MessagBox на JavaScript

Protected Sub ListView1_ItemCommand(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.ListViewCommandEventArgs) Handles ListView1.ItemCommand
        '...vwmr for view more; atcr for add to cart
        Dim var As String = e.CommandArgument.ToString
        Dim type As String = Left(var, 4)
        Dim ItemID As String = Replace(var, type, "")
        'Dim ad As LinkButton = CType(ListView1.FindControl("addToCart"), LinkButton)
        Dim myStringVariable As String = "You need to login first before adding anything to the cart!"

        If type = "vwmr" Then     '........go to view more page....
            Dim url As String = "~/dedicatedItemPage_aspx/dedicatedItemPage.aspx?typeOfItem=" & _
                                Request.QueryString("typeOfItem") & "&itemID=" & ItemID
            Response.Redirect(url, True)

        ElseIf type = "atcr" Then     '..........add the item to the cart.....
            If User.Identity.Name = "" Then
                Response.Write("<script type='javascript'>window.alert('" + myStringVariable + "')</script>")


                '            ClientScript.RegisterStartupScript(Me.[GetType](), "myalert", _
                '"alert('" + myStringVariable + "');" + _
                '"document.location = '" + ResolveUrl("~/login.aspx") + "';", True)
                '            'MsgBox("You need to login first before adding anything to the cart!", , "")
                'Response.Write("<script>alert('Hello')</script>")
                'ServerAlert.Show()

                'ClientScript.RegisterStartupScript(Me.[GetType](), "myalert", "alert('" + myStringVariable + "');", True)

                'ad.Attributes.Add("onclick", _
                ' "return confirm('Are you sure you want to delete?');")
                'Alert.Show("You need to login first before adding anything to the cart!")
                'MessageBox1.ShowInfo("You need to login first before adding anything to the cart!", 300, 400)
                'Response.Redirect("~/login.aspx", True)

            Else
                '..........pull details from database..........
                Dim con As New SqlClient.SqlConnection("Data Source=.\SQLEXPRESS;" & _
                                               "AttachDbFilename=|DataDirectory|\database.mdf;" & _
                                               "Integrated Security=True;User Instance=True")
                Dim sql As New SqlClient.SqlCommand("SELECT * FROM " & typeOfItems & " WHERE " & _
                                                    "itemID = '" & ItemID & "'", con)
                Dim reader As SqlClient.SqlDataReader
                con.Open()
                reader = sql.ExecuteReader
                reader.Read()
                Dim itemName As String = reader.Item("itemName")
                Dim itemPrice As String = reader.Item("ourPrice")
                Dim offer As String
                If reader.Item("offer").ToString = "" Then
                    offer = ""
                Else
                    offer = reader.Item("offer")
                End If
                con.Close()
                '.......................................................

                Dim userID As String = User.Identity.Name
                Dim sCart = New cart
                If sCart.CheckIfItemPresent(userID, ItemID, itemPrice, offer) = True Then
                    Exit Sub
                End If

                Dim buyNo As String = sCart.findLatestBuyNo(userID)
                Session("buyNo") = buyNo

                Session("buyNo") = sCart.AddToCart(ItemID, itemName, itemPrice, offer, buyNo, userID)
            End If
        End If   
    End Sub

я хочу заменить asp.net MsgBox предупреждением javascript... как я могу это сделать? закомментированные строки - это те, которые я уже пробовал... пожалуйста, помогите мне... срочно. asp.net MsgBox не работает на стороне клиента после публикации веб-сайта.