What is the difference between exposing a read-only collection and using an immutable collection in C#?

Read-only views versus immutable collections

Separates restricted mutation access from a stable immutable value and chooses snapshots or structural sharing deliberately.

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A read-only interface or wrapper prevents the receiver from mutating through that reference, but the owner may still change the backing collection. An immutable collection returns a new collection for each logical change, leaving the previous value unchanged. I expose a read-only view when one owner legitimately mutates state and consumers only need observation. I use an immutable value when readers need a stable snapshot that cannot change underneath them.

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Read-only wrappers and interfaces, backing collection mutation, immutable snapshots, structural sharing, publication, allocation, and ownership.

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Read-only access and immutable state solve different problems. One restricts what a caller can do through an API, while the other gives all observers a stable value.
A read-only interface or wrapper prevents the receiver from mutating through that reference, but the owner may still change the backing collection. An immutable collection returns a new collection for each logical change, leaving the previous value unchanged. I expose a read-only view when one owner legitimately mutates state and consumers only need observation. I use an immutable value when readers need a stable snapshot that cannot change underneath them.
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For example, List<T>.AsReadOnly() reflects later changes to the original list, so it is not a historical snapshot. Calling ToArray() creates a separate snapshot, although mutable objects inside it are still shared. and use structural sharing so updates reuse much of the old structure rather than copying everything. Their operation costs differ from or , so I do not substitute them mechanically in a write-heavy hot path.

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Calling a read-only wrapper immutable while its backing collection still changes.
Copying the outer collection and assuming mutable elements were deeply copied.
Using persistent immutable collections without considering write cost and allocation.
Publishing a mutable collection while another thread is still building it.

What is the difference between exposing a read-only collection and using an immutable collection in C#?

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