Test-Driven Development (TDD)

Overview

TDD is a workflow where tests are written before production code in short cycles: Red, Green, Refactor.

TDD Cycle

flowchart LR
    A[Red - Write Failing Test] --> B[Green - Minimal Code]
    B --> C[Refactor - Improve Design]
    C --> A

Practical Example

Step 1 - failing test:

[Fact]
public void AppliesLoyaltyDiscount()
{
    Assert.Equal(90m, ApplyDiscount(100m, "LOYAL"));
}

Step 2 - minimal implementation:

private static decimal ApplyDiscount(decimal total, string tier)
{
    return tier == "LOYAL" ? 90m : total;
}

Step 3 - refactor for clarity and extensibility.

Production Guidance

• Keep cycles short and focused
• Use tests to define behavior, not internal structure
• Refactor immediately after passing tests

Common Pitfalls

• Writing overly broad tests that validate too many rules
• Skipping refactor step after green
• Treating TDD as mandatory for every low-risk change

Key Takeaways

• TDD improves design feedback and regression safety.
• Small cycles reduce risk during feature development.
• Refactoring is a required part of the method.