WaitingForActivation
状态。我在使用.NET 4.5中的System.Net.Http.HttpClient时遇到了一些奇怪的行为 - 在等待(例如)
httpClient.GetAsync(...)
的调用结果时,将永远不会返回。仅在使用新的异步/等待语言功能和Tasks API时,在某些情况下才会发生此问题 - 当仅使用连续时,代码似乎总是可以正常工作。
以下是一些代码,可重现该问题 - 将其放入Visual Studio 11中的新“MVC 4 WebApi项目”中,以公开以下GET端点。
/api/test1
/api/test2
/api/test3
/api/test4
/api/test5 <--- never completes
/api/test6
这里的每个端点返回相同的数据(来自stackoverflow.com的响应头),除了/api/test5
,它永远不会完成。
我是否在使用HttpClient类时遇到了错误,还是以某种方式误用了API?
复现代码:
public class BaseApiController : ApiController
{
/// <summary>
/// Retrieves data using continuations
/// </summary>
protected Task<string> Continuations_GetSomeDataAsync()
{
var httpClient = new HttpClient();
var t = httpClient.GetAsync("http://stackoverflow.com", HttpCompletionOption.ResponseHeadersRead);
return t.ContinueWith(t1 => t1.Result.Content.Headers.ToString());
}
/// <summary>
/// Retrieves data using async/await
/// </summary>
protected async Task<string> AsyncAwait_GetSomeDataAsync()
{
var httpClient = new HttpClient();
var result = await httpClient.GetAsync("http://stackoverflow.com", HttpCompletionOption.ResponseHeadersRead);
return result.Content.Headers.ToString();
}
}
public class Test1Controller : BaseApiController
{
/// <summary>
/// Handles task using Async/Await
/// </summary>
public async Task<string> Get()
{
var data = await Continuations_GetSomeDataAsync();
return data;
}
}
public class Test2Controller : BaseApiController
{
/// <summary>
/// Handles task by blocking the thread until the task completes
/// </summary>
public string Get()
{
var task = Continuations_GetSomeDataAsync();
var data = task.GetAwaiter().GetResult();
return data;
}
}
public class Test3Controller : BaseApiController
{
/// <summary>
/// Passes the task back to the controller host
/// </summary>
public Task<string> Get()
{
return Continuations_GetSomeDataAsync();
}
}
public class Test4Controller : BaseApiController
{
/// <summary>
/// Handles task using Async/Await
/// </summary>
public async Task<string> Get()
{
var data = await AsyncAwait_GetSomeDataAsync();
return data;
}
}
public class Test5Controller : BaseApiController
{
/// <summary>
/// Handles task by blocking the thread until the task completes
/// </summary>
public string Get()
{
var task = AsyncAwait_GetSomeDataAsync();
var data = task.GetAwaiter().GetResult();
return data;
}
}
public class Test6Controller : BaseApiController
{
/// <summary>
/// Passes the task back to the controller host
/// </summary>
public Task<string> Get()
{
return AsyncAwait_GetSomeDataAsync();
}
}
HttpClient.GetAsync(...)
? - Benjamin Fox