# Skill 05 - Functions, Debugging, and Testing

## Function Design

- Follow DRY, but do not abstract too early.
- Keep functions small enough to explain in one sentence.
- Prefer pure functions for business rules when possible.
- Make side effects obvious in names and placement.
- Use parameters for required input and return values for computed output.
- Use tuples for small local groupings; use named types for public APIs or durable concepts.

## Async Rules

- Use `Task`/`Task<T>` for asynchronous operations.
- Suffix asynchronous methods with `Async`.
- Pass `CancellationToken` through I/O or long-running operations.
- Never use `.Result` or `.Wait()` in request-handling or UI code.

## Debugging Rules

- Reproduce the issue first.
- Use breakpoints, watches, locals, call stack, and trace/log output.
- Verify assumptions with tests rather than comments alone.
- Remove temporary diagnostic output before final code unless it becomes structured logging.

## Unit Testing Rules

- Use Arrange, Act, Assert.
- Test normal, boundary, invalid, and null cases.
- Unit-test pure logic without external services.
- Integration-test database, filesystem, HTTP, serialization, and DI wiring.
- Keep tests deterministic. Avoid reliance on local time, random values, order, or environment unless controlled.

## Common Commands

```bash
dotnet test
dotnet test --filter FullyQualifiedName~SomeTestName
dotnet test --collect:"XPlat Code Coverage"
```