Cross Page Postback Without Circular Reference Errors

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Introduction

This whole thing started out simple. All I wanted to do was pass a single variable back and forth between two pages in an admin section of a website using cross page postback. Were it not for my use of a master page, I would not be writing about this. Without a master, all you need to use is find the control. However, with a master page, things get more complicated. I’ll not go into it more than to say you need to find the control via the clientID, and you need to somehow get that ID in ways that I consider prone to error. Therefore, I opted for strong typing of the previous page. This nailed me with another issue. It works great one way, but not two ways. From x.aspx to y.aspx - no problem, but from y.aspx back to x.aspx - error! Well, it took me a while, but I figured out how to do it cleanly; in one word: Interface.

Let’s start by reviewing the three methods for cross page postback. I have two variables I need to pass, iItemID and iClientID, between two pages, x.aspx and y.aspx.

1. PreviousPage.FindControl("lbl_iItemID") where lbl_iItemID is a Label on the previous page with the value you want to get. This gave me problems with a master page, necessitating some wordy and sloppy code. Sloppy in at least the way I was doing it!
2. Strong typing of the previous page. Here you add a <%@ PreviousPageType VirtualPath="sourcepage.aspx" %> in the receiving page. This is a reference to the previous page, so you can just use a PreviousPage.lbl_iItemID.Text to get the value. The problem here is when you try to go back to the first page, you get an error noting a circular reference. In other words, x to y is on, but y to x is error.
3. This one is not common, but it should be! Use an interface. From here forward, this is what I will be using for all cross page post backs. It allows you to create a discrete number of strongly typed arguments that can be referenced by any page you decide that needs to receive or pass variables.

The interface method

I will outline how I consider it should be done:

1. Create the interface. This one is simple, with only two items.

using System;
using System.Web;
using System.Web.Services;
using System.Web.Services.Protocols;
using System.ComponentModel; 
public interface ICommonPostback 
{ 
  int IItemID 
  { get; set; } 
  int IClientUploadID 
  { get; set; } 
}

Save as ICommonPostback.cs.

2. Reference the interface in your two pages like this, by adding ICommonPostBack:

public partial class Admin_y : System.Web.UI.Page, ICommonPostback //(for y)
public partial class Admin_x : System.Web.UI.Page, ICommonPostback //(for x)

3. Create properties.

#region Cross page postback var
private int iItemID = -1;
private int iClientUploadID = -1;
public int IItemID
{
  get { return iItemID; }
  set { iItemID = value; }
}
public int IClientUploadID
{
  get { return iClientUploadID; }
  set { iClientUploadID = value; }
}
#endregion

This may not be necessary, but I consider it worth the effort to get cleaner code for all pages involved in the posting cycle.

4. Create a button on each page with the post back URL set to the other page.

<asp:Button ID="Button1" runat="server" Text="Button" 
    onclick="Button1_Click" PostBackUrl="~/Admin/y.aspx" />

5. Create a Button_Click event with the following (given that this is x.aspx). You set the values here to be referenced in the receiving page.

protected void Button1_Click(object sender, EventArgs e)
{
  iItemID = 22;
  iClientUploadID = 23;
}

On the receiving page (given that this is y.aspx).

protected void Page_Load(object sender, EventArgs e)
{
  if (PreviousPage != null)
  {
    ICommonPostback frm = PreviousPage as ICommonPostback;
    Label1.Text = frm.IItemID.ToString();
  }
}

6. Do the same on the other page, but change the variables to show that posting has occurred. See the attached code (uses ASP.NET and C# 3.5).
7. Note, for master pages, it will work the same, except you will need to reference the PreviousPage.Master, or if nested, the master page's PreviousPage.Master.Master.