probléme avec ViewState, ASP Javascript

    Private Property GridViewSortDirection() As String
        Get
            Return IIf(ViewState("SortDirection") = Nothing, "ASC", ViewState("SortDirection"))
        End Get

        Set(ByVal value As String)
            Session("SortDirection") = 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 Sub GridView2_Sorting(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewSortEventArgs) Handles GvTable.Sorting
        Label1.Text = GetSortDirection() 'Retourne toujours "ASC"
    End Sub

    Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
        btntext.Text = GetSortDirection()
    End Sub

Protected Sub GridView2_Sorting(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewSortEventArgs) Handles GvTable.Sorting
        btntext.Text = GetSortDirection()
End Sub