now I fetch dbref by following code:
BsonDocument doc = test.FindOne();
var l = doc.Elements.ElementAt(1);
var linkr=l.Value;//dbref there
MongoDBRefSerializer ser=new MongoDBRefSerializer();
BsonReader reader = BsonReader.Create(linkr.ToJson());
var refrefref= ser.Deserialize(reader, typeof(MongoDBRef), new opt());
MongoDBRef a = (MongoDBRef)refrefref;
var referenced = db.FetchDBRef(a);

opt class
class opt : IBsonSerializationOptions
{
public void ApplyAttribute(IBsonSerializer serializer, Attribute attribute)

{ throw new NotImplementedException(); }

public IBsonSerializationOptions Clone()
{ throw new NotImplementedException(); }

public IBsonSerializationOptions Freeze()

{ throw new NotImplementedException(); }

}
i dont know why code so long and there is a lot of unnessesary types?

i think it would be better to add following method to fetchdbref:
BsonDocument doc = test.FindOne();
var l = doc.Elements.ElementAt(1);
var linkr=l.Value;//dbref there
MongoDBRef refer = linkr.AsDBRef() //
var profit = db.FetchDBRef(refer);

if possible.
thank you.