Error : The type or namespace name 'ScriptManager' does not exist in the namespace 'System.Web.UI' (are you missing an assembly reference?)

Error :

The type or namespace name 'ScriptManager' does not exist in the namespace 'System.Web.UI' (are you missing an assembly reference?)

Error Details :

Getting above error on addition ScriptManager to .aspx page

Solution :

Add the System.Web.Extensions.dll reference to project to resolve above error.