title: Add items to a collection if the collection does NOT already contain it by comparing a property of the items?
body: <p>Basically, how do I make it so I can do something similar to: <code>CurrentCollection.Contains(...)</code>, except by comparing if the item's property is already in the collection?</p>
  
  <pre><code>public class Foo
  {
      public Int32 bar;
  }
  
  
  ICollection&lt;Foo&gt; CurrentCollection;
  ICollection&lt;Foo&gt; DownloadedItems;
  
  //LINQ: Add any downloaded items where the bar Foo.bar is not already in the collection?
  </code></pre>