我很想看看问题中提供的方法和公认答案的结果，所以我对它进行了测试。
代码如下:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
namespace AsyncTest
{
class Program
{
class Worker
{
public int Id;
public int SleepTimeout;
public async Task DoWork(DateTime testStart)
{
var workerStart = DateTime.Now;
Console.WriteLine("Worker {0} started on thread {1}, beginning {2} seconds after test start.",
Id, Thread.CurrentThread.ManagedThreadId, (workerStart-testStart).TotalSeconds.ToString("F2"));
await Task.Run(() => Thread.Sleep(SleepTimeout));
var workerEnd = DateTime.Now;
Console.WriteLine("Worker {0} stopped; the worker took {1} seconds, and it finished {2} seconds after the test start.",
Id, (workerEnd-workerStart).TotalSeconds.ToString("F2"), (workerEnd-testStart).TotalSeconds.ToString("F2"));
}
}
static void Main(string[] args)
{
var workers = new List<Worker>
{
new Worker { Id = 1, SleepTimeout = 1000 },
new Worker { Id = 2, SleepTimeout = 2000 },
new Worker { Id = 3, SleepTimeout = 3000 },
new Worker { Id = 4, SleepTimeout = 4000 },
new Worker { Id = 5, SleepTimeout = 5000 },
};
var startTime = DateTime.Now;
Console.WriteLine("Starting test: Parallel.ForEach...");
PerformTest_ParallelForEach(workers, startTime);
var endTime = DateTime.Now;
Console.WriteLine("Test finished after {0} seconds.\n",
(endTime - startTime).TotalSeconds.ToString("F2"));
startTime = DateTime.Now;
Console.WriteLine("Starting test: Task.WaitAll...");
PerformTest_TaskWaitAll(workers, startTime);
endTime = DateTime.Now;
Console.WriteLine("Test finished after {0} seconds.\n",
(endTime - startTime).TotalSeconds.ToString("F2"));
startTime = DateTime.Now;
Console.WriteLine("Starting test: Task.WhenAll...");
var task = PerformTest_TaskWhenAll(workers, startTime);
task.Wait();
endTime = DateTime.Now;
Console.WriteLine("Test finished after {0} seconds.\n",
(endTime - startTime).TotalSeconds.ToString("F2"));
Console.ReadKey();
}
static void PerformTest_ParallelForEach(List<Worker> workers, DateTime testStart)
{
Parallel.ForEach(workers, worker => worker.DoWork(testStart).Wait());
}
static void PerformTest_TaskWaitAll(List<Worker> workers, DateTime testStart)
{
Task.WaitAll(workers.Select(worker => worker.DoWork(testStart)).ToArray());
}
static Task PerformTest_TaskWhenAll(List<Worker> workers, DateTime testStart)
{
return Task.WhenAll(workers.Select(worker => worker.DoWork(testStart)));
}
}
}
结果输出:
Starting test: Parallel.ForEach...
Worker 1 started on thread 1, beginning 0.21 seconds after test start.
Worker 4 started on thread 5, beginning 0.21 seconds after test start.
Worker 2 started on thread 3, beginning 0.21 seconds after test start.
Worker 5 started on thread 6, beginning 0.21 seconds after test start.
Worker 3 started on thread 4, beginning 0.21 seconds after test start.
Worker 1 stopped; the worker took 1.90 seconds, and it finished 2.11 seconds after the test start.
Worker 2 stopped; the worker took 3.89 seconds, and it finished 4.10 seconds after the test start.
Worker 3 stopped; the worker took 5.89 seconds, and it finished 6.10 seconds after the test start.
Worker 4 stopped; the worker took 5.90 seconds, and it finished 6.11 seconds after the test start.
Worker 5 stopped; the worker took 8.89 seconds, and it finished 9.10 seconds after the test start.
Test finished after 9.10 seconds.
Starting test: Task.WaitAll...
Worker 1 started on thread 1, beginning 0.01 seconds after test start.
Worker 2 started on thread 1, beginning 0.01 seconds after test start.
Worker 3 started on thread 1, beginning 0.01 seconds after test start.
Worker 4 started on thread 1, beginning 0.01 seconds after test start.
Worker 5 started on thread 1, beginning 0.01 seconds after test start.
Worker 1 stopped; the worker took 1.00 seconds, and it finished 1.01 seconds after the test start.
Worker 2 stopped; the worker took 2.00 seconds, and it finished 2.01 seconds after the test start.
Worker 3 stopped; the worker took 3.00 seconds, and it finished 3.01 seconds after the test start.
Worker 4 stopped; the worker took 4.00 seconds, and it finished 4.01 seconds after the test start.
Worker 5 stopped; the worker took 5.00 seconds, and it finished 5.01 seconds after the test start.
Test finished after 5.01 seconds.
Starting test: Task.WhenAll...
Worker 1 started on thread 1, beginning 0.00 seconds after test start.
Worker 2 started on thread 1, beginning 0.00 seconds after test start.
Worker 3 started on thread 1, beginning 0.00 seconds after test start.
Worker 4 started on thread 1, beginning 0.00 seconds after test start.
Worker 5 started on thread 1, beginning 0.00 seconds after test start.
Worker 1 stopped; the worker took 1.00 seconds, and it finished 1.00 seconds after the test start.
Worker 2 stopped; the worker took 2.00 seconds, and it finished 2.00 seconds after the test start.
Worker 3 stopped; the worker took 3.00 seconds, and it finished 3.00 seconds after the test start.
Worker 4 stopped; the worker took 4.00 seconds, and it finished 4.00 seconds after the test start.
Worker 5 stopped; the worker took 5.00 seconds, and it finished 5.00 seconds after the test start.
Test finished after 5.00 seconds.
