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[ImmutableObject(true)] is here for HybridCache and nothing else. It is inert at runtime; the attribute is how HybridCache is told it may hand the same instance to every caller instead of storing a serialized copy in L1 and deserializing it on each hit.

Without it a cache <i>hit</i> would pay a full JSON deserialize. That is the cost #66 declined to spend on the serialize side, and it would land on the endpoints whose expensive part — the SQL aggregation — the cache already skips.

<b>Be clear about what it does and does not claim.</b> This type is mutable — both properties have setters and Items is a mutable List<T> — so the attribute is a statement of convention, not a property the compiler enforces: nothing mutates a PagedList it read out of the cache, and nothing may start.

What it is not is a new risk. A cached PagedList was already shared by reference across concurrent requests under plain IMemoryCache; the attribute keeps that exact behaviour rather than introducing it. The only alternative is letting HybridCache serialize and re-deserialize per hit, which is a real cost paid on every request to buy protection against a mutation that does not happen. Treat anything returned from a cache lookup as read-only, and if a caller ever genuinely needs to mutate one, copy it first.