Gets the unique ID of the control that caused the asynchronous postback.
Assembly: System.Web.Extensions (in System.Web.Extensions.dll)
The property contains the value of the control ID that caused the asynchronous postback, which can be one of the following:
A postback from a control that is a trigger for an UpdatePanel control.
Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)
The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.