HTML
    
 
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>
 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
 <html xmlns="http://www.w3.org/1999/xhtml">
 <head runat="server">
   <title>AutoComplete Extender</title>
 </head>
 <body>
   <form runat="server">
   <div>
   <b style = "color:Red;">Enter Customer Name</b>
   <asp:TextBox ID="txtContactsSearch" runat="server"></asp:TextBox>
   <asp:AutoCompleteExtender ID="txtContactsSearch_AutoCompleteExtender" MinimumPrefixLength="1"
   ServiceMethod="SearchCustomers" CompletionInterval="100" EnableCaching="false"
   CompletionSetCount="10" TargetControlID="txtContactsSearch" runat="server" FirstRowSelected="false">
   </asp:AutoCompleteExtender>
   <asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true">
   </asp:ScriptManager>
   </div>
   </form>
 </body>
 </html>
   
      
       
  CODE BEHIND
    
 
using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Web;
 using System.Web.UI;
 using System.Web.UI.WebControls;
 using System.Data;
 public partial class AutoCompleteExtender : System.Web.UI.Page
 {
   protected void Page_Load(object sender, EventArgs e)
   {
  
   }
   private static List<Customer > GetStudent()
   {
   List<Customer > Customers = new List<Customer >()
    {
     new Customer  {Name = "Roman"},
     new Customer  {Name = "Iqbal"},
     new Customer  { Name = "Amin"},
     new Customer  {  Name = "Asad"},
     new Customer  {  Name = "Abul"},
     new Customer  {  Name = "Abaden"},
     new Customer  { Name = "M Ali"},
     new Customer  {  Name = "Ashikur Rhaman"},
     new Customer  {  Name = "Abdul"},
     new Customer  {  Name = "Asif"},
     new Customer  {  Name = "Aminur"},
     new Customer  {  Name = "Arifur Rahman"},
     new Customer  {  Name = "Asgor"},
     new Customer  {  Name = "Abul Momen"}  
  
   };
   return Customers;
   }
   public class Customer 
   {
    
     public string Name { get; set; }
       
   }
   [System.Web.Services.WebMethodAttribute(), System.Web.Script.Services.ScriptMethodAttribute()]
   public static List<string> SearchCustomers(string prefixText, int count)
   {
   List<Customer > CustomeList = GetStudent();
   var query = from m in CustomeList
   
   select m.Name.ToString();
   try
   {
   return (from customer in query 
   where customer.ToLower().StartsWith(prefixText.ToLower())
   select customer).Take (count).ToList<string>();
  
   }
   catch (Exception ex )
   {
  
   throw new Exception("Problem Loading in finding customer" + ex.Message);
   }  
  
   }
  
 }