Test-Driven Development (TDD)

What is TDD?

Test-Driven Development (TDD) is a development approach where you write tests before writing the implementation code.

Red-Green-Refactor Cycle

Benefits of TDD

• Better design: Forces you to think about API before implementation
• Fewer bugs: Catches issues early
• Confidence: Refactor without fear
• Documentation: Tests show how code should be used
• Testable code: Naturally leads to better architecture

TDD Workflow Example

Step 1: 🔴 Red - Write Failing Test

public class DiscountCalculatorTests
{
    [Fact]
    public void CalculateDiscount_TenPercent_ReturnsDiscountedPrice()
    {
        // Arrange
        var calculator = new DiscountCalculator();

        // Act
        var result = calculator.CalculateDiscount(100, 0.1m);

        // Assert
        Assert.Equal(90, result);
    }
}

// This test FAILS - DiscountCalculator doesn't exist yet!

Step 2: 🟢 Green - Make Test Pass

public class DiscountCalculator
{
    public decimal CalculateDiscount(decimal price, decimal discountPercent)
    {
        return price - (price * discountPercent);
    }
}

// Test now PASSES! ✅

Step 3: 🔵 Refactor - Improve Code

[Theory]
[InlineData(100, 0.1, 90)]
[InlineData(50, 0.2, 40)]
[InlineData(200, 0.5, 100)]
public void CalculateDiscount_VariousInputs_ReturnsCorrectPrice(
    decimal price, decimal discount, decimal expected)
{
    var calculator = new DiscountCalculator();
    
    var result = calculator.CalculateDiscount(price, discount);
    
    Assert.Equal(expected, result);
}

InvenTrack TDD Example

Let's build a stock validator using TDD.

🔴 Red: Write Test

[Fact]
public void IsInStock_QuantityGreaterThanZero_ReturnsTrue()
{
    var validator = new StockValidator();
    var product = new Product { Quantity = 10 };

    var result = validator.IsInStock(product);

    Assert.True(result);
}

🟢 Green: Implement

public class StockValidator
{
    public bool IsInStock(Product product)
    {
        return product.Quantity > 0;
    }
}

🔴 Red: Add Edge Case

[Fact]
public void IsInStock_QuantityZero_ReturnsFalse()
{
    var validator = new StockValidator();
    var product = new Product { Quantity = 0 };

    var result = validator.IsInStock(product);

    Assert.False(result);
}

// Test PASSES - implementation already handles this! ✅

TDD Best Practices

• Start simple: Begin with simplest test case
• One test at a time: Focus on one behavior
• Minimal code: Write just enough to pass
• Refactor often: Keep code clean
• Fast tests: Tests should run quickly
• Descriptive names: Test names explain behavior

Key Takeaways

• TDD: Write tests before code
• Red-Green-Refactor: The TDD cycle
• 🔴 Red: Write failing test
• 🟢 Green: Make it pass
• 🔵 Refactor: Improve code
• Benefits: Better design, fewer bugs, confidence