https://mongodb.github.io/mongo-csharp-driver/2.4/apidocs/html/P_MongoDB_Bson_BsonValue_AsBsonJavaScriptWithScope.htm
BsonValue.AsBsonJavaScriptWithScope Property
Casts the BsonValue to a BsonJavaScriptWithScope (throws an InvalidCastException if the cast is not valid).
bsonvalueproperty