如何正确使用 LongRunning Task

郑子铭

发布于 2025-07-03 14:36:29

10900

代码可运行

文章被收录于专栏：DotNet NB && CloudNativeDotNet NB && CloudNative

运行总次数：0

代码可运行

在上一篇文章《如何正确实现一个后

        protected override Task ExecuteAsync(CancellationToken stoppingToken)
        {
            return Task.Factory.StartNew(async () =>
            {
                while (!stoppingToken.IsCancellationRequested)
                {
                    // Simulate some work
                    Console.WriteLine("HostServiceTest_A is doing work.");

                    LongTermTask();

                    await Task.Delay(1000, stoppingToken); // Delay for 1 second
                }

                Console.WriteLine("HostServiceTest_A task done.");

            }, TaskCreationOptions.LongRunning);
        }

        private void LongTermTask()
        {
            // Simulate some work
            Console.WriteLine("LongTermTaskA is doing work.");
            Thread.Sleep(30000);
        }

但是被黑洞视界 大佬指出这个用法是错误的：以上用法并不能保证任务始终在同一个 Task(线程) 上执行。原因是当碰到第一个 await 之后运行时会从 ThreadPool 中调度一个新的线程来执行后面的代码，而当前线程被释放。这个时候就不符合我们使用 LongRunning 的期望了。

在 .NET 中，Task.Factory.StartNew 提供了 TaskCreationOptions.LongRunning 选项，很多开发者会用它来启动长时间运行的任务，并且想当然的认为它会永远执行在同一个线程上。但是事实上当遇到 asyncawait 的时候并想象的那么简单。

下面我们还是通过一个错误的示例开始讲解如何正确的使用它。

错误用法

很多人会直接在 Task.Factory.StartNew 里传入一个 async 方法：

// See https://aka.ms/new-console-template for more information
Console.WriteLine("Hello, World!");

var task = Task.Factory.StartNew(async () =>
{
    Console.WriteLine($"long running task starting. Thread id: {Thread.CurrentThread.ManagedThreadId}");
    var loopCount = 1;
    while (true)
    {
        Console.WriteLine($"\r\nStart: loop count: {loopCount}, Thread id: {Thread.CurrentThread.ManagedThreadId}");
        await LongRunningJob();
        Console.WriteLine($"End: loop count: {loopCount}, Thread id: {Thread.CurrentThread.ManagedThreadId} \r\n ");
        loopCount++;
    }

}, TaskCreationOptions.LongRunning);


static async Task LongRunningJob()
{
    Console.WriteLine($"task doing. Thread id: {Thread.CurrentThread.ManagedThreadId}");
    await Task.Delay(1000);
}

Console.ReadLine();

输出：

Hello, World!
long running task starting. Thread id: 12

Start: loop count: 1, Thread id: 12
task doing. Thread id: 12
End: loop count: 1, Thread id: 11


Start: loop count: 2, Thread id: 11
task doing. Thread id: 11
End: loop count: 2, Thread id: 11

可以看到，第一次循环后，线程 id 发生了变化。很明显 LongRunning 失效了。原因开篇已经讲了，不再赘述。

正确用法 1：同步方法

将 LongRunningJob 改为同步方法，避免异步切换线程：

var task = Task.Factory.StartNew(() =>
{
    Console.WriteLine($"long running task starting. Thread id: {Thread.CurrentThread.ManagedThreadId}");
    var loopCount = 1;
    while (true)
    {
        Console.WriteLine($"\r\nStart: loop count: {loopCount}, Thread id: {Thread.CurrentThread.ManagedThreadId}");
        LongRunningJob();
        Console.WriteLine($"End: loop count: {loopCount}, Thread id: {Thread.CurrentThread.ManagedThreadId} \r\n ");
        loopCount++;
    }

}, TaskCreationOptions.LongRunning);


static void LongRunningJob()
{
    Console.WriteLine($"task doing. Thread id: {Thread.CurrentThread.ManagedThreadId}");
    Thread.Sleep(1000);
}

输出：

Hello, World!
long running task starting. Thread id: 12

Start: loop count: 1, Thread id: 12
task doing. Thread id: 12
End: loop count: 1, Thread id: 12

线程 id 始终不变，说明始终运行在专用线程上。

正确用法 2：异步方法同步等待

如果必须用异步方法，可以用 .Wait() 让调用变为同步：

var task = Task.Factory.StartNew(() =>
{
    Console.WriteLine($"long running task starting. Thread id: {Thread.CurrentThread.ManagedThreadId}");
    var loopCount = 1;
    while (true)
    {
        Console.WriteLine($"\r\nStart: loop count: {loopCount}, Thread id: {Thread.CurrentThread.ManagedThreadId}");
        LongRunningJob().Wait();
        Console.WriteLine($"End: loop count: {loopCount}, Thread id: {Thread.CurrentThread.ManagedThreadId} \r\n ");
        loopCount++;
    }

}, TaskCreationOptions.LongRunning);


static async Task LongRunningJob()
{
    Console.WriteLine($"task doing. Thread id: {Thread.CurrentThread.ManagedThreadId}");
    await Task.Delay(1000);
}

输出：

Hello, World!
long running task starting. Thread id: 12

Start: loop count: 1, Thread id: 12
task doing. Thread id: 12
End: loop count: 1, Thread id: 12