Sql命令语法错误:System.Data.SqlClient。SqlException:“靠近','的语法不正确。”

我正在用C语言构建一个销售点系统,并通过一个对象将数据从接口传递到数据库。

private void btnUpdate_Click(object sender, EventArgs e) //UPDATE FUNCTION//
    {
            user_management_system user_mgnt = new user_management_system();
            user_mgnt.Username = txt_userName.Text;
            user_mgnt.Password = txt_password.Text;
            user_mgnt.First_name = txt_firstName.Text;
            user_mgnt.Last_name = txt_lastName.Text;
            user_mgnt.Nationality = txt_nationality.Text;
            user_mgnt.Email = txt_email.Text;
            user_mgnt.Age = (txt_age.Text);
            con.Open();
            SqlCommand cmd = con.CreateCommand();
            cmd.CommandType = CommandType.Text;
            cmd.CommandText = "UPDATE userlogin SET password = ('"+ user_mgnt.Password + "',first_name='" + user_mgnt.First_name + "',last_name='" + user_mgnt.Last_name + "',age='" + user_mgnt.Age +
                          "',nationality='" + user_mgnt.Nationality + "',email='" + user_mgnt.Email + "', WHERE username ='"+ user_mgnt.Username+ "')";
           cmd.ExecuteNonQuery();
            con.Close();
            MessageBox.Show("Data has been successfuly updated");
            displaydata();