public string ChangeDoctorPassword(ChangePassword changePassword)
        {
            bool validOldPassword = aDoctorGateway.IsOldPasswordValid(changePassword.DoctorId, changePassword.OldPassword);

            if (validOldPassword)
            {
                int rowAffected = aDoctorGateway.ChangeDoctorPassword(changePassword);
                if (rowAffected > 0)
                {
                    return("Success");
                }
                else
                {
                    return("Your password changing failed.");
                }
            }
            else
            {
                return("Your old password is wrong. Try again!");
            }
        }