以下是有问题的代码：var raw = Enumerable.Range(1, 10);var candidates = raw.AsParallel();var p1 = candidates.AsParallel() .SelectMany( ii => { Debug....

以下是有问题的代码：

var raw = Enumerable.Range(1, 10);

var candidates = raw.AsParallel();

var p1 = candidates.AsParallel()
    .SelectMany(
        ii =>
        {
            Debug.Log(">>>>> " + ii);

            Thread.Sleep(1000);

            Debug.Log("<<<<< " + ii);

            return new[] { ii };
        }
    );

            var p2 = p1.Select(
                (v, i) => v);

            var chosen = p2.FirstOrDefault();

执行后，给出以下日志：

>>>>> 1
<<<<< 1
>>>>> 2
<<<<< 2
>>>>> 3
<<<<< 3
>>>>> 4
<<<<< 4
>>>>> 5
<<<<< 5
>>>>> 6
<<<<< 6
>>>>> 7
<<<<< 7
>>>>> 8
<<<<< 8
>>>>> 9
<<<<< 9
>>>>> 10
<<<<< 10

执行耗时 10 秒。理论上应该耗时 1 秒， >>>>> x 执行后立即到达。

这种行为的原因是什么以及如何解决它？

更新1： 我已经试验了第一个答案的解决方案：

            var raw = Enumerable.Range(1, 10);

            var candidates = raw.AsParallel()
                .WithExecutionMode(ParallelExecutionMode.ForceParallelism);

            var p1 = candidates.AsParallel()
                .SelectMany(
                    ii =>
                    {
                        Debug.Log(">>>>> " + ii);

                        Thread.Sleep(1000);
                        Debug.Log("<<<<< " + ii);
                        return new[] { ii };
                    }
                );

            var p2 = p1.Select(
                (v, i) => v);

            var chosen = p2.FirstOrDefault();

结果仍然失败：

>>>>> 3
<<<<< 3
>>>>> 2
<<<<< 2
>>>>> 1
<<<<< 1
>>>>> 4
<<<<< 4
>>>>> 5
<<<<< 5
>>>>> 6
<<<<< 6
>>>>> 7
<<<<< 7
>>>>> 8
<<<<< 8
>>>>> 9
<<<<< 9
>>>>> 10
<<<<< 10

• plinq