This appears to occur with any projection built with Builders<T>.Projection.

The model class has an Id member which is a string, but no annotation attributes, for example:

public class SomeClass { public string Id

{get;set;} public string Name {get;set;}

}

var result = _collection.FindOneAndUpdateAsync(filter, updateDef, new FindOneAndUpdateOptions
{
IsUpsert = true,
Projection = Builders<T>.Projection.Expression(u => new

{ U.Id }

),
ReturnDocument = ReturnDocument.Before
}).Result;

The exception is thrown in StringSerializer.DeserializeValue. In this case, the local variable bsonType is ObjectId, and _representation is String.

Am I correct in thinking a check for this, followed by a call to bsonReader.ReadString(), would be the fix?

If so, I'd like to take the opportunity to build a Pull Request, but am unsure of the testing/submission guidelines, since the page linked to in the docs either no longer exists or has moved from where GitHub points to. Could you advise please?