mongodb/mongo-csharp-driver — pull request #136 by brianbanet (2012-10-24 00:29:57+00:00)
Title: Adding support for implicit convert of ObjectId to String

By being able to support conversion to String, you insulate any wrapper/test classes from having to directly reference the BSON assembly. Your domain class can expose a String property called ObjectId which simply returns the Id property as a string.