tdd

Test-Driven Development Skill

Overview

TDD transforms testing from an afterthought into the foundation of development. Tests are proof that code works correctly.

When to Invoke

• Writing new features
• Fixing bugs (Prove-It pattern)
• When asked to write tests
• Before any implementation work

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Core Cycle: RED-GREEN-REFACTOR

RED Phase

Write a test that describes expected behavior. It must fail.

public class DiscountCalculatorTests
{
    private readonly DiscountCalculator _sut;

    public DiscountCalculatorTests()
    {
        _sut = new DiscountCalculator();
    }

    [Fact]
    public void Calculate_WhenOrderOverThreshold_AppliesTenPercentDiscount()
    {
        // Arrange
        var orderAmount = 150m;

        // Act
        var discount = _sut.Calculate(orderAmount);

        // Assert
        discount.Should().Be(15m);
    }
}

Run: dotnet test → Should FAIL

GREEN Phase

Write minimal code to pass the test. No extras.

public class DiscountCalculator
{
    public decimal Calculate(decimal amount)
    {
        if (amount > 100)
            return amount * 0.1m;
        return 0;
    }
}

Run: dotnet test → Should PASS

REFACTOR Phase

Improve code while keeping tests green.

public class DiscountCalculator
{
    private const decimal DiscountThreshold = 100m;
    private const decimal DiscountRate = 0.1m;

    public decimal Calculate(decimal amount)
    {
        if (amount <= DiscountThreshold)
            return 0;
        
        return amount * DiscountRate;
    }
}

Run: dotnet test → Should still PASS

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Prove-It Pattern (Bug Fixes)

Step 1: Write Failing Test

[Fact]
public void GetTotal_WhenCartIsEmpty_ReturnsZeroWithoutError()
{
    // Arrange — This test should FAIL with buggy code
    var cart = new Cart(items: Array.Empty<CartItem>());

    // Act
    var act = () => cart.GetTotal();

    // Assert
    act.Should().NotThrow();
    cart.GetTotal().Should().Be(0);
}