任务.开始奇怪的行为 [英] Task.Start strange behavior

问题描述

你好，我有下面的代码创建一个任务.然后将其设置为开始.该任务旨在将响应添加到 ConcurrentBag 列表中.但是等待似乎并不在等待所有任务完成.但是它们被标记为完成.列表中只有一项任务.使用 Task.Run ！

Hello I have the following code that creates a task. Then sets it to start. The task is meant to add a response to the ConcurrentBag list. But the await does not seem to be waiting for all the tasks to complete. But they are being marked as completed. There is only one task in the list. It works fine when using Task.Run!

public async void RunT1()
{          
    DoesNotWork();
}

public async void RunT2()
{          
    DoesWork();
}

public async void DoesNotWork()
{
    ConcurrentBag<string> concurrentBag = new ConcurrentBag<string>();
    List<Task> taskList = new List<Task>();

    Task task1 = new Task(async () =>
    {
        var xml = await LongWorkingMethod();
        concurrentBag.Add(xml);
    });

    taskList.Add(task1);

    taskList[0].Start();
    await Task.WhenAll(taskList);

    if (concurrentBag.Count > 0) //concurrentBag is empty,
        //even though the task has finished
    {
        Debug.Print("success");
    }
}

public async void DoesWork()
{
    ConcurrentBag<string> concurrentBag = new ConcurrentBag<string>();
    List<Task> taskList = new List<Task>();

    Task task1 = Task.Run(async () =>
    {
        var xml = await LongWorkingMethod();
        concurrentBag.Add(xml);
    });

    taskList.Add(task1);

    await Task.WhenAll(taskList);

    if (concurrentBag.Count > 0) //concurrentBag is NOT empty
    {
        Debug.Print("success");
    }
}

推荐答案

几乎没有理由直接使用 Task 构造函数.

There's almost never a reason to use the Task constructor directly.

在您的情况下， Task 构造函数不支持 async 委托(即 Action 而不是 Func< Task> )，当您将其中一个作为参数传递时，将其视为 async void .

In your case the Task constructor doesn't support an async delegate (i.e. Action instead of Func<Task>) and when you pass one as a parameter is it treated as async void.

这意味着调用 Start 即发即忘" 委托，由于没有 Task，它无法等待内部的异步操作完成到 await 上.

That means that calling Start "fires and forgets" the delegate and it can't wait for the asynchronous operation inside it to complete because there's no Task to await on.

var task = new Task(async () => await Task.Delay(1000));
task.Start();
await task; // completes immediately

在需要运行新的 Task 的情况下，请使用 Task.Run .如您所见， Task.Run 确实支持 async 委托，并等待整个操作完成

Use Task.Run in cases where you need to run a new Task. Task.Run, as you see, does support an async delegate correctly and waits for the entire operation to complete

var task = Task.Run(async () => await Task.Delay(1000));
await task; // completes after 1000 milliseconds

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