ASP.NET：バックグラウンドで処理をさせるには？

Public Class WebForm1

    Inherits System.Web.UI.Page



   Public Delegate Function HidoukiSyoriDelegate(ByVal duration As Integer) As String



   Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

      If Not IsPostBack Then

         Session("state") = "未処理"

         Label1.Text = Session("state").ToString()

      End If

   End Sub



   Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

      '処理クラス

      Dim objExec As HidoukiClass = New HidoukiClass

      'デリゲート

      Dim dlgt As HidoukiSyoriDelegate = New HidoukiSyoriDelegate(AddressOf objExec.HidoukiSyori)

      'コールバックメソッド

      Dim cbm As AsyncCallback = New AsyncCallback(AddressOf CallbackMethod)

      '非同期処理開始

      Dim ar As IAsyncResult = dlgt.BeginInvoke(Int32.Parse(TextBox1.Text), cbm, dlgt)



      Session("state") = "処理中"

      Label1.Text = Session("state").ToString()

   End Sub



   'コールバックメソッド

   Private Sub CallbackMethod(ByVal ar As IAsyncResult)

      Dim dlgt As HidoukiSyoriDelegate = CType(ar.AsyncState, HidoukiSyoriDelegate)

      Dim ret As String = dlgt.EndInvoke(ar)

      Session("state") = ret

   End Sub



   '状態チェック

   Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs)

      Label1.Text = Session("state").ToString()

   End Sub



End Class



'非同期処理対象クラス

Public Class HidoukiClass

   Public Function HidoukiSyori(ByVal duration As Integer) As String

      System.Threading.Thread.Sleep(duration)

      Return "処理完了"

   End Function

End Class