I've been building a set of libraries recently that make HTTP calls into the Force.com and Chatter REST APIs and I needed to find a way to test my methods and ensure that my HTTP Requests were constructed correctly. These libraries do a number of things to help make valid HTTP calls; any inadvertant changes (or introduced bugs) could cause a lot of problems. Fortunately, with help from Ryan Dunn, I was able to build some unit tests that are able to inspect my HttpRequestMessage an ensure that everything has been properly setup.

There are a few key pieces to solving this challenge:

  1. Accepting a callback in our class's constructor for a special HttpClient used by our unit tests.
  2. A custom DelegatingHandler that returns a mocked response for our special HttpClient.
  3. Using a lambda expression within the unit test to make our assertions.

Let me show you how it works. Or, if you simply want to go directly to the code, you can get it here on Github: https://github.com/wadewegner/Sample-UnitTestHttpClient

The Initial Sample

Let's take a relatively complex and real world example of a sample HTTP client.

We can easily call this client and method to get a response.

Pretty straightforward so far, right? You probably already seen some of the problems.

The Problems

This will work fine when we have some kind of server that is listening locally at port 1899. However, there are cases when this won't work so well. For example:

  1. Will this work when there's not an active server listening at port 1899?

  2. How do I create a test tyhat ensure that my User Agent is set properly?

  3. How do I check and ensure I'm attaching my authorization header properly?

  4. How do I check that the URL is getting formatted properly?

  5. How do I make sure that a developer (probably myself) doesn't accidentally mess this up in the future?

And so on. We need to find a way to test these things in our unit tests.

The Solution

Let's see how we can make a few changes to make this a lot easier to test.

First, we'll create a new local propery that we'll use for our HttpClient. Not that it's a callback. More on that in a moment.

We only plan to inject our own HttpClient into this class when we're running the unit tests. Let's update the existing constructor to simply create the HttpClient but then create a second constructor that we can use to inject our HttpClient when running our unit tests.

This gives us an overload we can use during our unit test.

We now have to update our HttpGet method to use our _builder property instead of newing up it's own HttpClient. Simply update the using statement.

Now it will use whichever client is defined when the class is constructed.

This is a great start and makes our class easier to test. Now let's look at what we have to do to write the unit test and create the delegating handler to give us a good response.

Let's first create our DelegatingHandler.

As you can see, it's pretty basic. The important thing to understand is that we're going to use this DelegatingHandler when we construct the HttpClient that we inject through our unit test. You'll see that I'm returning JsonContent as part of the response. You'll need the following class for this to work.

With all this in place, we're ready to write our unit test.

You'll see that we create our HttpClient using the SampleRouteHandler that we previously defined. We're also using a lambda expression to run all our unit test assertions. Once the client is created we assign it and pass it into the SampleHttpClient overload that takes the HttpClient injection.

Run your unit test. Put breakpoints inside the SampleRouteHandler and notice that you'll break into these assertions after the SampleHttpClient calls client.SendAsync(request). This is the perfect way to test and ensure that you've built your HttpRequestMessage properly.

You can find the full source code for this sample on Github here: https://github.com/wadewegner/Sample-UnitTestHttpClient

I hope this helps! Many thanks again to Ryan Dunn for his help!