Testing HTTP dependencies in modern web applications is a common problem, but it’s also something that can create difficulty for authoring reliable tests.

Today, we’re open-sourcing a library to help reduce the friction many developers have with this common requirement: JustEat.HttpClientInterception.

You can find the repository in our GitHub organisation and can find the package available to download at JustEat.HttpClientInterception on

The Problem

Many modern software applications integrate with external Application Programming Interfaces (APIs) to provide solutions for problems within their domain of responsibility, whether it be delivering your night in, booking a flight, trading financial instruments, or monitoring transport infrastructure in real-time.

These APIs are very often HTTP-based, with RESTful APIs that consume and produce JSON often being the implementation of choice. Of course integrations might not be with full-blown APIs, but just external resources that are available over HTTP, such as a website exposing HTML, or a file download portal.

In .NET applications, whether these are console-based, rich GUIs, background services, or ASP.NET web apps, a common go-to way of consuming such HTTP-based services from code is using the HttpClient class.

HttpClient provides a simple API surface for GET-ing and POST-ing resources over HTTP(S) to external services in many different data formats, as well as functionality for reading and writing HTTP headers and more advanced extensibility capabilities.

It is also commonly exposed as a dependency that can be injected into other services, such as third-party dependencies for tasks such as implementing OAuth-based authentication.

Overall this makes HttpClient a common and appropriate choice for writing HTTP-based integrations for .NET applications.

An important part of software development is not just implementing a solution to a given problem, but also writing tests for your applications. A good test suite helps ensure that delivered software is of a high quality, is functionally correct, is resilient in the face of failure, and provides a safety net against regression for future work.

When your application depends on external resources though, then testing becomes a bit more involved. You don’t want to have the code under test making network calls to these external services for a myriad of reasons. They make your tests brittle and hard to maintain, require a network connection to be able to run successfully, might cost you money, and slow down your test suite, to name but a few examples.

These issues lead to approaches using things like mocks and stubs. HttpClient, and its more low-level counterpart HttpMessageHandler, are not simple to mock however. While not difficult to do, their lack of interface and design lead to a requirement to implement classes that derive from HttpMessageHandler in order to override protected members to drive test scenarios, and to build non-primitive types by hand, such as HttpResponseMessage.

Another approach that can be used to simplify the ability to use mocks is to create your own custom IHttpClient interface and wrap your usage of HttpClient within an implementation of this interface. This creates its own problems in non-trivial integrations though, with the interface often swelling to the point of being a one-to-one representation of HttpClient itself to expose enough functionality for your use-cases.

While this mocking and wrapping is feasible, once your application does more than one or two simple interactions with an HTTP-based service, the amount of test code required to drive your test scenarios can balloon quite quickly and become a burden to maintain.

It is also an approach that only works in a typical unit test approach. As usage of HttpClient is typically fairly low down in your application’s stack, this does not make it a viable solution for other test types, such as functional and integration tests.

A Solution

Today we’re publishing a way of solving some of these problems by releasing our JustEat.HttpClientInterception .NET library as open-source to our organisation in under the Apache 2.0 licence.

A compiled version of the .NET assembly is also available from JustEat.HttpClientInterception on that supports .NET Standard 1.3 (and later) and .NET Framework 4.6.1 (and later).

JustEat.HttpClientInterception provides a number of types that allow HTTP requests and their corresponding responses to be declared using the builder pattern to register interceptions for HTTP requests in your code to bypass the network and return responses that drive your test scenarios.

Below is a simple example that shows registering an interception for an HTTP GET request to the Just Eat Public API:

The library provides a strongly-typed API that supports easily setting up interceptions for arbitrary HTTP requests to any URL and for any HTTP verb, returning responses that consist of either raw bytes, strings or objects that are serialized into a response as-and-when they are required.

Fault injection is also supported by allowing arbitrary HTTP codes to be set for intercepted responses, as well as latency injection via an ability to specify a custom asynchronous call-back that is invoked before the intercepted response is made available to the code under test.

With ASP.NET Core adding Dependency Injection as a first-class feature and being easy to self-host for use within test projects, a small number of changes to your production code allows HttpClientInterceptorOptions to be injected into your application’s dependencies for use with integration tests without your application needing to take a dependency on JustEat.HttpClientInterception for itself.

