In C# asp.net is it possible to send back a valid web page half way through a method?

Question

When I want to redirect a user who has lost session state back to the StartPage I find Response.Redirect("~/StartPage.aspx") causes the error "Response Redirect Cannot Be Called In A Page Callback". Therefore in a Page.IsCallback situation I want to add javascript to do the redirect - this works - see code below.

protected void ForceBackToStartPage()
{
    if (Page.IsCallback)
    {
        string fullLoginUrl = ConvertRelativeUrlToAbsoluteUrl( "~/StartPage.aspx" );
        string script = string.Format("window.location.href = '{0}';", fullLoginUrl);
        ClientScriptManager cm = Page.ClientScript;
        cm.RegisterClientScriptBlock(GetType(), "redirect", script, true);
        //Response.Flush();
        //Response.End();
    }
    else
    {
        Response.Redirect("~/StartPage.aspx");
    }

}

After I have added the javascript in this section the code continues to process normally leaving this function and going off to other processing sections of the webpage - probematic as the user has no session data. Ideally I'd want processing of this web page to finish - and for the page to return in a valid state so that the user gets redirected.

Is this possible?

I thought Response.Flush(); Response.End(); might do the trick - but the webpage sent back to the browser causes XML Parse errors.

Further info: I check for missing session data in a few places (depending on the webpage - this code is in lots of web pages) - for instance - PageLoad and from some submit button methods.

Before the new method I used this code e.g.:

protected void Page_Load(object sender, EventArgs e)
{
    if ((string)Session["OK"] != "OK")
    {
        // the session has timed out - take them back to the start page
        Response.Redirect("~/StartPage.aspx");
    }

...rest of processing

Now it's invoked like this:

protected void Page_Load(object sender, EventArgs e)
{
    if ((string)Session["OK"] != "OK")
    {
        // the session has timed out - take them back to the start page
        ForceBackToStartPage();
    }

...rest of processing

so I could obviously use

else { ...rest of processing }

but I was hoping not to have to do that as SVN Source Control takes that to mean I've re-written the whole 'rest of processing' section. This is problematic as when I merge from a branch into the trunk it does not check I am not overwriting mods because it assumes it's all new code coming in.

Answer 1

Response.Redirect issues a "302 Moved" command that cannot be served asynchronously. Given that you're just registering a script when IsCallback=true, could you not also do it when IsCallback= false? Register a script that hooks up an event to the loaded event of the window and redirect the page from script instead of from the server.