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POLINQ Except operator and object equality
 

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    <p>Here is an interesting issue I noticed when using the <code>Except</code> Operator: I have list of users from which I want to exclude some users:</p> <p>The list of users is coming from an XML file:</p> <p> </p> <p></p> <p>The code goes like this:</p> <pre><code>interface IUser { int ID { get; set; } string Name { get; set; } } class User: IUser { #region IUser Members public int ID { get; set; } public string Name { get; set; } #endregion public override string ToString() { return ID + ":" +Name; } public static IEnumerable&lt;IUser&gt; GetMatchingUsers(IEnumerable&lt;IUser&gt; users) { IEnumerable&lt;IUser&gt; localList = new List&lt;User&gt; { new User{ ID=4, Name="James"}, new User{ ID=5, Name="Tom"} }.OfType&lt;IUser&gt;(); var matches = from u in users join lu in localList on u.ID equals lu.ID select u; return matches; } } class Program { static void Main(string[] args) { XDocument doc = XDocument.Load("Users.xml"); IEnumerable&lt;IUser&gt; users = doc.Element("Users").Elements("User").Select (u =&gt; new User { ID = (int)u.Attribute("id"), Name = (string)u.Attribute("name") } ).OfType&lt;IUser&gt;(); //still a query, objects have not been materialized var matches = User.GetMatchingUsers(users); var excludes = users.Except(matches); // excludes should contain 6 users but here it contains 8 users } } </code></pre> <p>When I call <code>User.GetMatchingUsers(users)</code> I get 2 matches as expected. The issue is that when I call <code>users.Except(matches)</code> The matching users are not being excluded at all! I am expecting 6 users ut "excludes" contains all 8 users instead.</p> <p>Since all I'm doing in <code>GetMatchingUsers(IEnumerable&lt;IUser&gt; users)</code> is taking the <code>IEnumerable&lt;IUser&gt;</code> and just returning the <code>IUsers</code> whose ID's match( 2 IUsers in this case), my understanding is that by default <code>Except</code> will use reference equality for comparing the objects to be excluded. Is this not how <code>Except</code> behaves?</p> <p>What is even more interesting is that if I materialize the objects using <code>.ToList()</code> and then get the matching users, and call <code>Except</code>, everything works as expected!</p> <p>Like so:</p> <pre><code>IEnumerable&lt;IUser&gt; users = doc.Element("Users").Elements("User").Select (u =&gt; new User { ID = (int)u.Attribute("id"), Name = (string)u.Attribute("name") } ).OfType&lt;IUser&gt;().ToList(); //explicity materializing all objects by calling ToList() var matches = User.GetMatchingUsers(users); var excludes = users.Except(matches); // excludes now contains 6 users as expected </code></pre> <p>I don't see why I should need to materialize objects for calling <code>Except</code> given that its defined on <code>IEnumerable&lt;T&gt;</code>?</p> <p>Any suggesstions / insights would be much appreciated.</p>
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