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POUsing the Concurrent Dictionary - Thread Safe Collection Modification
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copied!<p>Recently I was running into the following exception when using a generic dictionary</p> <blockquote> <p>An InvalidOperationException has occurred. A collection was modified</p> </blockquote> <p>I realized that this error was primarily because of thread safety issues on the static dictionary I was using.</p> <p>A little background: I currently have an application which has 3 different methods that are related to this issue.</p> <ol> <li>Method A iterates through the dictionary using <code>foreach</code> and returns a value.</li> <li>Method B adds data to the dictionary.</li> <li>Method C changes the value of the key in the dictionary.</li> </ol> <p>Sometimes while iterating through the dictionary, data is also being added, which is the cause of this issue. I keep getting this exception in the <code>foreach</code> part of my code where I iterate over the contents of the dictionary. In order to resolve this issue, I replaced the generic dictionary with the <code>ConcurrentDictionary</code> and here are the details of what I did.</p> <p>Aim : My main objective is to completely remove the exception</p> <p><strong>For method B</strong> (which adds a new key to the dictionary) I replaced <code>.Add</code> with <code>TryAdd</code></p> <p><strong>For method C</strong> (which updates the value of the dictionary) I did not make any changes. A rough sketch of the code is as follows :</p> <pre><code> static public int ChangeContent(int para) { foreach (KeyValuePair<string, CustObject> pair in static_container) { if (pair.Value.propA != para ) //Pending cancel { pair.Value.data_id = prim_id; //I am updating the content return 0; } } return -2; } </code></pre> <p><strong>For method A</strong> - I am simply iterating over the dictionary and this is where the running code stops (in debug mode) and Visual Studio informs me that this is where the error occured.The code I am using is similar to the following</p> <pre><code> static public CustObject RetrieveOrderDetails(int para) { foreach (KeyValuePair<string, CustObject> pair in static_container) { if (pair.Value.cust_id.Equals(symbol)) { if (pair.Value.OrderStatus != para) { return pair.Value; //Found } } } return null; //Not found } </code></pre> <p>Are these changes going to resolve the exception that I am getting. </p> <p><strong>Edit:</strong></p> <p>It states on <a href="http://arbel.net/2013/02/03/best-practices-for-using-concurrentdictionary/" rel="nofollow">this page</a> that the method <code>GetEnumerator</code> allows you to traverse through the elements in parallel with writes (although it may be outdated). Isnt that the same as using foreach ? </p>

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