为什么这个程序只是挂起？

Question

I have the following program:

static void Main(string[] args) { RunTest(); }

    private static void RunTest() {
        DoIOWorkFiveTimesAsync().Wait();
    }

    private static async Task DoIOWorkFiveTimesAsync() {
        for (int i = 0; i < 5; ++i) {
            Console.WriteLine("Before: " + i);
            await DoIOWorkAsync();
            Console.WriteLine("After: " + i);
        }
    }

    private static Task DoIOWorkAsync() {
        Console.WriteLine("Doing work...");
        return new Task(() => Thread.Sleep(1500));
    }

I would expect to see:

  Before: 1
  Doing work...
  After: 1
  Before: 2
  Doing work...
  After: 2
  Before: 3
  Doing work...
  After: 3
  Before: 4
  Doing work...
  After: 4
  Before: 5
  Doing work...
  After: 5

But instead, it gets to:

Before: 1
Doing work...

And never gets any further. I have tried and tried to understand the async/await features in C#5, but always to no effect. Again, the explanation eludes me.

Answer 1

The problem is you're using return new Task(() => Thread.Sleep(1500)); instead of Task.Run .

new Task doesn't actually start the task, which will cause the await to never trigger.

Instead, try:

private static Task DoIOWorkAsync() {
    Console.WriteLine("Doing work...");
    return Task.Run(() => Thread.Sleep(1500));
}

Or, better yet:

private static async Task DoIOWorkAsync() {
    Console.WriteLine("Doing work...");
    await Task.Delay(1500);
}

Answer 2

Simple, you returned a Task , but you didn't start it.

If you change your code as follows:

private static Task DoIOWorkAsync()
{
    Console.WriteLine("Doing work...");
    Task work = new Task(() => Thread.Sleep(1500));
    work.Start();
    return work;
}

It works as you would expect.