c# - 为什么这个程序会挂起？

Question

我有以下程序:

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));
    }

我希望看到:

  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

但相反，它会:

Before: 1
Doing work...

而且永远不会更进一步。我已经尝试并尝试理解 C#5 中的异步/等待功能，但始终没有效果。同样，我无法解释。

Answer 1

问题是您正在使用 return new Task(() => Thread.Sleep(1500)); 而不是 Task.Run。

new Task 实际上并没有启动任务，这将导致 await 永远不会触发。

相反，尝试:

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

或者，更好的是:

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