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aspx_beginners thread: What's the problem with Page_Load?
Message #1 by "Jon Maz" <jonmaz@s...> on Mon, 13 May 2002 12:19:30
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Hi,
I have a simple UserControl that queries a DB and displays the query
result in a DropDownList. The page holding the UserControl should have
access to the value of the selected item in this DropDownList via a Public
Property of the UserControl called "PostedBy" (all code below).
The code seems to work fine if I access the .PostedBy property of the
UserControl in an "onclick" event handler subroutine. But if I try to
access this property (using the same code!) in Page_Load I get the
following error:
Object reference not set to an instance of an object.
My code seems fine to me, I can't work it out! Hope someone can tell me
what's going on!
Thanks in advance,
JON
*****************************************
MAINPAGE.aspx - displays the User Control
*****************************************
<%@ Page Language="VB" %>
<%@ Import Namespace="System.Data" %>
<%@ Import Namespace="System.Data.Oledb" %>
<%@ Register TagPrefix="PostedByDDL" TagName="UserControl"
src="POSTEDBY.ascx" %>
<script language="VB" runat="server">
Sub Page_Load(obj as object, e as EventArgs)
'***********************************
'* THIS DOES NOT WORK IN PAGE_LOAD *
'***********************************
Dim iExampleInteger as Integer = ucPostedByDDL.PostedBy
Response.Write("iExampleInteger is :" & iExampleInteger)
If not Page.IsPostBack
DataBind()
end if
end sub
Sub Cheese(obj as object, e as eventargs)
'****************************************
'* BUT IT WORKS FINE IN THIS SUBROUTINE *
'****************************************
Dim iExampleInteger as Integer = ucPostedByDDL.PostedBy
Response.Write("iExampleInteger is :" & iExampleInteger)
End sub
</script>
<html>
<form runat="server">
PostedBy: <PostedByDDL:UserControl id="ucPostedByDDL" runat="server" />
<br>
<asp:LinkButton runat="server" OnClick="Cheese" Text="Cheese" />
</form>
</html>
*****************************************
POSTEDBY.ascx - a simple User Control
*****************************************
<%@ Control Language="VB" %>
<%@ Import Namespace="System.Data" %>
<%@ Import Namespace="System.Data.Oledb" %>
<script language="VB" runat="server">
Dim objDatabase as new BusinessObjects.Database
Dim ds as new DataSet()
Sub Page_Load(obj as object, e as eventargs)
objDatabase.ConnectionString = ConfigurationSettings.AppSettings
("ConnectionString")
objDatabase.DataSet = ds
objDatabase.CreateDataTable("tblPostedBy", "SELECT * FROM tblPeople;")
ddPostedBy.DataSource = ds.Tables("tblPostedBy")
ddPostedBy.DataTextField = "People"
ddPostedBy.DataValueField = "PersonID"
If not Page.IsPostBack then
ddPostedBy.DataBind()
ddPostedBy.Items.FindByText("Smith, Mike").Selected = true
end if
end sub
Public Property PostedBy As Integer
Get
Return ddPostedBy.SelectedItem.Value
End Get
Set
ddPostedBy.SelectedItem.Selected = false
ddPostedBy.Items.FindByValue(value).Selected = true
End Set
End Property
</script>
<asp:DropDownList runat="server" id="ddPostedBy" />
Message #2 by "Minh T. Nguyen" <nguyentriminh@y...> on Mon, 13 May 2002 10:17:10 -0700
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Hi Jon,
I usually set up code in a different code-behind file, but I
have noticed that you have put your code in line. I am not quite sure,
but can it be that when the page is loaded for the first time, ASP.NET
doesn't know what the ucPostedByDDL control is, because the script is
loaded/read in chronologically first before the control comes? This is
just a wild guess, though.
Try putting your code in a code-behind file or move the script
part to the bottom.
Hope this works,
Minh.
-----Original Message-----
From: Jon Maz [mailto:jonmaz@s...]
Sent: Monday, May 13, 2002 12:20 PM
To: aspx_beginners
Subject: [aspx_beginners] What's the problem with Page_Load?
Hi,
I have a simple UserControl that queries a DB and displays the query
result in a DropDownList. The page holding the UserControl should have
access to the value of the selected item in this DropDownList via a
Public
Property of the UserControl called "PostedBy" (all code below).
