using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Security.Cryptography;
//  in Future TODO:
//  *   MD5-hashing password of users;
//  *   SQL-users, -logins;
//  *   create users in Windows-Forms;
namespace gild
{

    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Form2 f2 = new Form2();
            string log, pass;
            string username=null, code=null;

            SqlConnection conn = new SqlConnection("Data Source=USER12\\SQLEXPRESS;Initial Catalog=testdata;Integrated Security=True;");
            conn.Open();
            SqlCommand cmd = new SqlCommand("select * from testdata.dbo.logins where mode='true'", conn);
            SqlDataReader reader = cmd.ExecuteReader();
            while (reader.Read())
            {
                username = (string)reader[0];
                code = (string)reader[1];

                log = username.ToString();
                pass = code.ToString();
                //  *   *   *   *   *   *   *   *   *   *
               //   *   gethash
                MD5 md5Hasher = MD5.Create();
                byte[] data = md5Hasher.ComputeHash(Encoding.Default.GetBytes(textBox2.Text));

                StringBuilder sBuilder = new StringBuilder();

                for (int i = 0; i < data.Length; i++)
                 {
                    sBuilder.Append(data[i].ToString("x2"));
                 }
                string hashkey = sBuilder.ToString();
                //  *   ending gethash
                //  *   *   *   *   *   *   *   *   *   *

                //  *   verifyhash

                StringComparer comparer = StringComparer.OrdinalIgnoreCase;

                 if (0 == comparer.Compare(hashkey, pass))
                    {
            ...блабла ...еще дописываю )))
            }