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    <p>I <a href="http://philosopherdeveloper.com/posts/how-to-build-a-thread-safe-lock-free-resizable-array.html" rel="noreferrer">gave it a try a while back</a> (also: <a href="https://github.com/dtao/ConcurrentList" rel="noreferrer">on GitHub</a>). My implementation had some problems, which I won't get into here. Let me tell you, more importantly, what I learned.</p> <p>Firstly, there's no way you're going to get a full implementation of <code>IList&lt;T&gt;</code> that is lockless and thread-safe. In particular, random insertions and removals are <em>not</em> going to work, unless you also forget about O(1) random access (i.e., unless you "cheat" and just use some sort of linked list and let the indexing suck).</p> <p>What I <em>thought</em> might be worthwhile was a thread-safe, limited subset of <code>IList&lt;T&gt;</code>: in particular, one that would allow an <code>Add</code> and provide random <em>read-only</em> access by index (but no <code>Insert</code>, <code>RemoveAt</code>, etc., and also no random <em>write</em> access).</p> <p>This was the goal of <a href="http://philosopherdeveloper.com/posts/boy-can-dream.html" rel="noreferrer">my <code>ConcurrentList&lt;T&gt;</code> implementation</a>. But when I tested its performance in multithreaded scenarios, I found that <strong>simply synchronizing adds to a <code>List&lt;T&gt;</code> was faster</strong>. Basically, adding to a <code>List&lt;T&gt;</code> is lightning fast already; the complexity of the computational steps involved is miniscule (increment an index and assign to an element in an array; that's <em>really it</em>). You would need a <em>ton</em> of concurrent writes to see any sort of lock contention on this; and even then, the average performance of each write would still beat out the more expensive albeit lockless implementation in <code>ConcurrentList&lt;T&gt;</code>.</p> <p>In the relatively rare event that the list's internal array needs to resize itself, you do pay a small cost. So ultimately I concluded that this was the <em>one</em> niche scenario where an add-only <code>ConcurrentList&lt;T&gt;</code> collection type would make sense: when you want <em>guaranteed</em> low overhead of adding an element on <em>every single call</em> (so, as opposed to an amortized performance goal).</p> <p>It's simply not nearly as useful a class as you would think.</p>
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    1. PONo ConcurrentList<T> in .Net 4.0?
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    1. USDan Tao
    1. USDan Tao
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    1. COBravo! nicely done.
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      1. USAlan
    2. COAnd if you need something similar to `List<T>` that uses old-skool, monitor-based synchronisation, there's `SynchronizedCollection<T>` hidden away in the BCL: http://msdn.microsoft.com/en-us/library/ms668265.aspx
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      1. USLukeH
    3. COOne small addition: use the Capacity constructor parameter to avoid (as much as possible) the resizing scenario.
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      1. USHenk Holterman
 

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