如果 Task<HttpResponseMessage>.IsCompleted 为真，为啥 Task<HttpResponseMessage>.Result 会抛出异常？

Posted 2023-04-12

【中文标题】如果 Task<HttpResponseMessage>.IsCompleted 为真，为啥 Task<HttpResponseMessage>.Result 会抛出异常？

【英文标题】：Why does Task<HttpResponseMessage>.Result throw an exception if Task<HttpResponseMessage>.IsCompleted is true?如果 Task<HttpResponseMessage>.IsCompleted 为真，为什么 Task<HttpResponseMessage>.Result 会抛出异常？

【发布时间】：2020-08-10 07:34:18

【问题描述】：

这只发生在重负载下，所以我不能举一个简单的例子。这是一个系统测试，所以我试图避免等待，而是创建 2,000 个请求，然后轮询响应何时完成。

有问题的代码是：

Task<HttpResponseMessage> response = responseStatus.Item1;
if (!response.IsCompleted)
    continue;
HttpResponseMessage result = response.Result;

对 Result 的调用会引发以下情况：

System.AggregateException: One or more errors occurred.
StackTrace:       at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
   at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
   at System.Threading.Tasks.Task`1.get_Result()
   at SystemTests.V2.LoadTests.Test1000Requests() in C:\git\Jenova\restfulengine\SystemTests.V2\LoadTests.cs:line 121
INNER EXCEPTION:    System.Threading.Tasks.TaskCanceledException: A task was canceled.
StackTrace:    System.AggregateException: One or more errors occurred. ---> System.Threading.Tasks.TaskCanceledException: A task was canceled.
   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
   at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
   at System.Threading.Tasks.Task`1.get_Result()
   at SystemTests.V2.LoadTests.Test1000Requests() in C:\git\Jenova\restfulengine\SystemTests.V2\LoadTests.cs:line 121
---> (Inner Exception #0) System.Threading.Tasks.TaskCanceledException: A task was canceled.<---

2020-04-26 13:36:06,370 [MSTestAdapter Thread] WARN SystemTests.V2.LoadTests - v2/reports/19ea788d-ceb8-4a81-a974-f597494609dd failed (will retry)System.AggregateException: One or more errors occurred. ---> System.Threading.Tasks.TaskCanceledException: A task was canceled.
   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
   at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
   at System.Threading.Tasks.Task`1.get_Result()
   at SystemTests.V2.LoadTests.Test1000Requests() in C:\git\Jenova\restfulengine\SystemTests.V2\LoadTests.cs:line 121
---> (Inner Exception #0) System.Threading.Tasks.TaskCanceledException: A task was canceled.<---

【参考方案1】：

Task.IsCompleted 为“如果任务已完成，则为 true（即，任务处于三种最终状态之一：RanToCompletion、Faulted 或 Canceled）；否则为 false。”

因此，如果 HTTP 请求以无法创建可用HttpResponseMessage 的方式失败，您将无法访问结果。所以检查 Task.Status 看看它是什么。

【讨论】：

就是这样。我认为 IsCompleted 意味着成功。是的 - 我应该仔细阅读文档。

@DavidThielen 没错。在 .NET Core 中，他们添加了属性 IsCompletedSuccessfully。不幸的是，这个有用的属性在 .NET Framework 中不可用（并且可能永远不会）。