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POReturn value of IEnumerable<T>.Contains changes if called multiple times
 

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    <p>I'm having trouble understanding why multiple calls of Contains <strong>return different values for the same parameter on the same enumerable</strong>. While I understand that the collection can be modified, thus changing the result in a subsequent call, this can be ruled out here.</p> <p>Consider the following (stripped-down) code in an MVC view. The purpose of this will be to display a list of checkboxes (as there's no HTML-helper for that), and determining through the model's properties which ones should be checked when opening the view.</p> <pre><code>@foreach (var d in Model.AllDomains) { bool isChecked = Model.Project.Domains.Contains(d.ID); &lt;input @(isChecked ? "checked=\"checked\" " : "")type="checkbox" value="@d.ID" /&gt; // more stuff here } </code></pre> <p>Changing this to use an actual List makes the whole thing work as expected:</p> <pre><code>var tmp = Model.Project.Domains.ToList(); @foreach (var d in Model.AllDomains) { bool isChecked = tmp.Contains(d.ID); &lt;input @(isChecked ? "checked=\"checked\" " : "")type="checkbox" value="@d.ID" /&gt; // more stuff here } </code></pre> <p>The following is the model that is bound to my view (again simplified to make it more readable):</p> <pre><code>public ProjectVM GetByID(int id) { return new ProjectVM { Project = new Project { ... // Other properties here Domains = from d in MyObjectModel.Projects[id].Domains select d.ID }, AllDomains = from d in MyObjectModel.Domains orderby d.Name select new { ID = d.ID, Name = d.Name } }; } </code></pre> <p>Now, while from debugging I know that <code>Model.Project.Domains</code> will contain the correct number of entries, as well as the correct values, calling .Contains() on the method returns an <strong>arbitrary result</strong> - either true or false.</p> <p>In fact, if I put the line with the Contains() call into the debugger's "Watch" tab multiple times, even with an hard coded argument (e.g. 4) <strong>the result will alternate from <code>true</code> to <code>false</code></strong> with every call.</p> <p><strong>What is happening here, what am I overlooking?</strong></p> <p>Because of the way that Model.Project.Domains is instantiated, its actual type is a <code>WhereSelectEnumerableIterator&lt;T&gt;</code>, but this implements <code>IEnumerable&lt;T&gt;</code> so that shouldn't be an issue...</p>
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