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POCount() returns incorrect result on IEnumerable<T> filled using LINQ
 

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  1. POCount() returns incorrect result on IEnumerable<T> filled using LINQ
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    <p>To begin with, I have this inexplicable affection to LINQ and lambda expressions :) <br/> So I wrote a quite straightforward code using LINQ, which is supposed to get files from a directory according to a certain name pattern, order them and accumulate until the total length of accumulated files exceeds a certain threshold:</p> <pre><code>IEnumerable&lt;FileInfo&gt; l_allFiles = new DirectoryInfo(l_sDirName).GetFiles().Where(l_fileInfo =&gt; ms_pattern.IsMatch(l_fileInfo.Name)).OrderBy(l_fileInfo =&gt; l_fileInfo.CreationTime); int l_nFilesTotal = l_allFiles.Count(); if (nFilesTotal &gt; 0) { long l_nAccumulatedCmdLength = 0; IEnumerable&lt;FileInfo&gt; l_selectedFiles = l_allFiles.TakeWhile(l_fileInfo =&gt; (l_nAccumulatedCmdLength += l_fileInfo.Length) &lt;= Settings.Default.Threshold); int l_nNumOfSelected = l_selectedFiles.Count(); if (l_nNumOfSelected &gt; 0) { l_ret = new A { Files = l_selectedFiles }; } } </code></pre> <p><br/> Well, this code works fine when all found files together do not exceed the threshold.<br/> As soon as not all found files are selected to <code>l_selectedFiles</code>, in most cases <code>l_selectedFiles.Count()</code> returns <strong>0</strong> even though <code>l_selectedFiles</code> is not empty. In rare cases, when <code>l_selectedFiles.Count()</code> returns the correct value, the consecutive call to <code>Files.Count()</code> in the <em>A</em> class returns <strong>0</strong>.<br/> To add to the mistery, the debugger always updates the value of <code>l_nAccumulatedLength</code> not after execution of the <code>TakeWhile()</code> method, but after executing the next <code>l_selectedFiles.Count()</code> statement. In cases that not all files are selected, the value of <code>l_nAccumulatedLength</code> is not always as expected...<br/> To complete the picture, I use <em>Microsoft Visual Studio 2010 Ultimate</em> on <em>Windows 7 Professional</em> and my project targets to <em>.NET4.0</em>.</p> <p>Can anybody give an explanation or a hint to this behavior and/or how to fix it? Because I am quite lost and cannot even imagine how exactly I debug and resolve this issue, and the community is my last hope.</p> <p>Thank you all in advance for your replies and comments.</p>
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