When the GridView.Sorting event is raised, you can use the GridViewSortEventArgs.SortExpression property to determine the sort expression that will be used to sort the System.Web.UI.WebControls.GridView control when sort operation is performed. By default, the System.Web.UI.WebControls.GridView control sorts a single column at a time. The sort expression simply contains the name of the field to sort. You can also sort multiple columns at a time by programmatically setting this property to a comma-separated list of field names.