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What is the difference between value and reference equality for strings? What is the difference between 'var' and an explicitly typed variable declaration in C#? What are auto-implemented properties, and what does the compiler generate behind the scenes for one? What is the difference between checked and unchecked arithmetic? 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 string interpolation compile, and when should you avoid it? 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?
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What is the difference between out, ref and in parameters?

Intermediate

Answer

ref passes a variable by reference (must be initialised, can be read/written); out must be assigned inside the method (need not be initialised by caller); in passes by reference read-only for performance.
bool TryParse(string s, out int value);
void Swap(ref int a, ref int b);
double Length(in Vector v); // read-only
Real-world example TryParse patterns use out to return both success and the parsed value.
All 16 Fundamentals questions