how to write logic for gridview sorting ?

Protected Sub GridView1_Sorting(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewSortEventArgs) Handles GridView1.Sorting

GridViewSortExpression = e.SortExpression
Dim pageIndex As Integer = GridView1.PageIndex
GridView1.DataSource = SortDataTable(GridView1.DataSource, False)
GridView1.DataBind()
GridView1.PageIndex = pageIndex
End Sub

Protected Sub GridView1_PageIndexChanging(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewPageEventArgs) Handles GridView1.PageIndexChanging

GridView1.DataSource = SortDataTable(GridView1.DataSource, True)
GridView1.PageIndex = e.NewPageIndex
GridView1.DataBind()
End Sub

Private Property GridViewSortDirection() As String

Get
Return IIf(ViewState("SortDirection") = Nothing, "ASC", ViewState("SortDirection"))
End Get
Set(ByVal value As String)
ViewState("SortDirection") = value
End Set
End Property

Private Property GridViewSortExpression() As String

Get
Return IIf(ViewState("SortExpression") = Nothing, String.Empty, ViewState("SortExpression"))
End Get
Set(ByVal value As String)
ViewState("SortExpression") = value
End Set
End Property

Private Function GetSortDirection() As String

Select Case GridViewSortDirection
Case "ASC"
GridViewSortDirection = "DESC"
Case "DESC"
GridViewSortDirection = "ASC"
End Select
Return GridViewSortDirection
End Function

Protected Function SortDataTable(ByVal dataTable As DataTable, ByVal isPageIndexChanging As Boolean) As DataView

If Not dataTable Is Nothing Then
Dim dataView As New DataView(dataTable)
If GridViewSortExpression <> String.Empty Then
If isPageIndexChanging Then
dataView.Sort = String.Format("{0} {1}", GridViewSortExpression, GridViewSortDirection)
Else
dataView.Sort = String.Format("{0} {1}", GridViewSortExpression, GetSortDirection())
End If
End If
Return dataView
Else
Return New DataView()
End If
End Function