Question

WEB applications based on. NET MVC 3 to establish the C # asynchronous operating value error, code as follows:

public ActionResult Contact ()
            {
                    / / Create an asynchronous processing operations
                    Task task = new Task (() => {
                            string [] testTexts = new string [10] {"a", "b", "c", "d", "e", "f", "g", "h" , "i", "j"};
                            foreach (string text in testTexts)
                            {
                                    / / The following line does not have a problem
                                    System.IO.File.AppendAllText (Server.MapPath ("/ test.txt"), text);
                                    / / The following line to be a problem, find a solution. Because some other program of practical application in my project set to use System.Web.HttpContext.Current
                                    / / System.IO.File.AppendAllText (System.Web.HttpContext.Current.Server.MapPath ("/ test.txt"), text);
                                    / / Thread to hang five seconds to simulate asynchronous time difference
                                    System.Threading.Thread.Sleep (5000);
                            }
                    });
                    / / Asynchronous processing
                    task.Start ();
                    return View ();
            }

Answer 1

Since the HttpContext is bound to the current request, once you return it will no longer be available. But your asynchronous task continues to run in the background and when it attempts to access it, it is no longer available. For this reason you should pass all dependencies to the task as a parameter:

public ActionResult Contact()
{
    // everything that depends on an HttpContext should be done here and passed
    // as argument to the task
    string p = HttpContext.Server.MapPath("~/test.txt");

    // Create an asynchronous processing operations
    Task task = new Task(state =>
    {
        var path = (string)state;
        var testTexts = new[] { "a", "b", "c", "d", "e", "f", "g", "h", "i", "j" };
        foreach (string text in testTexts)
        {
            System.IO.File.AppendAllText(path, text);

            // Thread to hang five seconds to simulate asynchronous time difference
            Thread.Sleep(5000);
        }
    }, p);

    // Asynchronous processing
    task.Start();
    return View();
}