Could not load file or assembly 'AjaxControlToolkit' or one of its dependencies.

I am always getting this type of errors while publishing the sites like Could not load file or assembly 'AjaxControlToolkit' or one of its dependencies. like

Line 1:  <%@ Page Language="C#" AutoEventWireup="true" CodeFile="index.aspx.cs" Inherits="index" %>
Line 2:  
Line 3: <%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>Line 4:  
Line 5:  

To resolve this issue i had followed these steps, might be this will helpful to others also

1. Registered the assembly by using the following code:

<%@ Register Assembly="AjaxControlToolkit"
Namespace="AjaxControlToolkit" TagPrefix="ajaxControl" %>

You have already done the first step.

2. Copy the AjaxControlToolkit.dll into the /bin folder of the web site application or download library file from this location Download.

3. You must have included ScriptManager in your files

4. If Yet this problem isn't resolved please add/replace this line in your config file
<add assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>