Newtonsoft.Json.JsonSerializationException: 'Error converting value "5b5b4df5b8e2e208efb2c0da" to type 'MongoDB.Bson.ObjectId'. Path 'Id', line 1, position 64.'
InvalidCastException: Invalid cast from 'System.String' to 'MongoDB.Bson.ObjectId'.

public class TblUser
{
    public ObjectId Id { get; set; }
    public string Username { get; set; }
    public string Password { get; set; }
}

public static string ObjToStr(Object obj)
{
    return JsonConvert.SerializeObject(obj);
}

public static T StrToObj<T>(string str)
{
    return JsonConvert.DeserializeObject<T>(str);
}