List<T> vs Span<T> vs ReadOnlySpan<T> in C#: when to reach for which
List<T> is a growable heap collection; Span<T> and ReadOnlySpan<T> are stack-only views over memory you already own. Use List<T> for anything you store, return from async, or grow; Span<T> for a mutable allocation-free view in a synchronous method; ReadOnlySpan<T> for read-only parsing over strings, u8 literals, and slices.