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POADO.NET DataTable/DataRow Thread Safety
 

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  1. POADO.NET DataTable/DataRow Thread Safety
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    <p><strong>Introduction</strong></p> <p>A user reported to me this morning that he was having an issue with inconsistent results (namely, column values sometimes coming out null when they should not be) of some parallel execution code that we provide as part of an internal framework. This code has worked fine in the past and has not been tampered with lately, but it got me to thinking about the following snippet:</p> <p><strong>Code Sample</strong></p> <pre><code>lock (ResultTable) { newRow = ResultTable.NewRow(); } newRow["Key"] = currentKey; foreach (KeyValuePair&lt;string, object&gt; output in outputs) { object resultValue = output.Value; newRow[output.Name] = resultValue != null ? resultValue : DBNull.Value; } lock (ResultTable) { ResultTable.Rows.Add(newRow); } </code></pre> <p>(No guarantees that that compiles, hand-edited to mask proprietery information.)</p> <p><strong>Explanation</strong></p> <p>We have this cascading type of locking code other places in our system, and it works fine, but this is the first instance of cascading locking code that I have come across that interacts with ADO .NET. As we all know, members of framework objects are usually not thread safe (which is the case in this situation), but the cascading locking should ensure that we are not reading and writing to ResultTable.Rows concurrently. We are safe, right?</p> <p><strong>Hypothesis</strong></p> <p>Well, the cascading lock code does not ensure that we are not reading from or writing to ResultTable.Rows <strong>at the same time</strong> that we are <strong>assigning</strong> values to columns in the new row. What if ADO .NET uses some kind of buffer for assigning column values that is not thread safe--even when different object types are involved (DataTable vs. DataRow)?</p> <p>Has anyone run into anything like this before? I thought I would ask here at StackOverflow before beating my head against this for hours on end :)</p> <p><strong>Conclusion</strong></p> <p>Well, the consensus appears to be that changing the cascading lock to a full lock has resolved the issue. That is not the result that I expected, but the full lock version has not produced the issue after many, many, many tests.</p> <p>The lesson: be wary of cascading locks used on APIs that you do not control. Who knows what may be going on under the covers!</p>
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    1. USTheVillageIdiot
    1. USAllen E. Scharfenberg
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    1. COAllen...I could use a little more information. Are you using DataSet objects in a disconnected manner ? What is the data store and how are the updates scheduled ? ADO.NET is a many headed beast :)
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      1. USRusty
    2. COVery good question! Yes, this is a DataTable used in a disconnected manner. There is no data store in this case (all in memory). Threads are dispatched via an instance of an internal thread pool that we use all over the place. In this particular scenario, the thread pool is throttled at 8 threads via a Semaphore. Anyway, I hope that answers your question sufficiently!
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      1. USAllen E. Scharfenberg
    3. COMost excellent. I am working with some disconnected DataSets at this very moment...I'll try and get in a few tests. Are you running 3.5 or 4.0 ?
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