This is my problem(it's killing me!!)

I have a master page for my homepage(site.master):
<asp:ContentPlaceHolder ID="Content1" runat="server">
   <asp:ContentPlaceHolder ID="mainContentLeft" runat="server"></asp:ContentPlaceHolder>
   <asp:ContentPlaceHolder ID="mainContentRight" runat="server"></asp:ContentPlaceHolder>
</asp:ContentPlaceHolder>

I then have an inherited master page for my content pages(content.master):
<%@ Master MasterPageFile="~/site.master" %>
<asp:Content ID="Content1" ContentPlaceHolderID="Content1" runat="server">
   <asp:ContentPlaceHolder ID="mainContent" runat="server"></asp:ContentPlaceHolder>
</asp:Content>

I have a content page called events.aspx, which contains a hidden field and a button which posts to signup.aspx.
I want to get the value of the hidden field from the events page in the codebehind of signup. This is only causing me issues because of the inherited master page. This is my code so far in signup.aspx.vb:
(FYI, i tested if I get into the If statement, and i do, just not getting the value. Always get the same error: Object reference not set to an instance of an object.
If PreviousPage IsNot Nothing AndAlso PreviousPage.IsCrossPagePostBack Then
Dim EventID As HiddenField
   'Have tried alot of different variations of PreviousPage.Master.Master etc.
   EventID = CType(PreviousPage.FindControl("hdnEvent"), HiddenField)
   lblError.Text = EventID.Value.ToString()
End If

Hey Micky,

Calling PreviousPage after posting from a content page will return a value that's typed to the top-level master page.  In your case, to get the content of the hidden field, you should replace your call to PreviousPage.FindControl() with:

CType(PreviousPage.Form.FindControl("ContentPlaceHolder1").FindControl("ContentPlaceHolder1").FindControl("HiddenField1"), HiddenField)

Of course, you'll probably need to replace the control names used here with those from your actual application.  The above should return the expected control instance.

HTH,
Clay

According to my initial code, and your response Clay, I start at the root and work down.
So, my root content placeholder is Content1, and the contents page holder is mainContent.

Dim EventID As HiddenField
EventID = CType(PreviousPage.Form.FindControl("Content1").FindControl("mainContent").FindControl("hdnEvent"), HiddenField)

Still gives me an error. I am not sure if I'm interpreting your comment, and my code correctly.|

My display string after the CType is
lblMessage.Text = EventID.Value.ToString()

Any ideas as to why this isn't working?
M

In my own investigation, I used the default Visual Studio template for the top-level master, which contains its own <form> tags, and my response assumed you had the same structure.  However, a careful re-reading of your original post suggests otherwise, and judging from your source code it appears that the <form> is in your content page.  Sorry about not catching that earlier.

This is relevant because the control on which you need to call FindControl() needs to be no farther above the control in the page's control tree than the lowest-level naming container that contains the control -- naming containers being the controls whose IDs are strung together to form the unique ID for the control that you see when you view source.   If the Form is in the content page's Content control, than the form is the naming container, otherwise the Content control is. 

That may be a litle hard to follow, but the impact on you is easy.  Instrad of calling the above, you would then be able to simply call:

EventID = CType(PreviousPage.Form.FindControl("hdnEvent"), HiddenField)

And that should work for you (again, assuming the <form> is where I think it is.)

Thanks,
Clay

Like I said, this problem is going to kill me!! Thanks for all the help so far, I think I'll give you some further code on my project.

site.master (i took out my div's for ease of reading. The homepage is default.aspx)
<form id="form1" runat="server">
<asp:ContentPlaceHolder ID="Content1" runat="server">
   <asp:ContentPlaceHolder ID="mainContentLeft" runat="server"></asp:ContentPlaceHolder>
   <asp:ContentPlaceHolder ID="mainContentRight" runat="server"></asp:ContentPlaceHolder>
</asp:ContentPlaceHolder>
</form>

content.master (all content pages use this as their master file)
<%@ Master MasterPageFile="~/site.master" %>
<asp:Content ID="Content1" ContentPlaceHolderID="Content1" runat="server">
   <asp:ContentPlaceHolder ID="mainContent" runat="server"></asp:ContentPlaceHolder>
</asp:Content>

My page events.aspx has a hidden field and a link button to signup.aspx
<asp:HiddenField ID="hdnEvent" runat="server" Value="2" />
<asp:LinkButton ID="LinkButton1" runat="server" PostBackUrl="signup.aspx">Signup</asp:LinkButton>

I have a suggestion, if events.aspx page is the only page which submits to signup.aspx then add this directive to your signup.aspx:
<%@ PreviousPageType VirtualPath="~/events.aspx"%>

Also expose your hidden field in your events.aspx by a public readonly property

then use is like this
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
   If PreviousPage IsNot Nothing AndAlso PreviousPage.IsCrossPagePostBack Then
      Dim EventID As HiddenField
      EventID = PreviousPage.EventIDHiddenField
      lblMessage.Text = EventID.Value

   End If
End Sub

Try it and inform us with the results, also this is a reference for the mentioned directive above
http://msdn2.microsoft.com/ms228169.aspx

Good luck

Some more helpful(maybe) info.

I actually have a textbox within the site.master file. It is used for searching the site. This posts to search.aspx(which uses content.master). In my codebehind for search.aspx, I have this...

Dim SearchTerm As TextBox
lblSearch.Text = PreviousPage.Master.FindControl("txtSearchTerm").ToString()

For the first post, i.e. from the homepage(site.master) it works fine. The next post, from search.aspx, gives me the error again. I know this is related to my issue of having content.master inherited from site.master. I have tried numerous variations to the above to my original problem, some educated, some not, and I can't figure it out.
HELP!!!!

Mick

Thanks for the suggestion on exposing the hidden field. As it turns out, signup.aspx always comes from events.aspx, so it works perfectly. However, I would still like to know how to reference the object without doing this, once, just so I can understand it, and because I believe I will need to know that for other requirements on my website.

Thanks again mohammed,

Mick

In my joy of finding a solution, I have since realised this might not work. With the hiddenfield placed on the page, it works no problem, however my hiddenfield is within a FormView, as it is generated dynamically. Once I place the hiddenfield inside the FormView, my public readonly return hdnEvent returns an error or hdnEvent is not declared. Within the ItemTemplate my hdnEvent is as such:

<asp:HiddenField ID="hdnEvent" runat="server" Value='<%# Eval("EventID") %>' />

Is it because this hiddenfield is dynamic that the public readonly won't find it? Now I guess I must go back to my original scenario, which still isn't resolved!! LOL
Gotta love it,

Mick