Task.Delay是如何工作的？

Question

They say Task.Delay() is an async Thread.Sleep(). To test this I wrote below code. I expect to print immediately "One" then 3 seconds later result variable (15) will be printed. 2 seconds after this, "Two" will be printed. But it doesnt seem so. "One" is not immediately printed. "One" is printed 3 seconds later. Why does it wait 3 seconds to print "One" ?

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApplication31
{
class Program
{
    public static int Multiply(int a, int b)
    {
        return a * b;
    }

    public static async Task DoRunAsync(int m, int n)
    {
        int result = await Task.Run(() => Multiply(m, n));
        await Task.Delay(3000);
        Console.WriteLine("One");
        Console.WriteLine(result);
    }

    static void Main(string[] args)
    {
        Task t = DoRunAsync(3, 5);
        Thread.Sleep(5000);
        Console.WriteLine("Two");
    }
}
}

Answer 1

Modifying your DoRunAsync method as follows will make things work as intended:

public static async Task DoRunAsync(int m, int n)
{
    int result = await Task.Run(() => Multiply(m, n));
    Console.WriteLine("One"); // before the delay, not after...
    await Task.Delay(3000);
    Console.WriteLine(result);
}

Your code behaves as follows:

await Task.Delay(3000); // Await 3 seconds...
Console.WriteLine("One"); // Once you are done awaiting, print the string...

If you await before printing the string, you can't expect it to be printed immediately...

Answer 2

It takes 3 seconds to print "One" because you await -ed Task.Delay too soon.

Change the code as follows to get the result that you expected:

int result = await Task.Run(() => Multiply(m, n));
var taskDelay = Task.Delay(3000); // No blocking here, so
Console.WriteLine("One");                              // printing can proceed.
await taskDelay; // This causes a block for the remainder of 3 seconds
Console.WriteLine(result);

When you start the delay task prior to printing "One" without await -ing it, subsequent WriteLine could complete without a delay.

Answer 3

The job of await is to suspend the current method until whatever awaitable you pass to it is complete . How, why and when that awaitable was created and started are of no relevance to what the await keyword accomplishes.

I think you're under the impression that await starts something, when in fact it's the opposite - it marks a point in your code where you wait for something to finish .