<asp:DropDownList id="MyDrop" runat="server" 
       onselectedindexchanged="MyDrop_SelectedIndexChanged">
</asp:DropDownList>

Triggers="{ControlID:btnSubmit;Parameter:},{ControlID:btnReset;Parameter:},{ControlID:MyDrop;Parameter:}"

private void Page_Load(object sender, System.EventArgs e)
{
            TextBox1.Style["width"] = "200px";
            if (!CallbackPanel1.IsCallbackByMe)
            {
                MyDrop.Items.Add("Uno");
                MyDrop.Items.Add("Due");
                MyDrop.Items.Add("Tre");
            }
}

<%@ Control language="C#" AutoEventWireup="false" CodeFile="Demo.ascx.cs" Inherits="EO.Web.Demo.Demos.Callback.Features.Auto_Disable_Contents.Demo" %>
<%@ Register TagPrefix="eo" NameSpace="EO.Web" Assembly="EO.Web" %>
<p>EO.Web CallbackPanel can automatically disable its contents during callback. 
	Enter something and click "Submit" to see it in action.
</p>
<eo:callbackpanel id="CallbackPanel1" runat="server" LoadingHTML="Processing data, this may take a while..."
	Height="160px" 
    Triggers="{ControlID:btnSubmit;Parameter:},{ControlID:btnReset;Parameter:},{ControlID:MyDrop;Parameter:}" LoadingPanelID="panel1"
	AutoDisableContents="True">
	<asp:Panel id="panResult" Runat="server" Visible="False">
		<P>
			<asp:Label id="lblResult" Runat="server"></asp:Label></P>
		<asp:Button id="btnReset" Text="Try Again" Runat="server"></asp:Button>
	</asp:Panel>
	<asp:Panel id="panForm" Runat="server">
		<TABLE style="BORDER-COLLAPSE: collapse" borderColor="gainsboro" cellPadding="2" width="400"
			border="1">
			<TR>
				<TD colSpan="2">An Important Form - Can Not Be Submitted Twice</TD>
			</TR>
			<TR>
				<TD>Important Text:</TD>
				<TD>
					<asp:TextBox id="TextBox1" runat="server" MaxLength="100"></asp:TextBox></TD>
			</TR>
			<TR>
				<TD>Important Choice:</TD>
				<TD>
					<asp:DropDownList id="DropDownList1" runat="server">
						<asp:ListItem Value="Choice 1">Important Choice 1</asp:ListItem>
						<asp:ListItem Value="Choice 2">Important Choice 2</asp:ListItem>
					</asp:DropDownList></TD>
			</TR>
			<TR>
				<TD style="PADDING-LEFT: 10px; PADDING-TOP: 10px" colSpan="2">
					<asp:Button id="btnSubmit" Text="Submit" Runat="server" >
					</asp:Button></TD>
			</TR>
		</TABLE>
		<asp:DropDownList id="MyDrop" runat="server" 
            onselectedindexchanged="MyDrop_SelectedIndexChanged">
		</asp:DropDownList>
		
	</asp:Panel>
	<asp:Panel id="Panel1" Runat="server"></asp:Panel>
</eo:callbackpanel>

namespace EO.Web.Demo.Demos.Callback.Features.Auto_Disable_Contents
{
	using System;
	using System.Data;
	using System.Drawing;
	using System.Web;
	using System.Web.UI.WebControls;
	using System.Web.UI.HtmlControls;

	/// <summary>
	///		Summary description for Demo.
	/// </summary>
	public partial class Demo : System.Web.UI.UserControl
	{
		private void Page_Load(object sender, System.EventArgs e)
		{
			TextBox1.Style["width"] = "200px";
		}

		#region Web Form Designer generated code
		override protected void OnInit(EventArgs e)
		{
			//
			// CODEGEN: This call is required by the ASP.NET Web Form Designer.
			//
			InitializeComponent();
			base.OnInit(e);
            MyDrop.Items.Add("Uno");
            MyDrop.Items.Add("Due");
            MyDrop.Items.Add("Tre");
        }
		
		/// <summary>
		///		Required method for Designer support - do not modify
		///		the contents of this method with the code editor.
		/// </summary>
		private void InitializeComponent()
		{
			this.btnReset.Click += new System.EventHandler(this.btnReset_Click);
			this.btnSubmit.Click += new System.EventHandler(this.btnSubmit_Click);
			this.Load += new System.EventHandler(this.Page_Load);
		}
		#endregion

		private void btnSubmit_Click(object sender, System.EventArgs e)
		{
			//Make the process to take a while
			System.Threading.Thread.Sleep(2000);

			panForm.Visible = false;
			panResult.Visible = true;
			lblResult.Text = string.Format("Your random confirmation number is {0}.", new Random().Next());
		}

		private void btnReset_Click(object sender, System.EventArgs e)
		{
			panResult.Visible = false;
			panForm.Visible = true;
		}
        
        protected void MyDrop_SelectedIndexChanged(object sender, EventArgs e)
        {
            TextBox1.Text = MyDrop.SelectedValue;
        }
}
}