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POReturn an empty collection when Linq where returns nothing
 

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    <p>I am using the below statement with the intent of getting all of the machine objects from the <code>MachineList</code> collection (type IEnumerable) that have a <code>MachineStatus</code> of <em>i</em>. The <code>MachineList</code> collection will not always contain machines with a status of <em>i</em>.</p> <p>At times when no machines have a <code>MachineStatus</code> of <em>i</em> I'd like to return an empty collection. My call to <code>ActiveMachines</code> (which is used first) works but <code>InactiveMachines</code> does not.</p> <pre><code>public IEnumerable&lt;Machine&gt; ActiveMachines { get { return Customer.MachineList .Where(m =&gt; m.MachineStatus == "a"); } } public IEnumerable&lt;Machine&gt; InactiveMachines { get { return Customer.MachineList .Where(m =&gt; m.MachineStatus == "i"); } } </code></pre> <p><strong>Edit</strong></p> <p>Upon further examination it appears that any enumeration of <code>MachineList</code> will cause subsequent enumerations of <code>MachineList</code> to throw an exeception: <code>Object reference not set to an instance of an object</code>.</p> <p>Therefore, it doesn't matter if a call is made to <code>ActiveMachines</code> or <code>InactiveMachines</code> as its an issue with the <code>MachineList</code> collection. This is especially troubling because I can break calls to <code>MachineList</code> simply by enumerating it in a Watch before it is called in code. At its lowest level <code>MachineList</code> implements <code>NHibernate.IQuery</code> being returned as an <code>IEnumerable</code>. What's causing <code>MachineList</code> to lose its contents after an initial enumeration?</p>
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    1. CONo, nothing special happens at enumeration; it would be interesting to see what "inactive" etc are in the debugger. I also wonder whether (for example) `MachineStatus` is a facade property that is throwing an exception - i.e. `public string MachineStatus { get {return someInnerField.Status;}}` and `someInnerField` is `null`.
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      1. USMarc Gravell
 

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