我偶然发现Task.WhenAll的一个重载,一个以IEnumerable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。
在测试课程中:
// 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
显然每项任务都会运行两次!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!
答案 0 :(得分:9)
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()
不执行&#34;查询&#34;立即,但返回一个普查员。只有当您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,每次遍历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,结果序列Task<int>
存储在List<Task<int>>
中(当第二次枚举该列表时不再生成)。
当您致电Task.WhenAll(tasks)
时,此方法会迭代tasks
,从而启动每项任务。当您稍后再次迭代时(使用foreach
循环输出结果),查询将再次执行,从而启动新任务。