ASP.NET Core – How to unit test a model validation attribute

You can directly unit test a model validation attribute by creating an instance of the attribute class, and then testing the two primary methods: IsValid() and FormatErrorMessage(). Here’s an example of unit testing a custom validation attribute: You can also test the built-in validation attributes (such as [Range], [RegularExpression], etc..), which is useful when you … Read more

ASP.NET Core – How to unit test an action filter

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

C# – Trim a UTF-8 string to the specified number of bytes

Here’s the simplest way to efficiently trim a UTF-8 string to the specified number of bytes: A UTF-8 string can have a mix of characters between 1 to 4 bytes. When you only take part of the byte array, you may end up cutting multi-byte chars in half, which then get replaced with the replacement … Read more

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

Quick way to create a unit test project in Visual Studio

You don’t need to manually create a unit test project (through the UI or through the command line). Instead, you can can use the Create Unit Tests command in Visual Studio to generate a unit test project, add it to the solution, and wire up a test stub all at once. Note: This approach can … 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

Algorithm Explained: Get the max items you can purchase with a fixed budget

Problem statement: Given a fixed budget and a list of item prices. What is the max number of items you can purchase? You can only buy each item once. Note: This is the Mark and Toys problem from HackerRank. Example: You’re given $10 and a list of items to choose from: A cool coffee cup … Read more