GridViewSelectEventArgs::NewSelectedIndex Property
Gets or sets the index of the new row to select in the GridView control.
Assembly: System.Web (in System.Web.dll)
Because the SelectedIndexChanging event occurs before the GridView control performs the select operation, the SelectedIndex property of the control cannot be used to determine the index of the new row selected by the user. The SelectedIndex property still contains the index of the previously selected row. To determine the index of the new row selected by the user, use the NewSelectedIndex property. You can also use this property to programmatically override the row index selected by the user by setting it to another value.
Available since 2.0