ConfigureAwait(false) 未按预期使 HttpContext NULL

Question

在使用async/await时，为了避免死锁，建议一直使用ConfigureAwait(false)直到最后一层。我知道使用 ConfigureAwait(false) 也会使当前的 HttpContext 为空。 Article

下面是示例代码，只是为了了解为什么ConfigureAwait(false) 没有在预期时将 HttpContext 设为 NULL，而在未预期时将其设为 NULL。

控制器

public class MyController : Controller
{
    private readonly MyService _service = new MyService();
    private readonly MyMapper _mapper = new MyMapper();        


    public async Task<ActionResult> DoSomething()
    {
        var data = await _service.GetData().ConfigureAwait(false);

        // EXPECTED: Below, HttpContext should NOT NULL
        // ACTUAL: HttpContext is NOT NULL as expected
        if (HttpContext == null)
        {
            throw new ArgumentNullException("context");
        }

        var model = _mapper.Map(data);

        return View(model);
    }
}

将数据源异步调用到 GetData 的服务

public class MyService
{
    public async Task<string> GetData()
    {
        // EXPECTED: HttpContext should be NULL here since we are calling 
        //this method from the Controller with ConfigureAwait(false)     

        // ACTUAL: HttpContext is NOT NULL, WHY?
        if(HttpContext.Current == null)
        {
            throw new ArgumentNullException("context");
        }

        using (var client = new HttpClient())
        {
            var response = await client.GetAsync("http://url").ConfigureAwait(false);
            response.EnsureSuccessStatusCode();
            return await response.Content.ReadAsStringAsync().ConfigureAwait(false);
        }
    }
}

映射器类将数据映射到模型

public class MyMapper
{
    public MyModel Map(string data)
    {
        // EXPECTED:  HttpContext should NOT NULL here since async call is already done.
        // ACTUAL: HttpContext is NULL here. WHY?
        if (HttpContext.Current == null)
        {
            throw new ArgumentNullException("context");
        };

        return new MyModel()
        {
            Message = data
        };
    }
}

HttpContext 在MyService 中应为NULL，在MyMapper 中应为NOT NULL，但事实并非如此。请在 MyService 和 MyMapper 中查看我的内联 cmets

Answer 1

你有它倒退。 HttpContext.Current 将在上下文切换之后为空，而不是之前。

所以：

// ACTUAL: HttpContext is NOT NULL, WHY?
if(HttpContext.Current == null)
{
    throw new ArgumentNullException("context");
}

不，预计不是null，因为到目前为止还没有任何await（父await 不适用）。

// EXPECTED:  HttpContext should NOT NULL here since async call is already done.
// ACTUAL: HttpContext is NULL here. WHY?
if (HttpContext.Current == null)

切换发生在这一行之后

var response = await client.GetAsync("http://url").ConfigureAwait(false);

所以它应该是null。