ASP.Net Error: The type System.Web.UI.ExtenderControl is defined in an assembly that is not referenced

Refer

https://forums.asp.net/t/1291963.aspx?System+Web+UI+ExtenderControl+Missing

https://stackoverflow.com/questions/20657982/ajax-control-toolkit-compile-error-asp-net

Dear @dharmendr,

after using your references i am getting below error.

The type 'System.Web.UI.ExtenderControl' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'

Refer below link.

https://www.sitefinity.com/developer-network/forums/bugs-issues-/reference-to-assembly-system-web-extensions-version-4-0-0-0