{"ReadString cannot be called when BsonType is: ObjectId"} was the error I received a few times while learning MongoDB.

I was getting this often until I saw  a posting on Google Groups which reminded me that we need to be type conscious.  I was setting the “_id” type to string.  Realize that it needs to be set to ObjectId (which is a MongoDB C# driver type).

   1:      public class Thing

   2:      {

   3:          public Thing() { }

   4:          public string Id { get; set; }

   5:          public string author { get; set; }

   6:          public string title { get;set; }     

   7:      }

Change line #4 from “string” to “ObjectId” and you’re set.