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 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2
The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.