Hey Buddy,
It might be possible that you are missing something while applying the Ajax.
Please check all the things in this way:
1- If you want to apply Ajax 4.5 then you can download it from here:
http://www.c-sharpcorner.com/downloads/746/ajaxcontroltoolkit-binary-4-5.aspx[
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2- Now right click on "Reference" which would be present just below your Application Name and choose to "Add Reference".
3- Now Browse to the location where your Ajax dll is downloaded and add it.
4- Now go to your Toolbox and right click on Ajax Tools to choose Items.
5- Now it will take time to show all the Tools, Browse to the Ajax Toolkit dll and add it, add all the Tools of Ajax.
6- Now pass the reference of Ajax Toolkit just below the code where you page has been called, like this:
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm1.aspx.cs" Inherits="WebApplication26.WebForm1" %>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
<!DOCTYPE html>
7- Now you can use Ajax tools, they will not provide any error and will work perfectly.
Accept the answer if it helps you