System.InvalidOperationException: Unable to resolve service for type while attempting to activate

When the built-in dependency injection functionality is trying to create a type, it tries to resolve all of the constructor parameters. If it can’t resolve one of the parameters, it’ll throw a variation of one of these exceptions: InvalidOperationException: Unable to resolve service for type <Type Name> while attempt to activate <Type Name>. ArgumentException: Cannot … Read more

ASP.NET Core – How to unit test an ApiController

The key to unit testing an ApiController class is to mock out all of its dependencies, including the controller’s HttpContext property, like this: If the controller method you’re testing uses anything from the HttpContext, then you’ll want to swap in your own value. Otherwise HttpContext will be null and you’ll get a NullReferenceException. Fortunately Microsoft … Read more

Event-driven .NET: How to unit test code that depends on an event

Testing code with a dependency on an event can be tricky. You may be tempted to directly unit test the event handling method. However, that doesn’t prove that code under test is listening for the event. It’s better to really fire the event, and then test side effects of the event handling method. Firing the … Read more

ASP.NET – Async SSE endpoint

Server-Sent Events (SSE) allow a client to subscribe to messages on a server. The server pushes new messages to the client as they happen. This is an alternative to the client polling the server for new messages. In this article I’ll show how to implement the messaging system shown in the diagram below. This uses … Read more

C# – How to unit test code that uses HttpClient

HttpClient is a dependency. Like any other dependency, you need to pass it into your code (aka dependency injection). By passing it in, you can mock it out in unit tests. There are two approaches to mocking it out: Wrap the HttpClient and mock out the wrapper. Use a real HttpClient with a mocked out … Read more

Moq – How to return different values each time a mocked method is called

When you’re mocking a method that’s called multiple times, you may want to change the behavior of the method each time it’s called. The way you do this with Moq is by using SetupSequence(), like this: Note: You can also make it throw an exception in the sequence. Example of code I want to test … Read more

C# – How to unit test async methods

Let’s say you have the following async method you want to test: Here’s how to unit test this: This is awaiting the method you’re testing. To await it, you must make the unit test method return async Task. This example is a bit simplistic. In the real world when you are working with async methods, … Read more

How to mock static methods

The need to mock static methods in order to add a unit test is a very common problem. It’s often the case that these static methods are in third-party libraries. There are many utility libraries that are completely made up of static methods. While this makes them very easy to use, it makes them really … Read more