Asp.net – Error executing child request with Server.Transfer

Based on the OP’s problem-description, the long-and-short of it is that when a Server.Transfer() request is made to "~/pages/errors/404.aspx" something causes a NullReferenceException on one of these two lines (the OP didn’t specify exactly which line the stack-trace pointed at):

this.MP.IsErrorPage = true;
this.MP.SEOPageHeader.Title = "File not found";

The this.MP property returns the Page.Master property, cast to Master_Pages.Main.

It is possible but unlikely that this.MP is returning null – because that will only happen in Page_Load if the Page.MasterPageFile property is set to null despite ASP.NET setting it to a valid virtual-path due to the <%@ Page Master="" %> directive property.

It’s more likely that MP.SEOPageHeader is returning null, which means the next operation (setting .Title = "File not found") will fail with a NullReferenceException.

The OP declined to share the details of what exactly happens in the SEOPageHeader property’s get accessor – and their description only fuels speculation because it seems to have a big scope of responsibility "it’s a helper that sets various header meta tags for each page (canonical URLs, rel=next/prev etc etc), icon files, page titles etc etc".

Unable to diagnose the problem thoroughly, the next-best-thing I can do is suggest setting-up appropriate logging – and splitting-up can-fail operations (like this.MP.SEOPageHeader.Title) with intermediate variables which can be safely inspected for null and log accordingly:

Me: I suggest you rewrite _404’s Page_Load method to add some assertions and logging – so that the next time it happens, you’ll have a much better idea of what’s wrong (e.g. don’t directly access properties-of-properties, instead use a new local-variable to store the result of each property level/depth and test each return-value to ensure it’s not null).

Tom: i I think that’s a good solution

So I advocate updating Page_Load to this:

    using MainMasterPage = Master_Pages.Main;

//  public MainMasterPage MP => (MainMasterPage)base.Master;

    protected void Page_Load(object sender, EventArgs e)
    {
        try
        {
            this.SetUpMasterPage();
        }
        catch( Exception ex )
        {
            // TODO: Log this exception.
            throw; // <-- Be sure to re-throw this exception instead of swallowing it, because at-this-point-in-time we don't know exactly what went-wrong so we cannot preemptively decide that *any and all* exceptions can be safely ignored like this.
            // For example, supposing there's an exception caused by an ASP.NET Server Control invariant violation during the Init or Load stages in the pipeline/request-lifecycle: - swallowing the exception will make things worse because that same Server Control will then throw a new exception when ASP.NET tries to render it.
            // Instead, prefer to fail-fast, and keep an eye on your logs and frequently re-publish with appropriate graceful handling for each known exception type: https://stackoverflow.com/questions/2807241/what-does-the-expression-fail-early-mean-and-when-would-you-want-to-do-so
        }
    }

    private void SetUpMasterPage()
    {
        if( base.Master is MasterPage anyMaster )
        {
            if( anyMaster is MainMasterPage mp )
            {
                mp.IsErrorPage = true;
                if( mp.SEOPageHeader is TypeOfSEOPageHeader seoHeader )
                {
                    seoHeader.Title = "HTTP 404: Not Found";
                }
                else
                {
                    // TODO: Log warning that `base.Master.SEOPageHeader` is null - though this won't necesarily make whatever underlying problem there is any easier to diagnose...
                }
            }
            else
            {
                // TODO: Log warning that `base.Master` is non-null, but is also not of the expected type `Master_Pages.Main`
            }
        }
        else
        {
            // TODO: Log warning that `base.Master` is null.
        }
    }