**Course Title:** Mastering C#: From Fundamentals to Advanced Programming **Section Title:** Testing and Debugging in C# **Topic:** Write unit tests for a C# project and debug an existing application.(Lab topic) **Introduction:** In this lab topic, we will focus on writing unit tests for a C# project and debugging an existing application. Unit testing is an essential part of software development, as it ensures that individual components of the codebase work as expected. We will use the NUnit framework to write unit tests and Visual Studio's built-in debugging tools to debug an existing application. **Writing Unit Tests with NUnit:** Before we start writing unit tests, make sure you have NUnit installed in your Visual Studio project. You can install NUnit from the NuGet package manager. 1. Create a new test project in Visual Studio by selecting **File** > **New** > **Project...** and choosing **NUnit Test Project (.NET Core)** under the **Test** section. 2. Write your first test by creating a new class that inherits from **TestFixture**. Use the `[Test]` attribute to mark a method as a test. ```csharp [TestFixture] public class CalculatorTests { [Test] public void Add_TwoPositiveNumbers_ReturnsPositiveNumber() { // Arrange var calculator = new Calculator(); var num1 = 10; var num2 = 20; // Act var result = calculator.Add(num1, num2); // Assert Assert.That(result, Is.EqualTo(30)); } } ``` In this example, we're testing a simple `Add` method of a `Calculator` class. We arrange the test by creating a new instance of the `Calculator` class and setting up the input values. We then act on the object by calling the `Add` method and storing the result. Finally, we assert that the result is equal to the expected value using the `Assert.That` method. **Debugging an Existing Application:** To debug an existing application, we will use Visual Studio's built-in debugging tools. 1. Open the existing application project in Visual Studio. 2. Set a breakpoint in the code where you want to start debugging. You can do this by clicking on the left margin next to the line of code or pressing **F9**. 3. Start the application in debug mode by pressing **F5** or clicking the **Start Debugging** button. 4. Once the application hits the breakpoint, you can use the **Debug** > **Windows** menu to open various debugging windows, such as the **Locals** window, **Watch** window, or **Call Stack** window. For example, let's say we want to debug a simple console application that calculates the factorial of a number: ```csharp public class Program { public static void Main(string[] args) { var num = 5; var factorial = CalculateFactorial(num); Console.WriteLine($"Factorial of {num} is {factorial}"); } public static int CalculateFactorial(int number) { if (number == 0) { return 1; } else { return number * CalculateFactorial(number - 1); } } } ``` We can set a breakpoint inside the `CalculateFactorial` method and start debugging. Once we hit the breakpoint, we can use the debugging windows to inspect the values of the variables and step through the code to see how the method calculates the factorial. **Best Practices:** When writing unit tests and debugging an existing application, follow these best practices: * Keep your tests independent of each other. * Use descriptive names for your tests and test methods. * Use the **Arrange-Act-Assert** pattern in your tests. * Use debugging windows to inspect variable values and step through the code. * Use conditional breakpoints to break on specific conditions. **Conclusion:** In this lab topic, we learned how to write unit tests for a C# project using NUnit and debug an existing application using Visual Studio's built-in debugging tools. We also followed best practices for writing unit tests and debugging code. **External Resources:** * [NUnit documentation](https://docs.nunit.org/) * [Visual Studio debugging documentation](https://docs.microsoft.com/en-us/visualstudio/debugger/?view=vs-2022) **Exercise:** Write unit tests for a simple calculator class that has methods for addition, subtraction, multiplication, and division. Then, debug an existing application that uses the calculator class. **Leave a comment/Ask for help:** If you have any questions or need help with the exercise, leave a comment below.