Unit Testing

Overview

Unit testing verifies small, isolated pieces of logic, usually functions or classes, without external dependencies.

Characteristics

• Fast execution
• Deterministic results
• No network or database dependency
• Focused on one behavior per test

flowchart LR
    A[Test Case] --> B[Unit Under Test]
    C[Mocked Dependencies] --> B
    B --> D[Assertion]

Practical Example

private static decimal CalculateTax(decimal amount, decimal rate)
{
    return Math.Round(amount * rate, 2);
}

[Fact]
public void CalculateTax_ReturnsExpectedAmount()
{
    Assert.Equal(20.00m, CalculateTax(100m, 0.20m));
}

Production Guidance

• Name tests by behavior, not method name only
• Use fixtures for reusable setup
• Mock boundaries, not core business rules

Common Pitfalls

• Testing implementation details instead of outcomes
• Overusing mocks and hiding integration issues
• Writing flaky tests with time-based assumptions

Key Takeaways

• Unit tests provide fastest feedback loops.
• Isolated tests improve confidence during refactoring.
• Behavioral assertions keep tests robust over time.