Why ReadOnlyCollection<T> is not truly immutable and how ImmutableArray<T> fixes it:

• ReadOnlyCollection<T> is just a wrapper around an existing list. If another thread modifies the underlying List<T>, the ReadOnlyCollection changes underneath you (not thread-safe!).
• ImmutableArray<T> is a readonly struct wrapping a private immutable array. It is 100% thread-safe, has zero wrapper object heap allocation, and allows lock-free reads across all threads without risk of modification.

ImmutableArray<T> with Builder (builder.ToImmutable()) is the best choice for shared configuration states.

Because ImmutableArray<T> is a value-type struct, its memory footprint is identical to a raw array pointer.