UserManager: a class to manipulate local Windows users and groups

using System;
using System.Collections.Generic;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace MBeatini
{
    public partial class FormAddUser : Form
    {
        public FormAddUser()
        {
            InitializeComponent();

            // fill the combobox
            UserManager um = new UserManager();
            ArrayList al = um.ListGroupsInServer();
            al.Sort();
            for (int i = 0; i < al.Count; i++)
                comboBox1.Items.Add(al[i]);
            um = null;
        }

        private void button2_Click(object sender, EventArgs e)
        {
            Close();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (textBox1.Text.Trim().Length == 0)
            {
                MessageBox.Show("Username: mandatory field!", "Warning", MessageBoxButtons.OK);
                textBox1.Focus();
            }
            else if (textBox2.Text.Trim().Length == 0)
            {
                MessageBox.Show("Password: mandatory field!", "Warning", MessageBoxButtons.OK);
                textBox1.Focus();
            }
            else
            {
                UserManager um = new UserManager();
                if (!um.AddUser(textBox1.Text, textBox2.Text, textBox3.Text, comboBox1.SelectedItem.ToString()))
                    MessageBox.Show(um.ErrorMessage, "Warning", MessageBoxButtons.OK);
                else
                    MessageBox.Show("User " + textBox1.Text + " created", "Warning", MessageBoxButtons.OK);

                um = null;
            }
        }
    }
}