我只是偶然發現的Task.WhenAll重載之一,即需要一個IEnumerable作爲參數Task.WhenAll(IEnumerable):任務啓動兩次?
public static Task WhenAll(IEnumerable<Task<TResult>> tasks)
我想我會嘗試用下面的短節目此功能。
在測試類:
// 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);
}
}
最後主程序:
static void Main(string[] args)
{
var testClass = new TestClass();
Task t = Task.Run(() => testClass.TestAsync());
t.Wait();
}
輸出如下:
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
顯然,每個任務運行兩次!我在做什麼錯?
更奇怪的是:如果我在Task.Whenall之前使用ToList()
來列舉任務序列,則該函數按預期工作!
「請注意,此函數不是異步的,它不是可等待的」 - 它*是*可等待的。一個函數不需要被標記爲「async」是可以等待的 - 它只需要返回遵循awaitable-awaiter模式的東西。 「Task」是該模式的海報孩子。 –
Sprry,那是我的第一個版本。後來我意識到這可能是問題,我把所有事情都改變爲異步等待。更正了問題 –