With the library injected into the application, HTTP requests using HttpClient and/or HttpMessageHandler that are resolved by your IoC container of choice can be inspected and intercepted as-required before any network connections are made. You can also opt-in to behaviour that throws an exception for any un-intercepted requests, allowing you to flush out all HTTP requests made by your application from your tests.

Further examples of using the library can be found at these links:

The Benefits

We’ve used this library successfully with two internal applications we’re developing with ASP.NET Core (one an API, the other an MVC website) to really simplify our tests, and provide good code coverage, by using a test approach that is primarily a black-box approach.

The applications’ test suites self-host the application using Kestrel, with the service registration set-up to create a chain of DelegatingHandler implementations when resolving instances of HttpClient and HttpMessageHandler. With HttpClientInterceptorOptions registered to provide instances of DelegatingHandler by the test start-up code when the application is self-hosted, this allows all HTTP calls within the self-hosted application in the tests to be intercepted to drive the tests.

The tests themselves then either initiate HTTP calls to the public surface of the self-hosted server with a vanilla HttpClient in the case of the API, or use Selenium to test the rendered pages using browser automation in the case of the website.

This approach provides many benefits, such as:

  • Simple setup for testing positive and negative code paths for HTTP responses, such as for error handling.
  • Exercises serialization and deserialization code for HTTP request and response bodies.
  • Testing behaviour in degraded scenarios, such as network latency, for handling of timeouts.
  • Removes dependencies on external services for the tests to pass and the need to have access to an active network connection for services that may only be resolvable on a internal/private network.
  • No administrative permissions required to set-up port bindings.
  • Speeds up test execution by removing IO-bound network operations.
  • Allows you to skip set-up steps to create test data for CRUD operations, such as having to create resources to test their deletion.
  • Can be integrated in a way that other delegating handlers your application may use are still exercised and tested implicitly.
  • Allows us to intercept calls to IdentityServer for our user authentication and issue valid self-signed JSON Web Tokens (JWTs) in the tests to authenticate browser calls in Selenium tests.

In the case of the ASP.NET Core API using this test approach, at the time of writing, we’ve been able to achieve over 90% statement coverage of a several thousand line application with just over 200 unit, integration and end-to-end tests. Using our TeamCity server, the build installs the .NET Core runtime, restores its dependencies from NuGet, compiles all the code and runs all the tests in just over three-and-a-half minutes.

Some Caveats

Of course such a solution is not a silver bullet. Intercepting all of your HTTP dependencies isolates you from interface changes in your dependencies.

If an external service changes its interfaces, such as by adding a new API version or deprecating the one you use, adds new fields to the responses, or changes to require all traffic to support HTTPS instead of HTTP, your integration tests will not find such changes. It also does not validate that your application integrates correctly with APIs that require authentication or apply rate-limits.

Similarly, the black-box approach is relatively heavyweight compared to a simple unit test, so may not be suited to testing all of the edge cases in your code and low-level assertions on your responses.

Finally, your intercepted responses will only cater for the behaviour you’ve seen and catered-for within your tests. A real external dependency may change its behaviour over time in ways that your static simulated behaviours will not necessarily emulate.

A good mixture of unit, interception-based integration tests, and end-to-end tests against your real dependencies are needed to give you a good robust test suite that runs quickly and also gives you confidence in your changes as you develop your application over time. Shipping little and often is a key tenet of Continuous Delivery.

In Conclusion

We hope that you’ve found this blog post interesting and that you find JustEat.HttpClientInterception useful in your own test suites for simplifying things and making your applications even more awesome.

You can find the project in our organisation on GitHub and you can download the library to use in your .NET projects from the JustEat.HttpClientInterception package page on

Contributions to the library are welcome – check out the contributing guide if you’d like to get involved!

If you like solving problems at scale and don’t think testing is just for QAs, why not check out open positions in Technology at Just Eat?

About the Author

This blog post was written by Martin Costello, a Senior Engineer at Just Eat, who works on consumer-facing websites and the back-end services that power them.