IE8 freeze caused by long synchronous xmlhttprequest from silverlight - silverlight

Question

I'm having an issue where a long synchronous request will freeze Internet Explorer.
Let me explain the context : this is a web application which only supports IE8 and which can only use Synchronous*.
We have a Silverlight component with a save button. When the user presses the button, some information is sent to the server using a synchronous XMLHttpRequest, then some other actions are done client-side in the Silverlight component.
The server-side part includes calculations that will sometime take a while (several minutes).
In short, here is the code (c# silverlight part)
ScriptObject _XMLHttpRequest;
_XMLHttpRequest.Invoke("open", "POST", url, false);
_XMLHttpRequest.Invoke("send", data);
checkResponse(XMLHttpRequest);
doOtherThings();
I know that the server does its work properly because I can see in the verbose logs, the end of the page rendering for the "url" called from Silverlight.
However, in debug mode I can see that I never reach the "checkresponse" line. After calling the "send" line, IE will freeze forever, not unfreezing once the server log shows that "url" has been processed.
Also, I tried to add "_XMLHttpRequest.SetParameter("timeout", 5000)" between the "open" and the "send" lines. IE freezes for 5 seconds, then "checkresponse" and "dootherthings" are executed. Then IE freezes again while server-side calculations are processed and doesn't unfreeze once the server is done with its work.
IE timeout is supposed to be 3 hours (registry key ReceiveTimeout set to 10800000), and I also got rid of IE 2-connexions limit (MaxConnectionsPer1_0Server and MaxConnectionsPerServer set to 20).
Last important information : there is no issue when the server-side part only takes a few seconds instead of several minutes.
Do you know where the freeze could come from (IE bug, XMLHttpRequest bug, something I have done wrong) and how I can avoid this ?
Thank you !
Kévin B
*(while trying to solve my issue with the help of Google I found an incredible amount of "use asynch" and "synch is bad" posts; but I can't do this change in my app. Switching the application, ajax loads, and all server side calculations to asynchronous is a huge work which has been quoted for our client and is a long-term objective. I need a short-term fix for now)

Answer 1

Silverlight virtually requires that everything be done asynchronously. Any long running synchronous process will hang the browser if run on the UI thread. If you never reach the 'checkResponse' line of code it is possible that an unhandled exception was thrown on the previous line, and it is being swallowed. You can check in the dev tools of your browser to see if there are any javascript errors. I am surprised that calling XMLHttpRequest synchronously works at all since I would expect it to lock up the UI thread. But, the solution depends on your definition of async.
You can try:
calling the sync XHR request on a background thread and then marshalling to the UI thread (eg with Dispatcher.BeginInvoke) when you are ready
setting up an XMLHttpRequest wrapper that makes the call in async mode and raises a callback in Silverlight on completion
using HttpClient or WebClient
While these options are async, they don't require your server code to be written any differently (it can stay synchronous). I have seen a web server process a call for over an hour before finally returning a response, at which time the Silverlight app raised a callback. You could even use tools like the TPL await/async, or co-routines available in many mvvm frameworks to make the code appear very procedural/synchronous while still performing its actions asynchronously.
I hope you already solved your issue, but maybe this will be helpful to whomever may come across this post.