Asp.net login script

[iStyling]

i'm new to asp.net, i'm writing a login & registration script for learning database application. But the script seems not work. it stills can add duplicated username. Here is the script\[code\]using System;using System.Collections.Generic;using System.Linq;using System.Web;using System.Web.UI;using System.Web.UI.WebControls;using System.Data.SqlClient;using System.Configuration;public partial class Registration : System.Web.UI.Page{ static string temp; protected void Page_Load(object sender, EventArgs e) { if (IsPostBack) { SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["earchConnectionString"].ConnectionString); con.Open(); string cmdStr = "Select count(*) from [user] where UserName='" + TextBoxUN.Text + "'"; SqlCommand userExist = new SqlCommand(cmdStr, con); int temp = Convert.ToInt32(userExist.ExecuteScalar().ToString()); con.Close(); if (temp == 1) { Response.Write("User Name Already Exist....<br /> Please Choose Another User Name."); } } } protected void Submit_Click(object sender, EventArgs e) { SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["earchConnectionString"].ConnectionString); con.Open(); string insCmd = "Insert into [user] (UserName, Password, EmailAddress, FullName, level) values (@UserName,@Password,@EmailAddress, @FullName, @level)"; SqlCommand insertUser = new SqlCommand(insCmd, con); insertUser.Parameters.AddWithValue("@UserName", TextBoxUN.Text); insertUser.Parameters.AddWithValue("@Password", TextBoxPass.Text); insertUser.Parameters.AddWithValue("@EmailAddress", TextBoxEA.Text); insertUser.Parameters.AddWithValue("@FullName", TextBoxFN.Text); insertUser.Parameters.AddWithValue("@level", level.SelectedValue.ToString()); try { insertUser.ExecuteNonQuery(); con.Close(); //Response.Redirect("Login.aspx"); Label1.Text = temp; } catch (Exception er) { Response.Write("Something wrong"); } finally { //Any Special Action You Want To Add } }}\[/code\]Any can detect the problems?thanks