Task.WhenAll(IEnumerable):任务启动两次？

Question

我偶然发现了Task.WhenAll的一个重载,即将 IEnumerable作为参数的重载

public static Task WhenAll(IEnumerable<Task<TResult>> tasks)

我以为我会用以下短程序尝试这个功能.

在Test类中:

// contains the task numbers that has been run
private HashSet<int> completedTasks = new HashSet<int>();

// async function. waits a while and marks that it has been run:
async Task<int> Calculate(int taskNr)
{
     string msg = completedTasks.Contains(taskNr) ?
         "This task has been run before" :
         "This is the first time this task runs";
    Console.WriteLine($"Start task {i} {msg}");

    await Task.Delay(TimeSpan.FromMilliseconds(100));

    Console.WriteLine($"Finished task {taskNr}");
    // mark that this task has been run:
    completedTasks.Add(taskNr);
    return i;
}

// async test function that uses Task.WhenAll(IEnumerable)
public async Task TestAsync()
{
    Console.Write("Create the task enumerators... ");
    IEnumerable<Task<int>> tasks = Enumerable.Range(1, 3)
        .Select(i => Calculate(i));
    Console.WriteLine("Done!");

    Console.WriteLine("Start Tasks and await");
    await Task.WhenAll(tasks);
    Console.WriteLine("Finished waiting. Results:");

    foreach (var task in tasks)
    {
        Console.WriteLine(task.Result);
    }
}

最后主要方案:

static void Main(string[] args)
{
     var testClass = new TestClass();
     Task t = Task.Run(() => testClass.TestAsync());
     t.Wait();
}

输出如下:

Create the task enumerators... Done!
Start Tasks and wait
Start task 1 This is the first time this task runs
Start task 2 This is the first time this task runs
Start task 3 This is the first time this task runs
Finished task 2
Finished task 3
Finished task 1
Finished waiting. Results:
Start task 1 This task has been run before
Finished task 1
1
Start task 2 This task has been run before
Finished task 2
2
Start task 3 This task has been run before
Finished task 3
3

显然每个任务都运行两次!我究竟做错了什么？

更奇怪的是:如果我ToList()在Task.Whenall之前使用任务序列进行枚举,则该函数按预期工作!

Answer 1

您的问题是延迟执行.改变这一行

IEnumerable<Task<int>> tasks = Enumerable.Range(1, 3)
    .Select(i => Calculate(i));

至

var tasks = Enumerable.Range(1, 3)
    .Select(i => Calculate(i)).ToList();

Select()不立即执行 "查询",但返回枚举器.只有在使用此枚举器迭代任务时,才会为序列1 ... 3调用内部lambda.
在您的版本中,每次迭代时tasks,Calculate(i)都会再次调用并创建新任务.
与.ToList()执行枚举一次并且将所得的序列Task<int>被存储在List<Task<int>>(并且当该列表中列举的第二时间不再次生成).

当您调用Task.WhenAll(tasks)此方法时,迭代tasks并从而启动每个任务.当您稍后再次迭代(使用foreach循环输出结果)时,将再次执行查询,从而启动新任务.