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    <p>I'm not familiar with the <code>ConcurrentHashMap</code> you're talking about. Perhaps you could define what function it performs that you'd like to have in C# too.</p> <p>If you just want to guarantee that a collection is thread-safe for concurrent access then you're best off defining your own lock strategy to control access to it.</p> <p>The <code>Hashtable.Synchronised(...)</code> method seen in earlier versions of the .NET framework has been removed (along with the hiding of all ICollection.SyncRoot implementations via explicit implementations in <code>System.Collections.Generic</code>). This is because whenever you lock on a publicly accessible object, you're opening up the possibility for other callers to lock on your objects in ways that you can't control, and therefore deadlocks become possible.</p> <p>Also, a 'synchronised' collection can only guarantee that calls to individual members are safe, however many scenarios involve 'transactions' including many members. Consider the following example:</p> <pre><code>if (collection.Contains(item)) collection.Get(item); </code></pre> <p>The <code>map</code> object (whatever type it is) might guarantee that both the <code>Contains</code> and <code>Get</code> methods are thread-safe, but it can't guarantee that the item is still contained in the map by the time <code>Get</code> is called. Instead you should use code like this:</p> <pre><code>private readonly object _mapLock = new object(); public void Method() { lock (_mapLock) { if (collection.Contains(item)) collection.Get(item); } } </code></pre> <p>@Oskar suggests using <code>ReaderWriterLock</code>. This class has a terrible implementation with worse performance than simple locking. If you're using newer versions of the framework, use <code>ReaderWriterLockSlim</code> instead.</p>
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    1. POEquivalent of java's ConcurrentHashMap in C#?
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    1. USDrew Noakes
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    1. COJust a performance note - the lock operation is very expensive, which is why you should perform a double check to avoid unnecessary locks: if (Contains...) lock(_mapLock) if (Contains...) Get...
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      1. USAllon Guralnek
    2. CO@Allon, 1) lock isn't quite as expensive as everyone makes out, but yet, there is a cost. 2) how do you know that `Contains` is thread-safe? If you don't call it while the collection is locked by your thread, another thread may modify it (perhaps performing an internal resize) and the results may be unpredictable. I prefer to always lock unless the documentation for the object in question makes it clear that a particular member is threadsafe for multiple readers and one writer (as some framework members do).
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      1. USDrew Noakes
    3. CODrew, since .Contains() is a read-only operation (does not modify shared state), the worst that could happen is that it returns an incorrect result (e.g. it couldn't corrupt the shared state). If it returns an erroneous false, then the operation will not be performed this go-around, and nothing undesirable happens. If the operation returns an erroneous true, the object is locked and then re-checked to prevent such false-positives. This is one of the two reasons for the seconds check (the other being to prevent a race condition).
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