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What is the difference between out, ref and in parameters? How do 'ref', 'out', and 'in' parameter modifiers differ in how they pass arguments to a method? How does string interpolation work, and how does it compare to string.Format() and simple concatenation? How do expression-bodied members simplify simple method, property, and constructor definitions? How does pattern matching in an 'is' expression differ from a traditional type check with 'as' and a null check, and what does it enable? How does the C# compiler enforce 'definite assignment' analysis, and what compile error results from violating it? How do C# 12's primary constructors on regular classes (not just records) change how you write simple constructor-and-field boilerplate? How does the C# 'nameof' operator work, and why is it preferred over hardcoding a string literal for things like argument names in exceptions? How do target-typed 'new' expressions work, and what ambiguity do they resolve compared to always specifying the type twice?
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How does string interpolation compile, and when should you avoid it?

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Interpolated strings usually compile to string.Format (or a DefaultInterpolatedStringHandler on modern .NET). Avoid it in hot logging paths where the message may not be emitted — use structured logging instead.
string s = $"User {id} at {DateTime.UtcNow}";
// logging: prefer logger.LogInformation("User {Id}", id);
Real-world example Structured logs let you query by the Id field later instead of parsing a formatted string.
All 16 Fundamentals questions