When writing unit tests for our code, we often use mocking frameworks like Moq to simulate dependencies and verify that our code is functioning as expected. One important aspect of testing with Moq is ensuring that a method call is made exactly once. In this article, we'll explore how to use Moq to verify a method call and ensure it's only called once.
First, let's set the stage with a simple example. Imagine we have a class called Calculator that has a method called Add which takes in two numbers and returns their sum. Our goal is to write a unit test using Moq to verify that the Add method is only called once when we call it.
To begin, we need to create a mock object of our Calculator class using Moq. This mock object will allow us to verify the method calls on our class. We can do this by using the Mock.Of<T> method and passing in our Calculator class as a generic parameter. This will create a mock object with all the methods and properties of our Calculator class.
Next, we need to set up our mock to expect a call to the Add method. We can do this by using the Setup method on our mock object and passing in the Add method as a parameter. Inside the Setup method, we can use the Verifiable method to indicate that we want to verify this method call later on.
Now, we can write our test code. We'll create an instance of our Calculator class and call the Add method, passing in two numbers. Then, we'll use the Verify method on our mock object to ensure that the Add method was called exactly once. If the method was not called at all or was called more than once, our test will fail.
But what if we want to verify that the method was called with specific parameters? For example, what if we only want to verify that the Add method was called with the numbers 5 and 10? We can do this by using the Verify method with an additional parameter that specifies the expected parameters for the method call.
Another useful feature of Moq is the ability to verify that a method was called with a specific number of times. For example, if we want to ensure that the Add method was called twice, we can use the Times method on our Verify call and pass in the number 2 as a parameter.
But what if we don't care about the specific parameters of the method call and just want to verify that it was called at least once? We can use the Times.AtLeast method and pass in the number 1 as a parameter. This will ensure that the method was called at least once, but it could have been called more times as well.
Lastly, we can also use the Times.Never method to verify that a method was never called. This is useful when we want to ensure that a certain method is not being called in our code.
In conclusion, Moq is a powerful framework for verifying method calls in our unit tests. By using the Setup and Verify methods, we can ensure that our code is functioning as expected and that certain methods are only called once. By using the Times method, we can also specify the exact number of times we want a method to be called. With these tools, we can confidently test our code and catch any potential bugs before they make it into production.
