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What do ??, ??= and ?. do in C#?

C# Language Features Basic Fresher1–2 Yrs Coding 1 min read

Quick Answer

a ?? b — return a unless it's null, then b. a ??= b — assign b to a only if a is currently null. a?.b — access b only if a isn't null, otherwise the whole expression is null.

?? is a null fallback — x ?? y gives me x unless it's null, then y. ??= only assigns if the left side is currently null, which is handy for lazy initialisation. And ?. is null-conditional access — obj?.Property gives null instead of throwing if obj is null, and it chains, so a?.b?.c stops at the first null. A nice combination is value ?? throw new ArgumentNullException(...) for a one-line guard clause.

  • name = input ?? throw new ArgumentNullException(nameof(input)); — the throw-expression idiom for guard clauses.
  • list?.Count ?? 0 — safe navigation plus default.
  • _cache ??= BuildCache(); — lazy one-time init.
  • a?.b?.c() short-circuits at the first null; obj?.Method() won't call the method if obj is null.
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