title: Can't deserialize from ObjectId to String in a MongoDb collection query
body: <p>Whenever I am trying to get a list of Ids from a collection:</p>
  
  <pre><code>var messages = db.GetCollection&lt;MessageExchange&gt;(collectionName);
  var ids = messages
     .AsQueryable()
     .Where(_=&gt; true)
     .Select(x =&gt; x.Id)
     .ToList();
  </code></pre>
  
  <p>I am getting a cast error:</p>
  
  <blockquote>
    <p>System.FormatException   HResult=0x80131537   Message=Cannot
    deserialize a 'String' from BsonType 'ObjectId'.   Source=   StackTrace: </p>
  </blockquote>
  
  <p>I am not able to modify the definition of a class, because its implemented in a package I can't change. Therefore the solution of putting: </p>
  
  <pre><code>[BsonRepresentation(BsonType.ObjectId)] 
  </code></pre>
  
  <p>on it, is <strong><em>not possible</em></strong>. Based on <a href="https://stackoverflow.com/q/26868367/1257607">this</a>.</p>
  
  <p>Is it possible to cast this in the Linq query?</p>