Task.WhenAll(IEnumerable)：任务启动两次？

Question

我刚刚偶然发现Task.WhenAll 的重载之一，它以 IEnumerable 作为参数

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

我想我会用下面的小程序试试这个功能。

在测试类中：

// 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);
    }
}

最后是主程序：

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了两次！我做错了什么？

更奇怪的是：如果我在 Task.Whenall 之前使用 ToList() 枚举任务序列，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));

到

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

Select() 不会立即执行“查询”，而是返回一个枚举数。仅当您使用此枚举器遍历任务时，才会为序列 1...3 调用内部 lambda。
在您的版本中，每次迭代 tasks 时，都会再次调用 Calculate(i) 并创建新任务。
使用.ToList()，枚举器执行一次，Task<int> 的结果序列存储在List<Task<int>> 中（并且在第二次枚举该列表时不会再次生成）。

---

当您调用Task.WhenAll(tasks) 时，此方法将遍历tasks，从而启动每个任务。当您稍后再次迭代时（使用 foreach 循环输出结果），查询会再次执行，从而启动新任务。