Apply CSSClass For GridView Sorting

.SortedAscendingHeaderStyle, .SortedDescendingHeaderStyle
{
  background-color: #FFFFCC;
  background-repeat: no-repeat;
}

.SortedAscendingHeaderStyle
{
  background-image: url(Controls/Images/arrow_up.png);
}

.SortedDescendingHeaderStyle
{
  background-image: url(Controls/Images/arrow_down.png);
}

Private Function GetIndex(ByVal SortExp As String) As Integer

        Dim i As Integer = 0
        For Each c As DataControlField In GridViewRequests.Columns
            If c.SortExpression = SortExp Then
                Return i
            Else
                i += 1
            End If
        Next

        Return i
End Function

'Note:FillDataSet() is a Function will Fill DataSet with data
'     To use it as DataSource For My GridView
Protected Sub GridViewRequests_Sorting(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewSortEventArgs) Handles GridViewRequests.Sorting
       Dim t As DataTable = CType(FillDataSet().Tables(0), DataTable)
       Dim dv As DataView = t.AsDataView
       dv.Sort = e.SortExpression + " " + GetSortDirectionString(e.SortExpression)
       GridViewRequests.DataSource = dv
       GridViewRequests.DataBind()

       'Apply CssClass for the sorting column
       If ViewState("SortDirection") = "ASC" Then
           GridViewRequests.HeaderRow.Cells(GetIndex(e.SortExpression)).CssClass = "SortedAscendingHeaderStyle"
       Else
           GridViewRequests.HeaderRow.Cells(GetIndex(e.SortExpression)).CssClass = "SortedDescendingHeaderStyle"
       End If

   End Sub

Private Function GetSortDirectionString(ByVal column As String) As String

        ' By default, set the sort direction to ascending.
        Dim sortDirection = "ASC"

        ' Retrieve the last column that was sorted.
        Dim sortExpression = TryCast(ViewState("SortExpression"), String)

        If sortExpression IsNot Nothing Then
            ' Check if the same column is being sorted.
            ' Otherwise, the default value can be returned.
            If sortExpression = column Then
                Dim lastDirection = TryCast(ViewState("SortDirection"), String)
                If lastDirection IsNot Nothing AndAlso lastDirection = "ASC" Then
                    sortDirection = "DESC"
                End If
            End If
        End If

        ' Save new values in ViewState.
        ViewState("SortDirection") = sortDirection
        ViewState("SortExpression") = column

        Return sortDirection
    End Function