Generalizing Solutions with Test Driven Development (C#)

Welcome back to our course on Test-Driven Development (TDD) in C# using xUnit. In our previous lesson, we introduced the fundamentals of TDD and the Red-Green-Refactor workflow. Now, we will advance our TDD skills by focusing on generalizing solutions and enhancing the complexity of our testing scenarios.

As a brief reminder, TDD involves a repetitive cycle known as Red-Green-Refactor:

• Red: Write a failing test to clarify the new functionality you aim to implement.
• Green: Develop the smallest amount of code needed to make that test pass.
• Refactor: Clean up the code, enhancing its quality while maintaining its functionality and ensuring all tests remain passing.

In this lesson, we will expand upon the Sum method, demonstrating how to generalize it while following these TDD principles.

Before we dive into coding, let's review our current setup. You are already familiar with the Sum method from Math.cs and its corresponding test in MathTests.cs:

// MathTests.cs
using Xunit;

public class MathTests
{
    [Fact]
    public void Sum_ShouldAddTwoNumbersCorrectly()
    {
        var math = new Math();
        var result = math.Sum(2, 3);
        Assert.Equal(5, result);
    }
}

// Math.cs
public class Math
{
    public int Sum(int a, int b)
    {
        return 5;
    }
}

This existing setup serves as a foundation. Now, we'll focus on expanding your understanding by generalizing the approach using TDD principles. Understanding where you've come from will help ensure future changes enhance our function without straying too far from the core logic.

To embrace the Red phase, let's introduce a new test case that will fail.

Update MathTests.cs to include more input scenarios:

// MathTests.cs
using Xunit;

public class MathTests
{
    [Fact]
    public void Sum_ShouldAddTwoNumbersCorrectly()
    {
        var math = new Math();
        var result = math.Sum(2, 3);
        Assert.Equal(5, result);
    }

    [Fact]
    public void Sum_ShouldAddTwoMoreNumbersCorrectly()
    {
        var math = new Math();
        var result = math.Sum(5, 6);
        Assert.Equal(11, result);
    }
}

By including a new scenario with new inputs, this step is intentionally set to fail to define our target goal clearly.

Running this test:

Output:

[xUnit.net 00:00:00.48]     MathTests.Sum_AddsTwoMoreNumbersCorrectly [FAIL]
  Failed MathTests.Sum_AddsTwoMoreNumbersCorrectly [1 ms]
  Error Message:
   Assert.Equal() Failure
Expected: 11
Actual:   5

This failure confirms that the new functionality needs addressing.