处理异步并行任务的多个异常

Handling multiple exceptions from async parallel tasks

问题

多个任务 运行 并行,所有 none 或其中任何一个都可能抛出异常。当所有任务完成后,必须报告所有可能发生的异常(通过日志、电子邮件、控制台输出......等等)。

预期行为

我可以通过带有异步 lambda 的 linq 构建所有任务,然后等待它们 运行ning 与 Task.WhenAll(tasks) 并行。然后我可以捕获一个 AggregateException 并报告每个单独的内部异常。

实际行为

抛出 AggregateException,但它只包含一个内部异常,无论抛出多少个异常。

最小的完整可验证示例

static void Main(string[] args)
{
    try
    {
        ThrowSeveralExceptionsAsync(5).Wait();
    }
    catch (AggregateException ex)
    {
        ex.Handle(innerEx =>
        {
            Console.WriteLine($"\"{innerEx.Message}\" was thrown");
            return true;
        });
    }

    Console.ReadLine();
}

private static async Task ThrowSeveralExceptionsAsync(int nExceptions)
{
    var tasks = Enumerable.Range(0, nExceptions)
        .Select(async n =>
        {
            await ThrowAsync(new Exception($"Exception #{n}"));
        });

    await Task.WhenAll(tasks);
}

private static async Task ThrowAsync(Exception ex)
{
    await Task.Run(() => {
        Console.WriteLine($"I am going to throw \"{ex.Message}\"");
        throw ex;
    });
}

输出

请注意,"I am going to throw" 消息的输出顺序可能会因竞争条件而改变。

I am going to throw "Exception #0"
I am going to throw "Exception #1"
I am going to throw "Exception #2"
I am going to throw "Exception #3"
I am going to throw "Exception #4"
"Exception #0" was thrown

那是因为 await "unwraps" 聚合异常并且总是抛出第一个异常(如 await 的文档中所述),即使您等待 Task.WhenAll 显然可以导致多个错误。您可以像这样访问聚合异常:

var whenAll = Task.WhenAll(tasks);
try {
    await whenAll;
}
catch  {
    // this is `AggregateException`
    throw whenAll.Exception;
}

或者您可以只遍历任务并检查每个任务的状态和异常。

请注意,在修复之后您还需要做一件事:

try {
    ThrowSeveralExceptionsAsync(5).Wait();
}
catch (AggregateException ex) {
    // flatten, unwrapping all inner aggregate exceptions
    ex.Flatten().Handle(innerEx => {
        Console.WriteLine($"\"{innerEx.Message}\" was thrown");
        return true;
    });
}

因为 ThrowSeveralExceptionsAsync 返回的任务包含我们抛出的 AggregateException,包裹在另一个 AggregateException.