c# - ContinueWith 丢失 SynchronizationContext

Question

在下面的代码片段中，SynchronizationContext 丢失了，因此 CurrentCulture 和 CurrentUICulture 也丢失了。 Log() 来自 this answer .

public async Task<ActionResult> Index()
{
    Log("before GetAsync");
    await new HttpClient().GetAsync("http://www.example.com/")
        .ContinueWith(request =>
        {
            Log("ContinueWith");
            request.Result.EnsureSuccessStatusCode();
        }, TaskContinuationOptions.AttachedToParent);

    return View();
}

static void Log(string message)
{
    var ctx = System.Threading.SynchronizationContext.Current;
    System.Diagnostics.Debug.Print("{0}; thread: {1}, context: {2}, culture: {3}, uiculture: {4}",
        message,
        System.Threading.Thread.CurrentThread.ManagedThreadId,
        ctx != null ? ctx.GetType().Name : String.Empty,
        System.Threading.Thread.CurrentThread.CurrentCulture.Name,
        System.Threading.Thread.CurrentThread.CurrentUICulture.Name);
}

这是输出:

before GetAsync; thread: 56, context: AspNetSynchronizationContext, culture: nl, uiculture: nl
ContinueWith; thread: 46, context: , culture: nl-BE, uiculture: en-US

在 GetAsync 之前，culture 和 UI culture 具有我在 Application_BeginRequest 中设置的值。在 ContinueWith 中，缺少上下文，文化设置为浏览器提供的内容，UI 文化设置为一些默认值。

据我所知，所有与 AspNetSynchronizationContext 相关的事情都应该自动发生。我的代码有什么问题？

Answer 1

为了在请求上下文线程上强制调度延续，您需要指定调度延续时应该使用的 TaskScheduler。

public async Task<ActionResult> Index()
{
    Log("before GetAsync");
    await new HttpClient().GetAsync("http://www.example.com/")
        .ContinueWith(request =>
        {
            Log("ContinueWith");
            request.Result.EnsureSuccessStatusCode();
        }, 
        TaskContinuationOptions.AttachedToParent,
        CancellationToken.None,
        TaskScheduler.FromCurrentSynchronizationContext());

    return View();
}

但是，您使用的是 await，它会自动将延续编码到当前的 SynchronizationContext。你应该能够做到这一点:

public async Task<ActionResult> Index()
    {
        Log("before GetAsync");
        HttpResponseMessage request = await new HttpClient().GetAsync("http://www.example.com/");

        //everything below here is you 'continuation' on the request context
        Log("ContinueWith");
        request.EnsureSuccessStatusCode();

        return View();
    }