PreviousPage.IsCrossPagePostBack

workidoo · November 2nd, 2008, 09:19 AM

Has just started a little bit with exercises in the book and has encountered a problem already in the beginning, Listing 1-13. I always get a following error when I try to go directly to page2.aspx and not go through page1.aspx. System.NullReferenceException: Object reference not set to an instance of an object. It somplains on line 5 in my codebehind: "If Not PreviousPage Is Nothing And PreviousPage.IsCrossPagePostBack Then"

This is how my code behind on page2.aspx.vb looks like:

Public Partial Class Page2
    Inherits System.Web.UI.Page

    Private Sub Page2_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        If Not PreviousPage Is Nothing And PreviousPage.IsCrossPagePostBack Then
            Dim pp_Textbox1 As TextBox
            Dim pp_Calendar1 As Calendar

            pp_Textbox1 = CType(PreviousPage.FindControl("Textbox1"), TextBox)
            pp_Calendar1 = CType(PreviousPage.FindControl("Calender1"), Calendar)

            Label1.Text = "Hello " & pp_Textbox1.Text & "<br /> Date Selected: " & pp_Calendar1.SelectedDate.ToShortDateString()
        Else
            Response.Redirect("Page1.aspx")
        End If
    End Sub
End Class

and this is my page2.aspx

<%@ Page Language="vb" AutoEventWireup="false" CodeBehind="Page2.aspx.vb" Inherits="ASPNET.Page2" %>
<%@ PreviousPageType VirtualPath="~/Page1.aspx" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" >
<head id="Head1" runat="server">
    <title>Second Page</title>
</head>
<body>
    <form id="form1" runat="server">
        <asp:Label ID="Label1" runat="server"></asp:Label>
    </form>
</body>
</html>

http://www.workidoo.com

workidoo · November 4th, 2008, 03:50 PM

Why is there no one who answer questions here???

http://www.workidoo.com

Steve Smith · January 9th, 2010, 04:29 PM

Hi,

I got the same problem when I added the code in Listing 1-13. The problem is in the IF statement:

Code:

If Not PreviousPage Is Nothing And PreviousPage.IsCrossPagePostBack Then

The problem arises when PreviousPage is nothing - you cannot check IsCrossPagePostBack for an object that does not exist. I got round it by nesting IF statements...

Code:

If Not PreviousPage Is Nothing Then
  If PreviousPage.IsCrossPagePostBack Then
    Label1.Text = "Hello..."
  Else
    Response.Redirect("Page1.aspx")
  End If
Else
  Response.Redirect("Page1.aspx")
End If