Preventing duplicate User Names with ASP.NET and jQuery

It's a common problem: you have a registration form, but you want to prevent user names or other values from being used more than once. You need a user-friendly way to prevent duplicate values being submitted. This is where the simplicity of jQuery excels.

User names and passwords are normally stored within a database. Commonly, developers wait until the form has been submitted before they perform any duplicate checking. This means that the user has to wait for a postback to complete before being informed that the user name they chose is already in use, and that they need to choose another one. Ajax tidies this up by allowing asynchronous querying of databases, so that the checking can be done behind the scenes while the user is still completing the registration form. I choose jQuery for my AJAX instead of ASP.NET AJAX because it is so much simpler to use in my opinion.

This example shows a very simple registration form:


<form id="form1" runat="server"> 

<div id="register"> 



    <div class="row">

      <span class="label">User Name:</span>

      <asp:TextBox ID="UserName" runat="server"></asp:TextBox><span id="duplicate"></span>


    <div class="row">

      <span class="label">Password:</span>

      <asp:TextBox ID="Password" runat="server"></asp:TextBox>


    <div class="row">

      <span class="label">&nbsp;</span>

      <asp:Button ID="btnRegister" runat="server" Text="Register" />






A web service will be used to house the method that checks the database for possible duplicates:


using System;

using System.Collections.Generic;

using System.Linq;

using System.Web;

using System.Web.Services;

using System.Web.Script.Services;

using System.Data.SqlClient;


/// <summary>

/// Summary description for UserService

/// </summary>

[WebService(Namespace = "")]

[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]


// To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line.


public class UserService : WebService {


    public UserService () {




    public int CheckUser(string username)


      string connect = @"Server=SERVER;Database=Database;Trusted_Connection=True;";

      string query = "SELECT COUNT(*) FROM Users WHERE UserName = @UserName";

      using(SqlConnection conn = new SqlConnection(connect))


        using(SqlCommand cmd = new SqlCommand(query, conn))


          cmd.Parameters.AddWithValue("UserName", username);


          return (int)cmd.ExecuteScalar();






There's nothing clever about this code. It is trimmed down to just show the working parts, and ignores error checking, for example - although it makes use of parameters to prevent SQL Injection. IF you are wondering why I don't close the connection after I'm done, that's what the using statement does for me behind the scenes. Any disposable object (one that implements IDisposable) can be instantiated within a using block which then takes care of Close() and Dispose() at the end of the block. One (Web) method - CheckUser() - accepts a string as an argument and then returns the number of rows in the database where that name is found. The [ScriptService] attribute is uncommented, so that the service is available to AJAX calls (not just ASP.NET AJAX, incidentally).

Now we'll look at the Javascript that uses jQuery to effect the call:


<script type="text/javascript" src=""></script> 

<script type="text/javascript"> 

  $(function() {




  function checkUser() {


      type: "POST",

      url: "UserService.asmx/CheckUser",

      data: {username: $('#UserName').val()},

      success: function(response) {


        if (response.d != "0") {

          $("#duplicate").html(' That user name has already been taken');








The first <script> tag brings in the jQuery library from Google's public code repository. Then an event handler is added to the element with the ID of UserName, which is the TextBox waiting for the user to put their chosen User Name in. The handler is wired up to the TextBox's onchange event, and calls the checkUser() function. Then the checkUser() function is defined in code.

When the user has added some text to the TextBox and then moves their cursor away, the "change" event is fired, and an AJAX call is made to the web service. If the response is not 0, then the web method has found at least one row that matches the user name that the user is attempting to submit. The <span> with the ID of "duplicate" is emptied of any messages resulting from previous attempts, and the message displayed to the user.

I've used ASP.NET controls for the inputs in the registration form, but jQuery is not an ASP.NET component (although it has been embraced by the ASP.NET team). So this approach will work with any server-side technology. One thing to note, though - if you place the registration form within a container, such as a ContentPlaceHolder in a Master Page, you will need to change the jQuery code that references its controls to counter the effects of INamingContainer (which is the bit that adds stuff to the ID of the control, such as ctl00_ContentPlaceHolder1_ControlID). The change needed is to use the control's ClientID property, so the first 3 lines of jQuery code will look like this:


  $(function() {

    $("#<%= UserName.ClientID %>").change(checkUser);



And of course, the ClientID will need to be used where controls are referenced in the the checkUser() function too.