C# – how to dump response headers in ASP.Net

I am using VSTS 2008 + C# + .Net 3.5 to develop ASP.Net. I want to dump all response headers returned to client for a specific aspx file. Any ideas how to do this easily?

I know how to use Response.Headers collection, but my confusion is where to enumerate to get the accurate response header? For example, if I enumerate in Page_Load, not all response headers could be enumerated, but if I enumerate after Response.Close, exception will be thrown.

Any advice?

EDIT1: Meeting with the following exception when using OnPreRender in VSTS 2008 debug mode (i.e. pressing F5 to debug)

{"This operation requires IIS integrated pipeline mode."}

protected override void OnPreRender(EventArgs e)
        {
            base.OnPreRender(e);
            using (StreamWriter writer = new StreamWriter("dump123.txt", true))
            {
                writer.WriteLine(DateTime.UtcNow + " Response headers");
                foreach (string item in HttpContext.Current.Response.Headers.Keys)
                {
                    writer.WriteLine(item + " : " + HttpContext.Current.Response.Headers[item]);
                }
            }

        }

thanks in advance,
George