Populating a drop down list using AJAX

I have 3 drop down boxes, created using the HTML select tag. On page load, the first box has a few names. Now, when I click on one of the names in the first box, some more names should appear in the second and when I click on a name in the second box, some more names should appear in the third box. How can I achieve this using AJAX? I have to use ASP.Net and MS SQL Server only. I'm a complete noob to AJAX and I've been educating myself about it,开发者_StackOverflow but nothing's working out. I've been looking for the code for close to a week. I looked up w3schools.com, but when I tried that code, it didn't work. Please help me out and please tell me step by step, the things required in order to make it work, and what goes where. I have a deadline that is fast approaching and am at my wit's end trying to make it work. HELP ME!!

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I would recommend placing the three dropdownlists in an UpdatePanel and adding a trigger to each for the updatepanel. Then, when the value changes, re-populate the dropdown and let the updatepanel push the update. Also keeps session-state in case you want to submit the values.

I don't have a compiler in front of me, but if need be just post a comment and I'll post the code tomorrow.

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Working Example

I'm using the "Traditional template" that comes with visual studio and the master page, so excuse the content place holders. But this should demonstrate what I mean:

<%@ Page Title="" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true" CodeBehind="MultiDropDown.aspx.cs" Inherits="AppSettingsRetrieval.MultiDropDown" %>
<asp:Content ID="Content1" ContentPlaceHolderID="HeadContent" runat="server">
    <script runat="server">
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!Page.IsPostBack)
            {
                String[] options = new[] { "ABC", "DEF", "GHI", "JKL", "MNO", "PQR", "STU", "VWX", "YZA" };
                foreach (String option in options)
                {
                    this.DropDownList1.Items.Add(new ListItem(option, option));
                }
            }
        }

        public void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
        {
            this.DropDownList2.Items.Clear();
            this.DropDownList2.Items.Add(new ListItem("--- Please Select One ---"));

            String[] options = new[] { "123", "456", "789", "101", "112", "131", "415", "161", "718" };
            foreach (String option in options)
            {
                var str = String.Format("{0} {1}", this.DropDownList1.SelectedValue, option);
                this.DropDownList2.Items.Add(new ListItem(str, str));
            }
            this.DropDownList2.Enabled = true;

            this.DropDownList3.Enabled = false;
        }

        public void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)
        {
            this.DropDownList3.Items.Clear();
            this.DropDownList3.Items.Add(new ListItem("--- Please Select One ---"));

            String[] options = new[] { "test", "testing", "tester", "foo", "bar", "foobar" };
            foreach (String option in options)
            {
                var str = String.Format("{0} {1}", this.DropDownList2.SelectedValue, option);
                this.DropDownList3.Items.Add(new ListItem(str, str));
            }
            this.DropDownList3.Enabled = true;
        }
    </script>
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
    <asp:ScriptManager runat="server" ID="ScriptManager1"></asp:ScriptManager>

    <asp:UpdatePanel runat="server" ID="UpdatePanel1" UpdateMode="Conditional">
        <ContentTemplate>
            <fieldset>
                <legend>Consecutive Dropdown List</legend>
                <p>
                    Primary Filter:
                    <asp:DropDownList runat="server" ID="DropDownList1" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged" AutoPostBack="true">
                        <asp:ListItem Text="---Please Select One---" />
                    </asp:DropDownList>
                </p>
                <p>
                    Secondary Filter:
                    <asp:DropDownList runat="server" ID="DropDownList2" OnSelectedIndexChanged="DropDownList2_SelectedIndexChanged" AutoPostBack="true" Enabled="false">
                        <asp:ListItem Text="---Please Select One---" />
                    </asp:DropDownList>
                </p>
                <p>
                    Final Filter:
                    <asp:DropDownList runat="server" ID="DropDownList3" Enabled="false">
                        <asp:ListItem Text="---Please Select One---" />
                    </asp:DropDownList>
                </p>
            </fieldset>
        </ContentTemplate>
    </asp:UpdatePanel>
</asp:Content>