If you have more than one version of AJAX Tool Kit in your System , there might be problem while developing an AJAX based pages ,
Error Message
Error 2 Assembly 'AjaxControlToolkit, Version=3.0.30512.17815, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e' uses 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' which has a higher version than referenced assembly 'System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'
Solution
This is due to the version Conflict , to solve this add the following tag in your web config file
<runtime>
<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
<dependentAssembly>
<assemblyIdentity name="System.Web.Extensions" publicKeyToken="31bf3856ad364e35" Culture="neutral"/>
<bindingRedirect oldVersion="1.0.61025.0" newVersion="3.5.0.0"/>
<publisherProfile apply="no"/>
</dependentAssembly>
</assemblyBinding>
</runtime>
