Following the question here: http://stackoverflow.com/questions/12088370/partial-update-in-mongodb-c-sharp-driver-dictionary-issue

The problem occurs when trying to do a partial update (using UpdateBuilder) to an object which holds a dictionary, no matter what types of key-value. The dictionary is not serializing well (at least not as expected) when using ToBsonDocument().
When trying to fetch the object after the update, we get de-serialization errors (depends on the type), the one I get in the test program is: Input string was not in the correct format.