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POThreadLocal<> and memory leak
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  1. POThreadLocal<> and memory leak
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    copied!<p>.Net 4. ThreadLocal&lt;> implements IDisposable. But it seems that calling Dispose() doesn't actually release references to thread local objects being held.</p> <p>This code reproduces the problem:</p> <pre><code>using System; using System.Collections.Generic; using System.Collections.Concurrent; using System.Linq; using System.Threading; namespace ConsoleApplication2 { class Program { class ThreadLocalData { // Allocate object in LOH public int[] data = new int[10 * 1024 * 1024]; }; static void Main(string[] args) { // Stores references to all thread local object that have been created var threadLocalInstances = new List&lt;ThreadLocalData&gt;(); ThreadLocal&lt;ThreadLocalData&gt; threadLocal = new ThreadLocal&lt;ThreadLocalData&gt;(() =&gt; { var ret = new ThreadLocalData(); lock (threadLocalInstances) threadLocalInstances.Add(ret); return ret; }); // Do some multithreaded stuff int sum = Enumerable.Range(0, 100).AsParallel().Select( i =&gt; threadLocal.Value.data.Sum() + i).Sum(); Console.WriteLine("Sum: {0}", sum); Console.WriteLine("Thread local instances: {0}", threadLocalInstances.Count); // Do our best to release ThreadLocal&lt;&gt; object threadLocal.Dispose(); threadLocal = null; Console.Write("Press R to release memory blocks manually or another key to proceed: "); if (char.ToUpper(Console.ReadKey().KeyChar) == 'R') { foreach (var i in threadLocalInstances) i.data = null; } // Make sure we don't keep the references to LOH objects threadLocalInstances = null; Console.WriteLine(); // Collect the garbage GC.Collect(); GC.WaitForPendingFinalizers(); GC.Collect(); Console.WriteLine("Garbage collected. Open Task Manager to see memory consumption."); Console.Write("Press any key to exit."); Console.ReadKey(); } } } </code></pre> <p>Thread local data stores a reference to a large object. GC doesn't collect these large objects if references are not nulled manually. I used Task Manager to observe memory consumption. I also run memory profiler. I made a snapshot after garbage was collected. The profiler showed that leaked object is rooted by GCHandle and was allocated with here:</p> <pre><code>mscorlib!System.Threading.ThreadLocal&lt;T&gt;.GenericHolder&lt;U,V,W&gt;.get_Boxed() mscorlib!System.Threading.ThreadLocal&lt;T&gt;.get_Value() ConsoleApplication2!ConsoleApplication2.Program.&lt;&gt;c__DisplayClass3.&lt;Main&gt;b__2( int ) Program.cs </code></pre> <p>That seems to be a flaw in ThreadLocal&lt;> design. The trick with storing all allocated objects for further cleanup is ugly. Any ideas on how to work around that?</p>
 

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