Explain integration testing vs unit testing

Unit Testing:

• Tests individual components in isolation
• Fast execution (milliseconds)
• No external dependencies
• Uses mocks/stubs for dependencies
• Runs frequently (every build)
• Large number of tests
• Focuses on single method/class behavior

[Fact]
public void CalculateDiscount_PremiumCustomer_Returns20Percent()
{
    var calculator = new DiscountCalculator();
    var result = calculator.Calculate(100, CustomerType.Premium);
    Assert.Equal(20, result);
}

Integration Testing:

• Tests how components work together
• Slower execution (seconds to minutes)
• Uses real dependencies (database, APIs, file system)
• Tests interactions between modules
• Runs less frequently (pre-commit, CI/CD)
• Fewer tests than unit tests
• Focuses on component collaboration

[Fact]
public async Task CreateOrder_ValidData_SavesToDatabase()
{
    // Uses real database (or test database)
    var dbContext = new OrderContext(connectionString);
    var repository = new OrderRepository(dbContext);
    var service = new OrderService(repository);
    
    var order = new Order { CustomerId = 1, Total = 100 };
    await service.CreateOrder(order);
    
    var saved = await dbContext.Orders.FindAsync(order.Id);
    Assert.NotNull(saved);
    Assert.Equal(100, saved.Total);
}

Comparison Table:

Best Practice: Use both! Follow the testing pyramid:

• 70% Unit Tests (base)
• 20% Integration Tests (middle)
• 10% E2E/UI Tests (top)