Enum Fields { Unknown = 0, Username = 1 << 0, Password = 1 << 1, Email 1 << 2...etc }
Dictionary<Fields, List<string>> errors = new Dictionary<Fields, List<string>>();
Fields mask = Fields.UserName | Fields.Password;
errors[mask] = "Input is empty";
List<string> errMsgs = errors[Fields.UserName];
// to get a list of all error messages associated with Fields.UserName
// including the one just entered.  A lookup on Fields.Password would
// return the same entry