C# – How to match an anonymous type parameter in a mocked method

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

C# – Unit testing code that does File IO

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

Moq – Verifying parameters passed to a mocked method

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

Moq – How to use Callback() to capture parameters passed to mocked methods

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: Logging method calls for troubleshooting. Simplifying assertions involving complex parameters. In this article, I’ll show examples of using Callback() in … Read more

Error CS0854 when you’re using Moq with optional parameters

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

C# – How to unit test code that reads and writes to the console

When you’re unit testing code that uses the console IO methods, you’ll need a way to automate the input and a way to capture the output. There are two ways to do this: Redirect the console IO. Wrap the console IO method calls and mock out the wrapper. In this article, I’ll show how to … Read more

C# – How to unit test code that uses Dapper

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

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

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