyaddress.com/someapi");
  SampleModel converted =
    JsonConvert.DeserializeObject<SampleModel>
                (await responseMessage.Content.ReadAsStringAsync());
  Assert.Equal("Stan Lee", converted.FullName);
}

Returning Different Status Codes

With GET calls, we should probably test to see what happens in our code when we get a 404 NotFound status. The question is, how would we set this up?

We are going to start by creating an example that is a little bit more realistic. We want to see our faked message handler in action. The best way to see that is to see the handler in use outside of a unit test. Let's create a class that might be called by a web controller.

public class ExampleControllerHandling
{
  private readonly HttpClient _httpClient;
  private const string BaseUrl = "http://www.goldlight-dummy.com/api/sample/";
  public ExampleControllerHandling(HttpClient httpClient)
  {
    _httpClient = httpClient;
  }

  public async Task<SampleModel> GetById(Guid id)
  {
    HttpResponseMessage response = await _httpClient.GetAsync(BaseUrl + id);
    switch (response.StatusCode)
    {
      case HttpStatusCode.OK:
        return JsonConvert.DeserializeObject<SampleModel>
               (await response.Content.ReadAsStringAsync());
      case HttpStatusCode.BadRequest:
        throw new Exception("Unable to find " + id);
    }
    return null;
  }
}

Let's create a test that exercises the BadRequest path through our code.

[Fact]
public async Task 
  GivenComplexController_WhenBadRequestIsExpected_ThenBadRequestIsHandled()
{
  FakeHttpMessageHandler fake = 
      new FakeHttpMessageHandler().WithStatusCode(HttpStatusCode.BadRequest);
  HttpClient httpClient = new HttpClient(fake);
  ExampleControllerHandling exampleController = 
                            new ExampleControllerHandling(httpClient);
  int called = 0;
  try
  {
    await exampleController.GetById(Guid.NewGuid());
  }
  catch (Exception e)
  {
    if (e.Message.StartsWith("Unable to find "))
    {
      called++;
    }
  }

  Assert.Equal(1, called);
}

As we saw in the code sample, the expected status code is added to our handler using the WithStatusCode call.

Handling Headers

It is not uncommon for our code to have to deal with response headers. To see how this works in practice, let's add a new method to the ExampleControllerHandling sample to return a list of all values when the status code is 200 and we have a response header called order66 that contains the value babyyoda.

public async Task<IEnumerable<SampleModel>> GetAll()
{
  HttpResponseMessage response = await _httpClient.GetAsync(BaseUrl);
  if (response.StatusCode == HttpStatusCode.OK && response.Headers.TryGetValues
                             ("order66", out IEnumerable<string> headers))
  {
    if (headers.First() == "babyyoda")
    {
      return JsonConvert.DeserializeObject<IEnumerable<SampleModel>>
                         (await response.Content.ReadAsStringAsync());
    }
  }

  return null;
}

To add the header, we are going to use WithResponseHeader to add a key/value pair as a header with our response. As we know that our message handler can be built with a fluent API, we are going to add multiple parts with one chain.

[Fact]
public async Task GivenMultipleInputsIntoController_WhenProcessing_ThenModelIsReturned()
{
  List<SampleModel> sample = new List<SampleModel>() 
                             {new SampleModel(), new SampleModel()};
  FakeHttpMessageHandler fake = 
          new FakeHttpMessageHandler().WithStatusCode(HttpStatusCode.OK)
    .WithResponseHeader("order66", "babyyoda").WithExpectedContent(sample);
  HttpClient httpClient = new HttpClient(fake);
  ExampleControllerHandling exampleController = 
                            new ExampleControllerHandling(httpClient);
  IEnumerable<SampleModel> output = await exampleController.GetAll();
  Assert.Equal(2, output.Count());
}

We can see in this sample that we are explicitly setting the status code (okay, while it's the default, I wanted to demonstrate adding multiple parts in one go) and the response header that we expect to trigger our response.

