C_SHARP_SKILLS/.ai/skills/04-control-flow-exceptions.md

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.