ASP.NET Core: BackgroundService停止（StopAsync）后无法重新启动（StartAsync）的问题

这里的 BackgroundService 是指：

Microsoft.Extensions.Hosting.BackgroundService

1. 问题复现

继承该BackgroundService，实现自己的MyService ：

    public class MyService : BackgroundService
    {
        private CancellationTokenSource _CancelSource;

        public async Task StartAsync()
        {
            _CancelSource = new CancellationTokenSource();
            await base.StartAsync(_CancelSource.Token);
            Console.WriteLine("Start");
        }

        public async Task CancelAsync()
        {
            await base.StopAsync(_CancelSource.Token);
            Console.WriteLine("Stop");
        }

        protected override async Task ExecuteAsync(CancellationToken stoppingToken)
        {
            while (!stoppingToken.IsCancellationRequested)
            {
                await Task.Delay(1000, stoppingToken).ContinueWith(tsk =>
                {
                    Console.WriteLine(string.Format("{0} working...", DateTime.Now.ToString("mm:ss")));
                });
            }
        }
    }

实例化后可以启动运行，然后停止。但是再次调用该实例的 StartAsync()就启动不了了。

当然，从 BackgroundService 实现的接口（IHostedService）来看，可能这个类本身就没打算让你手动控制启停。

2. 原因

一句话概括就是

作为函数输入参数的 CancellationToken 并没有用到

这也就是难怪网上的教程都是直接使用 CancellationToken.None 作为输入参数的原因。

下面是详细分析

直接贴下 BackgroundService 的 源码：

// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

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

namespace Microsoft.Extensions.Hosting
{
    /// <summary>
    /// Base class for implementing a long running <see cref="IHostedService"/>.
    /// </summary>
    public abstract class BackgroundService : IHostedService, IDisposable
    {
        private Task _executingTask;
        private readonly CancellationTokenSource _stoppingCts = new CancellationTokenSource();

        /// <summary>
        /// This method is called when the <see cref="IHostedService"/> starts. The implementation should return a task that represents
        /// the lifetime of the long running operation(s) being performed.
        /// </summary>
        /// <param name="stoppingToken">Triggered when <see cref="IHostedService.StopAsync(CancellationToken)"/> is called.</param>
        /// <returns>A <see cref="Task"/> that represents the long running operations.</returns>
        protected abstract Task ExecuteAsync(CancellationToken stoppingToken);

        /// <summary>
        /// Triggered when the application host is ready to start the service.
        /// </summary>
        /// <param name="cancellationToken">Indicates that the start process has been aborted.</param>
        public virtual Task StartAsync(CancellationToken cancellationToken)
        {
            // Store the task we're executing
            _executingTask = ExecuteAsync(_stoppingCts.Token);

            // If the task is completed then return it, this will bubble cancellation and failure to the caller
            if (_executingTask.IsCompleted)
            {
                return _executingTask;
            }

            // Otherwise it's running
            return Task.CompletedTask;
        }

        /// <summary>
        /// Triggered when the application host is performing a graceful shutdown.
        /// </summary>
        /// <param name="cancellationToken">Indicates that the shutdown process should no longer be graceful.</param>
        public virtual async Task StopAsync(CancellationToken cancellationToken)
        {
            // Stop called without start
            if (_executingTask == null)
            {
                return;
            }

            try
            {
                // Signal cancellation to the executing method
                _stoppingCts.Cancel();
            }
            finally
            {
                // Wait until the task completes or the stop token triggers
                await Task.WhenAny(_executingTask, Task.Delay(Timeout.Infinite, cancellationToken));
            }

        }

        public virtual void Dispose()
        {
            _stoppingCts.Cancel();
        }
    }
}

可以看到上面 StartAsync 函数调用 ExecuteAsync 时给它赋的参数直接是一个内部的只读变量，你在外部调用 StartAsync 给它输入的参数根本就没有用到。

结果就是，调用 StopAsync 之后，_stoppingCts 触发了Cancel请求，那么 _stoppingCts.IsCancellationRequested 就变成了 true，因为是只读的，所以再次调用StartAsync 来启动，进入 ExecuteAsync  之后 while判断直接就是false跳出了。

3. 解决办法

方法一：跳过StartAsync、StopAsync ，直接调用 ExecuteAsync ；

方法二：仿照官方的 BackgroundService，实现 IHostedService 接口，自己写一个 BackgroundService

方法三：使用 BackgroundWorker