Functional Testing Harness in ASP.NET Core

The ASP.NET Core team has made testing a first class system within their ecosystem, and it really shows. One of the aspects they made a few steps easier is functional testing of web applications. Both ASP.NET and ASP.NET Core have similar goals of creating a TestClient (GetTestClient) which can be used to perform the actual functional testing, but the ASP.NET Core teams HostBuilder pattern makes it just a touch easier to configure your TestServer.

One of the very cool things with ASP.NET Core is that if you are writing functional tests for your web application, you can use your actual Startup.cs class to configure your TestServer. This saves a lot of configuration overhead that’s involved in setting up unit tests (which still should be created). With functional tests, you usually want to see how many parts of the system work together, but you still want to stub/mock some of the external connections.

So, how can you let the system wire itself up and then mock out just the external connections? The HostBuilder makes it pretty easy to do just that. You can use your normal  Startup.cs class to configure the system and then add on an extra .ConfigureServices() function which will add in your mocks.

Here’s what that might look like:

And here’s what some code that uses it could look like: