如何在没有新线程或异步和等待关键字但仅任务的情况下获得异步

Question

I wonder how to accomplish the same thing the below program does without using extra threads or await and async keywords but only Tasks. A sample code would be awesome. It seems to me that we need to use TaskCompletionSource and Async versions of the IO-bound operations or any long-running operations.

static void Main(string[] args)
{
  Task t = Go();
  Console.WriteLine("Hello World");
  Task.Delay(1000).GetAwaiter().OnCompleted(() => { Console.WriteLine("Completed"); });
  Console.ReadLine();
}

static async Task Go()
{
  var task = PrintAnswerToLife();
  await task;
  Console.WriteLine("Done");
}

static async Task PrintAnswerToLife()
{
  var task = GetAnswerToLife();
  int answer = await task;
  Console.WriteLine(answer);
}

static async Task<int> GetAnswerToLife()
{
  var task = Task.Delay(2000);
  await task;
  int answer = 21 * 2;
  return answer;
}

Answer 1

You can do a pretty straightforward translation of async / await into Task by using ContinueWith . Other translations are also possible, eg, Task.Delay becomes System.Threading.Timer .

The basic pattern is, for any async method that does an await :

static async Task Go()
{
  var task = PrintAnswerToLife();
  await task;
  Console.WriteLine("Done");
}

becomes:

static Task Go()
{
  var tcs = new TaskCompletionSource<object>();
  var task = PrintAnswerToLife();
  task.ContinueWith(_ =>
  {
    Console.WriteLine("Done");
    tcs.SetResult(null);
  });
  return tcs.Task;
}

Correct error handling is a lot more work.