The code seems to work fine if I access the .PostedBy property of the
UserControl in an "onclick" event handler subroutine. But if I try to
access this property (using the same code!) in Page_Load I get the
following error:
Object reference not set to an instance of an object.
My code seems fine to me, I can't work it out! Hope someone can tell me
what's going on!
Thanks in advance,
JON
*****************************************
MAINPAGE.aspx - displays the User Control
*****************************************
<%@ Page Language="VB" %>
<%@ Import Namespace="System.Data" %>
<%@ Import Namespace="System.Data.Oledb" %>
<%@ Register TagPrefix="PostedByDDL" TagName="UserControl"
src="POSTEDBY.ascx" %>
<script language="VB" runat="server">
Sub Page_Load(obj as object, e as EventArgs)
'***********************************
'* THIS DOES NOT WORK IN PAGE_LOAD *
'***********************************
Dim iExampleInteger as Integer = ucPostedByDDL.PostedBy
Response.Write("iExampleInteger is :" & iExampleInteger)
If not Page.IsPostBack
DataBind()
end if
end sub
Sub Cheese(obj as object, e as eventargs)
'****************************************
'* BUT IT WORKS FINE IN THIS SUBROUTINE *
'****************************************
Dim iExampleInteger as Integer = ucPostedByDDL.PostedBy
Response.Write("iExampleInteger is :" & iExampleInteger)
End sub
</script>
<html>
<form runat="server">
PostedBy: <PostedByDDL:UserControl id="ucPostedByDDL" runat="server"
/>
<br>
<asp:LinkButton runat="server" OnClick="Cheese" Text="Cheese" />
</form>
</html>
*****************************************
POSTEDBY.ascx - a simple User Control
*****************************************
<%@ Control Language="VB" %>
<%@ Import Namespace="System.Data" %>
<%@ Import Namespace="System.Data.Oledb" %>
<script language="VB" runat="server">
Dim objDatabase as new BusinessObjects.Database
Dim ds as new DataSet()
Sub Page_Load(obj as object, e as eventargs)
objDatabase.ConnectionString = ConfigurationSettings.AppSettings
("ConnectionString")
objDatabase.DataSet = ds
objDatabase.CreateDataTable("tblPostedBy", "SELECT * FROM
tblPeople;")
ddPostedBy.DataSource = ds.Tables("tblPostedBy")
ddPostedBy.DataTextField = "People"
ddPostedBy.DataValueField = "PersonID"
If not Page.IsPostBack then
ddPostedBy.DataBind()
ddPostedBy.Items.FindByText("Smith, Mike").Selected = true
end if
end sub
Public Property PostedBy As Integer
Get
Return ddPostedBy.SelectedItem.Value
End Get
Set
ddPostedBy.SelectedItem.Selected = false
ddPostedBy.Items.FindByValue(value).Selected = true
End Set
End Property
</script>
<asp:DropDownList runat="server" id="ddPostedBy" />
Message #3 by "Jon Maz" <jonmaz@s...> on Mon, 13 May 2002 20:02:38
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Hi Minh,
Thanks for your reply. Swapping the order of Page_Load and the Cheese
subroutine (I'm starting to regret choosing a silly name for it now!)
makes no difference, I get the same error...
I haven't tried putting it in a code-behind file yet, but I am not
hopeful! After all, the same line of code works perfectly in the Cheese
subroutine, so the code itself is OK, it's just something about being in
Page_Load that is messing things up.
My suspicions chime with yours, that in some way (which I don't
understand) the UserControl hasn't been loaded into the page yet when it
is referenced from Page_Load.
Thanks for the help so far,
JON
Message #4 by "Jon Maz" <jonmaz@s...> on Mon, 13 May 2002 22:05:40
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Hi All,
I have the answer, but don't know why it works! In MAINPAGE.aspx, if I
put the code:
Dim iExampleInteger as Integer = ucPostedByDDL.PostedBy
Response.Write("iExampleInteger is :" & iExampleInteger)
in Page_PreRender instead of Page_Load, it works as planned. Putting it
in Page_Init, which I also tried, does NOT work.
Does anybody out there have an explanation for this?
JON
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