In the last post we have discussed core principles of unit testing and TDD. In this post, we’ll walk through a simple example of how to use Moq as mocking framework and Dependency Injection with NUnit in the context of Test-Driven Development (TDD) in C#. This combination allows for clean, maintainable tests by mocking dependencies and focusing on the behavior of the system under test. Imagine we have an application where we process payments through a PaymentService . This service depends on an external IPaymentGateway interface to make the actual payment. Let’s break this down step by step. Step 1: Write the Test First (Before Code) We want to test the PaymentService , which depends on the IPaymentGateway . Our first test will focus on making sure the PaymentService behaves correctly when making a valid payment. Here's the test code: using Moq; using NUnit.Framework; [TestFixture] public class PaymentServiceTests { private Mock<IPaymentGateway>...