What is TDD (Test-Driven Development)?

Test-Driven Development is a software development approach where tests are written before the actual code. It follows a cycle called Red-Green-Refactor:

The TDD Cycle:

1. Red: Write a failing test

   • Define what you want to achieve
   • Test fails because functionality doesn't exist yet

2. Green: Write minimal code to make the test pass

   • Focus on making it work, not perfect
   • Get to green as quickly as possible

3. Refactor: Improve the code

   • Clean up implementation
   • Remove duplication
   • Tests ensure behavior is preserved

Example TDD Process:

// Step 1: RED - Write failing test
[Fact]
public void Deposit_PositiveAmount_IncreasesBalance()
{
    var account = new BankAccount(100);
    account.Deposit(50);
    Assert.Equal(150, account.Balance);
}

// Step 2: GREEN - Make it pass (minimal code)
public class BankAccount
{
    public decimal Balance { get; private set; }
    
    public BankAccount(decimal initialBalance)
    {
        Balance = initialBalance;
    }
    
    public void Deposit(decimal amount)
    {
        Balance += amount;
    }
}

// Step 3: REFACTOR - Improve (add validation)
public void Deposit(decimal amount)
{
    if (amount <= 0)
        throw new ArgumentException("Amount must be positive");
    
    Balance += amount;
}

Benefits of TDD:

• Better design (code is inherently testable)
• Comprehensive test coverage
• Less debugging time
• Documentation through tests
• Confidence in refactoring
• Focus on requirements

Challenges:

• Initial learning curve
• Slower initial development
• Requires discipline
• Not suitable for all scenarios (e.g., UI, exploratory work)