How to allow sorting with gridview header column in asp.net

Here is complete code for allow sorting in asp.net gridview:

 public SortDirection dir
    {
        get
        {
            if (ViewState["dirState"] == null)
            {  ViewState["dirState"] = SortDirection.Ascending;
            }
            return (SortDirection)ViewState["dirState"];
        }
        set
        {        ViewState["dirState"] = value;
        }

    }
        protected void gvShowRecord_Sorting(object sender, GridViewSortEventArgs e)
        {
            string sortingDirection = string.Empty;

            if (dir == SortDirection.Ascending)
            {
                dir = SortDirection.Descending;

                sortingDirection = "Desc";
            }

            else
            {
                dir = SortDirection.Ascending;
                sortingDirection = "Asc";
            }

            DataView sortedView = new DataView(BindGridOfParticipantDetail());

            sortedView.Sort = e.SortExpression + " " + sortingDirection;

            gvShowRecord.DataSource = sortedView;

            gvShowRecord.DataBind();
        }
    }


and design page require some settings  as


1. AllowSorting="True"   onsorting="gvShowRecord_Sorting"

 2. <asp:BoundField HeaderText="First Name" DataField="FirstName" SortExpression="FirstName" />