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PONull Reference while handling a List in multiple threads
 

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  1. PONull Reference while handling a List in multiple threads
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    <p>Basically, i have a collection of objects, i am chopping it into small collections, and doing some work on a thread over each small collection simultaneously.</p> <pre><code>int totalCount = SomeDictionary.Values.ToList().Count; int singleThreadCount = (int)Math.Round((decimal)(totalCount / 10)); int lastThreadCount = totalCount - (singleThreadCount * 9); Stopwatch sw = new Stopwatch(); Dictionary&lt;int,Thread&gt; allThreads = new Dictionary&lt;int,Thread&gt;(); List&lt;rCode&gt; results = new List&lt;rCode&gt;(); for (int i = 0; i &lt; 10; i++) { int count = i; if (i != 9) { Thread someThread = new Thread(() =&gt; { List&lt;rBase&gt; objects = SomeDictionary.Values .Skip(count * singleThreadCount) .Take(singleThreadCount).ToList(); List&lt;rCode&gt; result = objects.Where(r =&gt; r.ZBox != null) .SelectMany(r =&gt; r.EffectiveCBox, (r, CBox) =&gt; new rCode { RBox = r, // A Zbox may refer an object that can be // shared by many // rCode objects even on different threads ZBox = r.ZBox, CBox = CBox }).ToList(); results.AddRange(result); }); allThreads.Add(i, someThread); someThread.Start(); } else { Thread someThread = new Thread(() =&gt; { List&lt;rBase&gt; objects = SomeDictionary.Values .Skip(count * singleThreadCount) .Take(lastThreadCount).ToList(); List&lt;rCode&gt; result = objects.Where(r =&gt; r.ZBox != null) .SelectMany(r =&gt; r.EffectiveCBox, (r, CBox) =&gt; new rCode { RBox = r, // A Zbox may refer an object that // can be shared by many // rCode objects even on different threads ZBox = r.ZBox, CBox = CBox }).ToList(); results.AddRange(result); }); allThreads.Add(i, someThread); someThread.Start(); } } sw.Start(); while (allThreads.Values.Any(th =&gt; th.IsAlive)) { if (sw.ElapsedMilliseconds &gt;= 60000) { results = null; allThreads.Values.ToList().ForEach(t =&gt; t.Abort()); sw.Stop(); break; } } return results != null ? results.OrderBy(r =&gt; r.ZBox.Name).ToList():null; </code></pre> <p>so, My issue is that SOMETIMES, i get a null reference exception while performing the OrderBy operation before returning the results, and i couldn't determine where is the exception exactly, i press back, click the same button that does this operation on the same data again, and it works !! .. If someone can help me identify this issue i would be more than gratefull. NOTE :A Zbox may refer an object that can be shared by many rCode objects even on different threads, can this be the issue ? as i can't determine this upon testing, because the error happening is not deterministic.</p>
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    1. PO
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    1. This table or related slice is empty.
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    1. USAlex
    1. USSiraj Mansour
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    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
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    2. PO
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    3. PO
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    1. VO
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      1. PONull Reference while handling a List in multiple threads
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
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      1. This table or related slice is empty.
      1. VTUpMod
    1. COSounds like you have a very typical threading style bug.
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      1. USTony The Lion
    2. COAre the different threads able to alter the value of `r.ZBox` at any given time?
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      1. PONull Reference while handling a List in multiple threads
      1. USTony The Lion
    3. COyou are modifying the results List<> from all threads at the same time. This is a bad idea, as List<T> is not thread safe. You will never know what you will end up with when you manipulate your list like this.
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      1. PONull Reference while handling a List in multiple threads
      1. USDirk Trilsbeek
 

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