Context.Items clears during page refresh/transfer

Question

I am working on a class project and I've run into a problem I can't figure out. I have a feeling it's actually pretty easy, but I've been working on stuff so long I can't think straight anymore.

I have a login page that allows a user to login and pass 2 data items to the next page using Context.Items and Server.Transfer. Here is the code snippet:

Context.Items["preferred"] = true;
Context.Items["pageNum"] = 1;
Server.Transfer("ProductsShelf.aspx");

On the "ProductsShelf" page I can access those two items and use the data like so:

pageNumber = (int)Context.Items["pageNum"];

I am then using a switch-statement with pageNumber to display certain information:

switch (pageNumber)
{
    case 1:
        imgProd.ImageUrl = "assets/laptop.bmp";
        lbl_Name.Text = "Laptop";
        lbl_desc.Text = "This is a cheap laptop!";
        lbl_price.Text = "199.99";
        break;
}

Obviously there's other entries I'm omitting. What I want to do is click a next or previous button and use the event to change the Context.Items["pageNum"] data so the Page_Load() event uses different data in the switch-statement. Hope that makes sense. Here is one of the button click events:

protected void btn_Prev_Click(object sender, EventArgs e)
{
        if (pageNumber == 1 || pageNumber == 2)
        {
            Context.Items["pageNum"] = 1;
        }
        else if (pageNumber == 3)
        {
            Context.Items["pageNum"] = 2;
        }
        Context.Items["preferred"] = preferredCustomer;
        Server.Transfer("ProductsShelf.aspx");
}

The problem is that before the button click event fires, the form posts and clears the Context.Items and pageNumber values. This means that the button event if-statements never fire and it results in:

pageNumber = (int)Context.Items["pageNum"];

Being null, throwing an exception and making me very sad. So my question is, how can I go about retaining the values? Should I switch to Response.Redirect and have something like ?page=1 in the URL? Or will that clear too when the form posts? Hopefully I'm not doing this completely wrong.

If TL;DR, here's a quick summary:

1. Context.Items has 2 values passed with Server.Transfer
2. These values determine what's shown on the next page
3. The form clears Context.Items and variables before button click event fires
4. The values are null, the if-statement doesn't run, and the app throws an exception
5. Question: how should I go about retaining those values?

Thanks a lot. :)

Answer 1

HttpContext items can be used within one request only - it will be recreated for next request so your values are bound to lose. You should use view-state to preserve data across post-backs. In page load, you should check if data exists in context and then copy it to view-state. Then in button click events, you can read the data from view-state, put into the context items and do server.transfer. Here's simple sample code:

private int PageNumber
{
   get 
   { 
    var value = ViewState["pageNum"]; 
    return null == value? 1: (int)value;
   }
   set
   {
     ViewState["pageNum"] = value;
   }
}

private bool IsPreferredCustomer
{
   get 
   { 
    var value = ViewState["preferred"]; 
    return null == value? false: (bool)value;
   }
   set
   {
     ViewState["preferred"] = value;
   }
}

protected void Page_Load(object sender, EventArgs e)
{
    var preferred = Context.Items["preferred"];
    if (null != preferred)
    {
      IsPreferredCustomer =  (bool)preferred;
    }
    var pageNum = Context.Items["pageNum"];
    if (null != pageNum )
    {
      PageNumber =  (int)Context.Items["pageNum "];
    }
}

Use the same PageNumber property in event code.