Assembly: System.Web (in System.Web.dll)
The page passes the value of the eventArgument parameter to the method of the control that implements the System.Web.UI.IPostBackEventHandler interface. This TreeView control also renders the HTML element that causes the postback event to occur. If the control renders client-side script to post back to the server, the argument from the script is passed in the eventArgument parameter. If the postback event is caused by a simple submit to the server, the eventArgument parameter is null.
The method provides the functionality for many events that are implemented by HTML and Web server controls.