C# – HttpWebResponse.GetResponse() fiddler says “Response Header parsing failed. “

One possible solution uses JavaScript on the client.

The client algorithm:

1. Generate a random unique token.
2. Submit the download request, and include the token in a GET/POST field.
3. Show the "waiting" indicator.
4. Start a timer, and every second or so, look for a cookie named "fileDownloadToken" (or whatever you decide).
5. If the cookie exists, and its value matches the token, hide the "waiting" indicator.

The server algorithm:

1. Look for the GET/POST field in the request.
2. If it has a non-empty value, drop a cookie (e.g. "fileDownloadToken"), and set its value to the token's value.

Client source code (JavaScript):

function getCookie( name ) {
  var parts = document.cookie.split(name + "=");
  if (parts.length == 2) return parts.pop().split(";").shift();
}

function expireCookie( cName ) {
    document.cookie = 
        encodeURIComponent(cName) + "=deleted; expires=" + new Date( 0 ).toUTCString();
}

function setCursor( docStyle, buttonStyle ) {
    document.getElementById( "doc" ).style.cursor = docStyle;
    document.getElementById( "button-id" ).style.cursor = buttonStyle;
}

function setFormToken() {
    var downloadToken = new Date().getTime();
    document.getElementById( "downloadToken" ).value = downloadToken;
    return downloadToken;
}

var downloadTimer;
var attempts = 30;

// Prevents double-submits by waiting for a cookie from the server.
function blockResubmit() {
    var downloadToken = setFormToken();
    setCursor( "wait", "wait" );

    downloadTimer = window.setInterval( function() {
        var token = getCookie( "downloadToken" );

        if( (token == downloadToken) || (attempts == 0) ) {
            unblockSubmit();
        }

        attempts--;
    }, 1000 );
}

function unblockSubmit() {
  setCursor( "auto", "pointer" );
  window.clearInterval( downloadTimer );
  expireCookie( "downloadToken" );
  attempts = 30;
}

Example server code (PHP):

$TOKEN = "downloadToken";

// Sets a cookie so that when the download begins the browser can
// unblock the submit button (thus helping to prevent multiple clicks).
// The false parameter allows the cookie to be exposed to JavaScript.
$this->setCookieToken( $TOKEN, $_GET[ $TOKEN ], false );

$result = $this->sendFile();

Where:

public function setCookieToken(
    $cookieName, $cookieValue, $httpOnly = true, $secure = false ) {

    // See: http://stackoverflow.com/a/1459794/59087
    // See: http://shiflett.org/blog/2006/mar/server-name-versus-http-host
    // See: http://stackoverflow.com/a/3290474/59087
    setcookie(
        $cookieName,
        $cookieValue,
        2147483647,            // expires January 1, 2038
        "/",                   // your path
        $_SERVER["HTTP_HOST"], // your domain
        $secure,               // Use true over HTTPS
        $httpOnly              // Set true for $AUTH_COOKIE_NAME
    );
}

I think this is very close to what @Zachary suggests. And it (seems to) work(s); actually I think using using as @Zachary does is even "nicer".
My main point being I cannot see the blocking behaviour of GetResponse() you (seem to) describe.

In addition the following code only roughly shows how everything works; it will not read the stream to the end for example (unless by coincidence :)). But it should work if you copy-n-paste it into an empty "Console Application"-project in Visual Studio.

You can try using some "shorter" URL for a test. The example here starts downloading an ISO of the debian distribution (a bit more than 600 MByte). Sorry debian, I did not mean to steal your bandwidth. -> Btw: is there something sensible one can use to test such a scenario?

The Code is strongly inspired by C# - How to read a continuous stream of XML over HTTP.

namespace StreamReadWebRequest
{
    using System;
    using System.Collections.Generic;
    using System.Text;
    using System.Net;
    using System.IO;

    class Program
    {
        static void Main(string[] args)
        {
            HttpWebRequest req;
            HttpWebResponse res = null;

            try
            {
                req = (HttpWebRequest)WebRequest.Create(
                        "http://cdimage.debian.org/debian-cd/5.0.4/i386/iso-cd/debian-504-i386-CD-1.iso");
                res = (HttpWebResponse)req.GetResponse();
                Stream stream = res.GetResponseStream();

                byte[] data = new byte[4096];
                int read;
                while ((read = stream.Read(data, 0, data.Length)) > 0)
                {
                    Process(data, read);
                }
            }
            finally
            {
                if (res != null)
                    res.Close();
            }
            Console.In.Read();
        }

        private static void Process(byte[] data, int read)
        {
            Console.Out.Write(ASCIIEncoding.ASCII.GetString(data));
        }
    }
}