MongoCollection<T>.FindAndModify doesn't work when updating an whole document.
The following code does no change to the database:
Document query =
new Document()
{

{ "_id", new Oid("4c7541d563804c1680000001") }

};
mongo["DB"].GetCollection<MyClass>("Collection").FindAndModify(theModifiedInstance, query);

On the other side, the following (supposedly equivalent) works fine:
Document command =
new Document()
{

{"findandmodify", "Collection"}

,

{"query", query}

,

{"update", theModifiedInstance}

,

{"new", false}

,

{"upsert", false}

};
mongo["DB"].SendCommand(command);

The bug seems to be on line 217 of MongoCollection_1.cs (as per version given in "Environment"):

{"update", EnsureUpdateDocument(document)}

EnsureUpdateDocument will create a new document with a $set. Clearly, from the example above, this should not be done.