Changing a session variable from a worker thread

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C#

ASP.NET

session

MVC4

Hi all,
I'm currently building some stuff into a web application and the application uses session state to 'remember' a user. It's an existing application, so I can't change that.
Now in the user object I want to update a property and all is well.
However, on one occassion I want to set it from a worker thread.

So, in my Controller I have something like this:

// Somewhere at login...
this.Session["user"] = myUser;

// In another method call...
var user = (User)this.Session["user"];
user.SomeProperty = myValue; // e.g. 1.

// In yet another method call...
var user = (User)this.Session["user"];
var myValue = user.SomeProperty; // myValue would be 1.
// myValue now has the value we stored earlier.

That works really great!

Now in another Controller I have the following:

// In some method call...
var user = (User)this.Session["user"];
Task.Factory.StartNew(() =>
{
    // Some long-running process...
    user.SomeProperty = myValue; // e.g. 2.
});
// return some value and handle it in JavaScript on client.
// The task will finish the long-running process on the server on a different thread and the user can switch pages, etc.

// In another method call AFTER the long-running process finishes...
var user = (User)this.Session["user"];
var myValue = user.SomeProperty; // myValue is 1.
// myValue doesn't have the value stored in the worker thread :(

So now I read that storing a value in the session is thread-dependent. I tried running a follow-up task and force it to run on the main thread, but the main thread is, of course, gone.
So is there any way to update this variable into my session on the worker thread?

Thanks.