C#

Quick Reference

Critical Rules

• ?. and ?? prevent NRE but ! overrides warnings — still crashes if null
• .Result or .Wait() on UI thread — deadlock, use await or ConfigureAwait(false)
• LINQ is lazy — query.Where(...) doesn't execute until iteration
• Multiple enumeration of IEnumerable — may re-query database, call .ToList() first
• Closure captures variable, not value — loop variable in lambda captures last value
• Struct in async method — copied, modifications lost after await
• String comparison culture — StringComparison.Ordinal for code, CurrentCulture for UI
• GetHashCode() must be stable — mutable fields break dictionary lookup
• Modifying collection while iterating — throws, use .ToList() to iterate copy
• decimal for money — float/double have precision loss
• readonly struct prevents defensive copies — use for performance
• sealed prevents inheritance — enables devirtualization optimization