As well as being able to set a single value for a header, there is an override that accepts an array of values.

Trailing Headers

In the same way we can set a header in a response, NetStandard2.1 gives us the ability to add trailing headers in a response. This feature is only available in targets that support .NetStandard2.1 such as .NET Core 3.1 and .NET 5. Use WithTrailingResponseHeader to add the trailing headers.

Setting Version Information

If we need to set version numbers for our REST calls, we have WithVersion to provide version information.

[Fact]
public async Task GivenValidRequestWithCustomVersion_WhenPostIsCalled_ThenCustomVersionIsSet()
{
  FakeHttpMessageHandler fake = 
          new FakeHttpMessageHandler().WithVersion(new Version(2, 1));
  HttpClient httpClient = new HttpClient(fake);
  HttpResponseMessage response = await httpClient.PostWrapperAsync("MyContent");
  Assert.Equal(response.Version, new Version(2, 1));
}

Extending the Calls With Your Own Tests

In some instances, we may want to perform additional testing that is hard to predict. Suppose we want to verify how many times we have called HttpClient, we need some mechanism to perform this. We could provide an InvocationCount property, but this means we are trying to predict the different ways the API will be used. Instead of doing this, we have opted to provide the ability to add pre and post invocation actions. The pre handler is called as the first operation in the SendAsync method and the post handler is called at just before the return. The first example here demonstrates adding the ability to count how many times the method was called using the WithPreRequest method.

[Fact]
public async Task GivenPreActionForController_WhenProcessing_ThenActionIsPerformed()
{
  int invocationCount = 0;
  List<SampleModel> sample = new List<SampleModel>() 
                             { new SampleModel(), new SampleModel() };
  FakeHttpMessageHandler fake = 
          new FakeHttpMessageHandler().WithPreRequest(() => invocationCount++)
    .WithExpectedContent(sample);
  HttpClient httpClient = new HttpClient(fake);
  ExampleControllerHandling exampleController = 
                            new ExampleControllerHandling(httpClient);
  IEnumerable<SampleModel> output = await exampleController.GetAll();
  Assert.Equal(1, invocationCount);
}

We can perform exactly the same invocation calculation using the WithPostRequest method.

[Fact]
public async Task GivenPostActionForController_WhenProcessing_ThenActionIsPerformed()
{
  int invocationCount = 0;
  List<SampleModel> sample = new List<SampleModel>() 
                             { new SampleModel(), new SampleModel() };
  FakeHttpMessageHandler fake = 
          new FakeHttpMessageHandler().WithPostRequest(() => invocationCount++)
    .WithExpectedContent(sample);
  HttpClient httpClient = new HttpClient(fake);
  ExampleControllerHandling exampleController = 
                   new ExampleControllerHandling(httpClient);
  IEnumerable<SampleModel> output = await exampleController.GetAll();
  Assert.Equal(1, invocationCount);
}

We can have multiple pre and post request actions. In the following example, we are deliberately going to throw an exception from our pre handler, verify that the pre handler was performed and check to ensure that the post handler was not reached because of the exception.

[Fact]
public async Task GivenPreAndPostActionForController_WhenProcessing_ThenActionIsPerformed()
{
  int invocationCount = 0;
  int postInvocationCount = 0;
  List<SampleModel> sample = new List<SampleModel>() 
                             { new SampleModel(), new SampleModel() };
  FakeHttpMessageHandler fake = new FakeHttpMessageHandler().WithPreRequest(() => 
                                                                  invocationCount++)
    .WithPreRequest(() => throw new Exception
                          ("Throwing deliberately"))
    .WithPostRequest(() => postInvocationCount++)
    .WithExpectedContent(sample);
  HttpClient httpClient = new HttpClient(fake);
  ExampleControllerHandling exampleController = new ExampleControllerHandling(httpClient);
  IEnumerable<SampleModel> output = await exampleController.GetAll();
  Assert.Equal(1, invocationCount);
  Assert.Equal(0, postInvocationCount);
}