我该如何创建一个控制台应用程序，以实现异步/等待

Question

I'm creating a weather app that polls temperature from a service I've made:

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

namespace Weather
{
    class Program
    {
        static BackgroundWorker bgw;

        static void Main(string[] args)
        {
            bgw = new BackgroundWorker();
            bgw.DoWork += bgw_DoWork;
            bgw.RunWorkerAsync();
        }

        static async void bgw_DoWork(object sender, DoWorkEventArgs e)
        {
            Weather bot = new Weather();

            if (bot.IsRunning)
            {
                await bot.Update();
            }
        }
    }

    public class Weather
    {
        public bool IsRunning { get; set; }

        private DateTime lastUpdated;

        public Weather()
        {
            IsRunning = true;

            lastUpdated = DateTime.Now.AddDays(-1);
        }

        public async Task<bool> Update()
        {
            if (lastUpdated < DateTime.Now)
            {
                lastUpdated = DateTime.Now.AddSeconds(30);

                // temperature
                double value = await GetLatestValue("New York");
            }

            return true;
        }

        private async Task<double> GetLatestValue(string city)
        {
            string url = "http://www" + city;

            var client = new WebClient();
            string data = await client.DownloadStringTaskAsync(url);

            return 4.3;
        }
    }
}

The problem here is that it does not seem to work? The GetLatesValue function is just jibberish, will just return 4.3 for testing purposes.

What happens is that on await GetLatestValue the console application just quits.

Answer 1

The problem is simpler than you might think: you are running a BackgroundWorker , which basically wraps a thread that has .IsBackground = true . Such threads will not keep a process alive - they will be shut down automatically when the process exits. The process will exit when all non -background threads are completed.

Your Main method starts the BackgroundWorker , but then does nothing else - Main exits, and the application is complete. The BackgroundWorker is then shut down at whatever point it's reached. There's nothing wrong with the code it's running - but the app is shutting down without letting it complete.

EDIT: if you want to test this, simply put a Console.ReadLine() at the end of your Main - it'll keep the application alive until you press Enter, and so you should see your thread run until you do.

Answer 2

In addition to Dan Puzey's answer , there's not much sense in assigning an async void method as an event handler for BackgroundWorker , in the first place.

Your worker method bgw_DoWork will return and the background thread will be finished as soon as the execution point hits the first await inside bgw_DoWork . The bot.Update task most likely still will be pending at that point.

You don't need a BackgroundWorker here. The code can be as simple as this:

static void Main(string[] args)
{
    DoWorkAsync().Wait();
}

static async Task DoWorkAsync()
{
    Weather bot = new Weather();

    if (bot.IsRunning)
    {
        await bot.Update();
    }
}