HttpContext.Current.Session is null in MVC 3 appli

I have a bilingual MVC 3 application, I use cookies and session to save "Culture" in Session_start method inside Global.aspx.cs file, but direct after it, the session is null.

This is my code:

    protected void Session_Start(object sender, EventArgs e)
    {
        HttpCookie aCookie = Request.Cookies["MyData"];

        if (aCookie == null)
        {
            Session["MyCulture"] = "de-DE";
            aCookie = new HttpCookie("MyData");
            //aCookie.Value = Convert.ToString(Session["MyCulture"]);
            aCookie["MyLang"] = "de-DE";
            aCookie.Expires = System.DateTime.Now.AddDays(21);
            Response.Cookies.Add(aCookie);
        }
        else
        {
            string s = aCookie["MyLang"];
            HttpContext.Current.Session["MyCulture"] = aCookie["MyLang"];
        }
 }

and second time it goes into the "else clause" because the cookie exists; inside my Filter, when it tries set the culutre, Session["MyCulture"] is null.

   public void OnActionExecuting(ActionExecutingContext filterContext)
    {

        System.Threading.Thread.CurrentThread.CurrentUICulture = new System.Globalization.CultureInfo(HttpContext.Current.Session["MyCulture"].ToString());
        System.Threading.Thread.CurrentThread.CurrentCulture = System.Globalization.CultureInfo.CreateSpecificCulture(HttpContext.Current.Session["MyCulture"].ToString());
    }

Why are you using HttpContext.Current in an ASP.NET MVC application? Never use it. That's evil even in classic ASP.NET webforms applications but in ASP.NET MVC it's a disaster that takes all the fun out of this nice web framework.

Also make sure you test whether the value is present in the session before attempting to use it, as I suspect that in your case it's not HttpContext.Current.Session that is null, but HttpContext.Current.Session["MyCulture"]. So:

public void OnActionExecuting(ActionExecutingContext filterContext)
{
    var myCulture = filterContext.HttpContext.Session["MyCulture"] as string;
    if (!string.IsNullOrEmpty(myCulture))
    {
        Thread.CurrentThread.CurrentUICulture = new CultureInfo(myCulture);
        Thread.CurrentThread.CurrentCulture = CultureInfo.CreateSpecificCulture(myCulture);
    }
}

So maybe the root of your problem is that Session["MyCulture"] is not properly initialized in the Session_Start method.