title: how to manage _id field when using POCO with mongodb c# driver
body: <p>If I want to read and write mongo data with a POCO</p>
  
  <pre><code>public class Thingy
  {
       public string Foo {get;set;}
  }
  ...
  coll.Insert(new Thing(Foo = "hello"));
  </code></pre>
  
  <p>When I read back I get a failure saying that _id is an unexpected attribute (which it is). So then I added a field called _id to the class. Now the insert doesnt work saying that the _id field cannot be null. A tried BsonIgnoreIfNull attribute, that didnt work.</p>