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Articles showing how to unit test code in different scenarios.
In this article, I’ll show how to unit test a custom InputFormatter. The main thing to test is the output of the ReadRequestBodyAsync() method. To test this, you have to pass in an InputFormatterContext object containing the request body. As an example, I’ll show how to unit test the following ReadRequestBodyAync() method: Note: This is … Read more
You can unit test a validation attribute by creating an instance of it and then testing the two methods: In this article, I’ll show examples of unit testing these methods in a custom validation attribute and in a built-in validation attribute (i.e. [Range]). Unit testing a custom validation attribute Consider the following custom validation attribute … Read more
To unit test an action filter, you have to pass in an action filter context object (which requires a lot of setup). Action filter methods are void, so you have to verify the behavior by inspecting the context object (or dependencies, like a logger, if you are injecting those). Here’s an example of doing the … Read more
When an anonymous type is defined in one assembly, it won’t match an anonymous type defined in another assembly. This causes problems when you’re unit testing and trying to mock a method that has an an anonymous type parameter. For example, let’s say you’re trying to unit test the following method: To unit test this, … Read more
If your code does File IO, such as reading text from a file, then it’s dependent on the file system. This is an external dependency. In order to make the unit tests fast and reliable, you can mock out the external dependencies. To mock out the file system dependency, you can wrap the File IO … Read more
When you need to verify that the code under test called a method with the expected parameters, you can mock the method with Moq and use Verify() + It.Is<T>() to check the parameters passed in. Verify() asserts that the method call happened as expected with the specified parameters. Here’s an example. This is verifying that … Read more
When you’re using Moq to set up a mocked method, you can use Callback() to capture the parameters passed into the mocked method: There are two main use cases for capturing parameters in a test: In this article, I’ll show examples of using Callback() in those two scenarios, and then I’ll explain some problems to … Read more
Problem Optional parameters aren’t optional when you’re using Moq. When you’re setting up or verifying a method call on a mock object, and the method has an optional parameter that you didn’t specify a value for, you’ll get the following compiler error: Error CS0854 – An expression tree may not contain a call or invocation … Read more
There’s two ways to unit test code that writes to the console (Console.WriteLine() / Console.Write()): In this article, I’ll show how to do both options. Option 1 – Capture the output with Console.SetOut() Let’s say you want to unit test the following code that outputs to the console with Console.WriteLine(): You can unit test this … Read more
There are three requirements for unit testing a middleware class: Mock out RequestDelegate by defining a lambda that returns a Task (Task.FromException, Task.FromResult, or Task.FromCanceled). Mock out HttpContext by using DefaultHttpContext. The middleware function needs to be awaited, so your unit test needs to be defined with async Task. Here’s an example: This is a … Read more
FluentAssertions is a library that improves unit tests by providing better failure messages, simplifies assertions in many scenarios, and provides a fluent interface (which improves code readability). In this article, I’ll show a few examples of how FluentAssertions can improve unit tests by comparing it with the built-in assertions (from Microsoft.VisualStudio.TestTools.UnitTesting). Install FluentAssertions To get … Read more
Here’s an example of adding a parameterized unit test in xUnit: To parameterize a unit test, you have to do three things: Add the [Theory] attribute. Add the parameters to the unit test method. In the example above, there are four parameters. Add one [InlineData] for each combo of data you want to test. If … Read more
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
Problem You have a unit test that doesn’t finish, and it prevents other tests from running. There’s no indication that the test passed or failed. It just stops running. When you run all of the tests together, some tests might finish, but once this one bad test stops, it prevents other tests from running. This … Read more
When you want to initialize something for all unit tests in a test class, you can apply the ClassInitialize attribute to a method. This initialization method only runs once and is ran before any of the unit tests. Here’s an example of how to add ClassInitialize: Note: ClassCleanup is the opposite of ClassInitialize. It runs … Read more
Dapper makes your code difficult to unit test. The problem is that Dapper uses static extension methods, and static methods are difficult to mock out. One approach is to wrap the Dapper static methods in a class, extract out an interface for that wrapper class, and then dependency inject the wrapper interface. In the unit … Read more
When you’re testing if two strings are equal, you can simply use Assert.AreEqual(). When you’re testing if a string contains a substring or a pattern, typically developers use Assert.IsTrue() with a substring method or regex. You should use StringAssert instead, because it gives better failure messages. The key problem is Assert.IsTrue() gives useless information. You … Read more
An event handler is a method that is registered to listen to an event. When the event is invoked, the event handler method is called. You may be tempted to directly call the event handler to unit test it. It’s better to actually raise the event though, and then check the side effects of the … Read more
The purpose of parameterized tests is to eliminate duplicated tests. There are two ways to pass parameters into a parameterized test: the DataRow attribute and the DynamicData attribute. With DataRow, the problem is you can only pass in constants and arrays. You can’t pass in reference types. When you try to pass in reference types, … Read more
To disable a test temporarily, you can add the [Ignore] attribute to the test method, like this: When you add the [Ignore] attribute, the test will be ignored by the test runner. It will show up in Test Explorer with a warning icon and will count as Skipped. Why would you want to disable a … Read more
Use Assert.ThrowsException<T>() in a unit test to verify that the code throws a specific type of exception. Here’s an example of asserting that ArgumentNullException is thrown: You specify the type of exception you’re expecting and then pass in a lambda that calls the code you’re testing. The assertion succeeds if and only if the code … Read more
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
I have a parameterized unit test with decimal parameters. When I run the test, I get the following exception: System.ArgumentException: Object of type ‘System.Double’ cannot be converted to type ‘System.Decimal’. Solution Change the parameters to doubles and convert them to decimals inside the test method. Why is it throwing an exception? You have to pass … Read more
Here’s an example of how to parameterize your tests using the built-in MSTest v2 test framework: There are 3 steps: Add parameters to your test method. Use [DataTestMethod] instead of [TestMethod]. For each test case, add [DataRow(…)] to pass in the parameters for that test case. What parameters can you pass in? You pass in … Read more
Let’s say you’re using DateTime.Parse() to convert a date string into a DateTime object. By default, DateTime.Parse() uses CultureInfo.CurrentCulture to figure out the date format. The current culture ultimately comes from your OS settings. So when you run this code on a computer that is using the en-US locale, the current culture will automatically default … Read more
