# Skill 04 - Control Flow, Pattern Matching, and Exceptions

## Selection and Iteration

- Use `if` for simple decisions.
- Use `switch` expressions for mapping values to results.
- Use pattern matching for type, property, relational, and logical shape checks.
- Use `foreach` for enumerating collections unless an index is required.
- Use `for` when index control matters.
- Avoid deeply nested conditionals; use guard clauses and extract methods.

## Guard Clause Rules

- Validate public method parameters at the top.
- Return early for invalid or no-op states when it reduces nesting.
- Throw argument exceptions for invalid caller input.
- Throw invalid operation exceptions for invalid object state.

## Exception Rules

- Exceptions are for exceptional or invalid states, not normal branching.
- Preserve stack traces. Use `throw;` when rethrowing inside a catch block.
- Add context when wrapping exceptions.
- Do not catch broad exceptions unless the boundary can recover, log, translate, or shut down cleanly.
- Do not expose sensitive details in web responses.

## Overflow and Defensive Coding

- Use `checked` where overflow would corrupt calculations.
- Validate external input before conversion or persistence.
- Prefer result objects for expected validation outcomes and exceptions for programming/runtime failures.