控制台应用程序中的 C# async/await

Question

I want to test async and await in a C# console application, so I write this test code. when I use async/await in a Winforms application, for example: call this doSth in a ButtonClicked event handler function. it will do delay 3000ms in another thread, and output "doSth execute finished" in MainThread. but in this console application, it output "doSth execute finished" in the working thread, why?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp12
{
    class Program
    {
        static void Main(string[] args)
        {
            doSth();
            Console.WriteLine("doSth call finished.");

            Thread.Sleep(5000);

            Console.WriteLine("Main finished.");
        }

        private static async void doSth()
        {
            Console.WriteLine("doSth execute Started...");

            await Task.Delay(3000);

            Console.WriteLine("doSth execute finished.");
        }
    }
}

this is the test code in Winforms application

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApp4
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            doSth();
            Console.WriteLine("doSth call finished.");

            Thread.Sleep(5000);

            Console.WriteLine("Main finished.");
        }

        private static async void doSth()
        {
            Console.WriteLine("doSth execute Started...");

            await Task.Delay(3000);

            Console.WriteLine("doSth execute finished.");
        }
    }
}

this is the stack trace in console application, the finally output is :

doSth execute Started...
doSth call finished.
doSth execute finished.
Main finished.
请按任意键继续. . .

this is the stack trace in Winforms application, the output is :

doSth execute finished.
doSth execute Started...
doSth call finished.
Main finished.

if I delete Thread.Sleep(5000) and make the main thread don't sleep, and in console application, it will lose the output and the second output of doSth function will not be called. in Winforms application, there is no effect.

Answer 1

First of all, async/await does not mean that there will be other threads executing. If await operator is hit, then the execution of that piece of code stops there (and awaits), while this operation waits, the same thread can pick up some more work (some more tasks to do).

So asynchronous code is rather about reusing the same threads as much as possible and not about spanning execution of code across as many threads as possible.

When deleting Thread.Sleep you simply leave work undone. You just finish running application while there task in the background still running. But it's terminated with your application.

Answer 2

Ultimately, any time you type async void , you're probably making a mistake (with a slight caveat around event-handlers on winforms); the fix here is to await things (if your only non-background thread, ie the Main entry-point, exits: then a console application terminates):

static async Task Main(string[] args)
{
    await doSthAsync();
    // ...
}

private static async Task doSthAsync() {...}

Note that async and concurrency/parallelism are very different things ; if your intentions were to achieve concurrency, then maybe:

static async Task Main(string[] args)
{
    var pending = Task.Run(doSthAsync);
    // ...
    await pending; // to allow parallel work to finish
}

Answer 3

Other answers explain why the Thread.Sleep keeps the application alive until you can see all the output.

I'll answer your first question:

call this doSth in a ButtonClicked event handler function. it will do delay 3000ms in another thread, and output "doSth execute finished" in MainThread. but in this console application, it output "doSth execute finished" in the working thread, why?

Side note: the delay is not done "in another thread". It's just a timer.

The reason the thread is different is because UI apps have a "context". This "context" is captured by the await and used to resume executing the async method (as I describe on my blog). The UI context in UI applications will resume executing async methods on the main UI thread. In contrast, console applications do not have a "context", so async methods resume executing on a thread pool thread.