C # locking an account on a specified computer only

Add a field mac in the database to write mac information or IP information of the corresponding user to this field. Use the following code to bind the account to the computer! Using System. Security. Cryptography;) must be added to the md5 encryption password in red ;): String username = tb_username.Text.ToString (). Trim (); // Byte [] result = Encoding. Default. GetBytes (this. tb_password.Text.Trim ()); // MD5 = new MD5CryptoServiceProvider (); // Byte [] output = md5.ComputeHash (result ); // String userpass = BitConverter. ToString (output). Replace ("-",""); String userpass = tb_password.Text.ToString (). Trim (); String SQL = "select id, username, userpass, mac from p_users where username = '" + username + "' and userpass = '" + userpass + "'"; SqlCommand cmd = new SqlCommand (SQL, conn ); Try { Conn. Open (); SqlDataReader dr = cmd. ExecuteReader (); If (dr. Read ()) { String mac = getMacAddr_Local (); // string ip = GetIP (); if (mac! = Dr ["mac"]. ToString ()) { MessageBox. Show ("Please use this program on the Right computer! "); Application. Exit (); } Else { Id = dr ["id"]. ToString (); // Name = dr ["truename"]. ToString (); // Userrole = dr ["userrole"]. ToString (); This. Close (); }} Else { MessageBox. Show ("incorrect user name or password! "," Error ", MessageBoxButtons. OK ); } } Catch (Exception ex) { Ex. ToString (); } Finally { Conn. Close (); }