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    <p>I would fully <em>expect</em> the write-lock to function as a memory barrier (within the write-lock, in particular), but I can't prove it off-hand.</p> <p>Whether you need the complexity of <code>ReaderWriterLockSlim</code> depends on the context; <code>Interlocked</code>, <code>volatile</code>, <code>lock</code> or <code>[MethodImpl]</code> might each do the job far more simply. You mainly need <code>ReaderWriterLock[Slim]</code> if you have lots of readers and few writers.</p> <p>However, the <code>get</code> is not currently protected by the lock; you'll need to us an explicit property implementation and take out a read lock yourself if you ever need multiple operations to be spanned by the write-lock (without readers seeing intermediate values).</p> <p>As an aside, the <code>Count++</code> usage would probably be more familiar to people.</p> <p>You should also use <code>try</code>/<code>finally</code> to ensure you release the lock on exception.</p> <p>To avoid the issue of taking write then read locks, perhaps:</p> <pre><code> private ReaderWriterLockSlim rwLock = new ReaderWriterLockSlim(); private int count; public int Count { get { rwLock.EnterReadLock(); int tmp = count; rwLock.ExitReadLock(); return tmp; } } public void Add(object x) { rwLock.EnterWriteLock(); try { // do some processing count++; } finally { rwLock.ExitWriteLock(); } } </code></pre> <hr> <p>Updated re your edit;</p> <p>That looks pretty solid. A <code>List&lt;T&gt;</code> would be my recommendation (over a <code>T[]</code> array), since it will do all the doubling etc internally, saving you lots of code. Since only one thread can update <code>Count</code> at a time, there is no need for <code>Interlocked</code>, and this property saves the need for a lock when reading <code>Count</code>, as long as you're fine for callers to get the old <code>Count</code> while another thread is adding rows (rather than being blocked).</p>
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    1. PODoes using ReaderWriterLockSlim cause a memory barrier?
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    1. USMarc Gravell
    1. USMarc Gravell
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    1. COYou might also consider marking count volatile (as opposed to a full read lock), depending on what you're actually using it for. If you need to be 100% sure the write is 100% finished before seeing the new value, use a read lock. If it doesn't matter, volatile should provide adequate protection.
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      1. USEric Rosenberger
    2. COOn the other hand, if your property is something more complex than, for instance, an int, you will need a read lock; volatile won't prevent you from seeing partially-updated complex structures.
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    3. CONote: for a collection like you've implied here, a read lock is probably the way to go; but you should evaluate case-by-case how much protection you really need.
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