与WebAPI异步时保留HttpContext(中等信任)

Question

我正在构建一组ASP.Net托管的WebAPI服务,必须使用一个很大程度上依赖于HttpContext.Current的旧库.我无法确保在参与异步调用的所有方法中保留上下文.我在下面的代码中尝试了await/Task.Wait和TaskScheduler.FromCurrentSynchronizationContext()的几个变体.

    [HttpGet]
    public Task<IEnumerable<string>> ContinueWith()
    {
        Thread.CurrentThread.CurrentCulture = new CultureInfo("fr-FR");  //or another culture that is not the default on your machine
        Thread.CurrentThread.CurrentUICulture = Thread.CurrentThread.CurrentCulture;

        var output = new List<string> { TestOutput("Action start") };

        var task = Task.Factory.StartNew(() =>
            {
                Thread.Sleep(1000);
                return TestOutput("In Task");
            }).ContinueWith(slowString =>
            {
                output.Add(slowString.Result);

                output.Add(TestOutput("Action end"));
                return output as IEnumerable<string>;
            });

        output.Add(TestOutput("Action Mid"));

        return task;
    }

    private string TestOutput(string label)
    {
        var s = label + " ThreadID: " + Thread.CurrentThread.ManagedThreadId.ToString(CultureInfo.InvariantCulture);
        s += " " + Thread.CurrentThread.CurrentCulture.EnglishName;
        s += HttpContext.Current == null ? " No Context" : " Has Context";
        Debug.WriteLine(s);
        return s;
    }

我希望能够确保CurrentCulture是fr-FR,并且在调用TestOutput的每个点上HttpContext.Current都不为null.我没有成功地使用我尝试的任何东西进行"In Task"调用.另外在我的一些测试线程中,id永远不会变化,这表明我已经有效地删除了该方法的异步性.如何确保在每次调用TestOutput时都保留culture和HttpContext.Current,并且代码可以在不同的线程上自由运行？

在闭包中捕获HttpContext.Current然后再次设置它将不适合我,因为我需要支持Medium Trust,它会在调用HttpContext.Current setter时抛出安全异常.

Answer 1

一个小小的注意事实,HttpContext.Current是可写的.

var context = HttpContext.Current;
var task = Task.Factory.StartNew(() => {
    HttpContext.Current = context;
    // You may want to set CultureInformation here too.

    return TestOutput("In Task");
});

Answer 2

每当您await执行任务时都会保留上下

你看到的是,有线程池的任务(没有上下文Task.Run,TaskFactory.StartNew或就此BackgroundWorker或Thread或Delegate.BeginInvoke).这是正常的和预期的.

所以,不要使用线程池任务.您的示例代码似乎想要使用具有多个线程的并行处理HttpContext,这是不可能的.

async如果需要,可以执行并发方法,但这要求您Thread.Sleep实际上可以使用async方法而不是基于CPU的方法:

[HttpGet]
public async Task<IEnumerable<string>> Test()
{
  Thread.CurrentThread.CurrentCulture = new CultureInfo("fr-FR");
  Thread.CurrentThread.CurrentUICulture = Thread.CurrentThread.CurrentCulture;

  var output = new List<string> { TestOutput("Action start") };

  var task = SlowStringAsync();
  output.Add(TestOutput("Action Mid"));
  output.Add(await task);
  output.Add(TestOutput("Action end"));
  return output;
}

public async Task<string> SlowStringAsync()
{
  await Task.Delay(1000);
  return TestOutput("In Task");
}

如果您的旧图书馆不受控制而您无法成功async,那么您必须同步调用它.async在以下情况下从方法调用同步方法是可以接受的:

[HttpGet]
public async Task<IEnumerable<string>> Test()
{
  Thread.CurrentThread.CurrentCulture = new CultureInfo("fr-FR");
  Thread.CurrentThread.CurrentUICulture = Thread.CurrentThread.CurrentCulture;

  var output = new List<string> { TestOutput("Action start") };

  output.Add(TestOutput("Action Mid"));
  Thread.Sleep(1000);
  output.Add(TestOutput("Not Really In Task"));
  output.Add(TestOutput("Action end"));
  return output;
}