When would you choose ReaderWriterLockSlim over lock, and what costs would make you avoid it?

When ReaderWriterLockSlim helps and when it hurts

Evaluates whether read-heavy access justifies a reader/writer lock instead of a simpler lock or immutable snapshot.

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TL;DR

ReaderWriterLockSlim allows multiple readers at the same time while still giving writers exclusive access. I would consider it when reads are frequent, read sections are meaningful, writes are uncommon, and a normal lock shows measurable contention. I would keep using lock for small critical sections because it is simpler and often faster at low contention. I would not hold a reader/writer lock across asynchronous I/O because it is a synchronous thread-based primitive.

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Read/write contention, lock overhead, writer starvation, upgradeable reads, async incompatibility, measurement, and immutable alternatives.

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Why this question matters
A reader/writer lock sounds ideal for every read-heavy cache, but its bookkeeping and more complex acquisition rules can cost more than the protected work. The decision needs a workload, not only a read/write ratio.
ReaderWriterLockSlim allows multiple readers at the same time while still giving writers exclusive access. I would consider it when reads are frequent, read sections are meaningful, writes are uncommon, and a normal lock shows measurable contention. I would keep using lock for small critical sections because it is simpler and often faster at low contention. I would not hold a reader/writer lock across asynchronous I/O because it is a synchronous thread-based primitive.
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Details

Choosing a reader/writer lock only because the code has more reads than writes.
Holding the lock while performing database, file, or network I/O.
Using upgradeable mode for most reads and serializing the workload accidentally.
Forgetting that an immutable snapshot may remove read locking entirely.

When would you choose ReaderWriterLockSlim over lock, and what costs would make you avoid it?

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