Task.WaitAll - Not Waiting?

Question

I am trying to get my head around Task.WaitAll(..) in the Task Parallel Library.

I am running a test using the following code to try to simulate two tasks, one that will run longer (10 seconds) than the specified amount of time to wait for (10,, and one to run less (3 seconds) than the specified amount of time to wait for. The specified amount of time is 5 seconds. The code I am using is the following:

Task<int>[] tasks = new Task<int>[]
{
    Task.Factory.StartNew<int>(()=>
        {
            Thread.Sleep(10000);
            return 1;
        }),
    Task.Factory.StartNew<int>(()=>
        {
            Thread.Sleep(3000);
            return 2;
        })
};

TimeSpan timeSpan = new TimeSpan(0, 0, 5);
Task.WaitAll(tasks,timeSpan);

int[] results = new int[tasks.Length];
for(int i = 0; i < tasks.Length;i++)
{
    Console.WriteLine(tasks[i].Result);
}

As far as the timeSpan, I have also tried directly passing in 5000 instead of the timeSpan variable, but it is not working. I am getting the following output:

1
2

I would expect to only get the following output because the other thread is executing longer than the expected amount of time to wait.

Am I misunderstanding this, or doing this test wrong?

IMHO, you accepted the wrong answer. :) – James Manning Sep 10 '12 at 18:09 — James Manning, Sep 10 '12 at 18:09
Remember, sometimes the UI takes time to draw. – Jerry Nixon Jul 19 '13 at 20:28 — Jerry Nixon, Jul 19 '13 at 20:28

score 11 · Answer 1 · answered Sep 07 '12 at 15:39

While Task.Delay is what you'd generally want to use instead (so you're not wasting the thread during the sleep), your issue actually isn't related to that.

What's happening here is that when you go to print out the results, you're accessing the Result property of each task. That blocks on the task completing, so you're waiting 5 seconds in your WaitAll, then another 5 seconds when you print out the result of the 10 second task.

Based on your stated intention, you need to check the task status before accessing the Result, since your intent is not to block on it, but only to print it out if it happens to have already finished:

int[] results = new int[tasks.Length];
for (int i = 0; i < tasks.Length; i++)
{
    if (tasks[i].IsCompleted)
    {
        Console.WriteLine(tasks[i].Result);
    }
}

BTW, you can show that the 'block on Result' is what's happening in the original code by just adding some simple time checks (for instance, using Stopwatch, like this)

Task<int>[] tasks = new Task<int>[]
{
    Task.Factory.StartNew<int>(()=>
        {
            Thread.Sleep(10000);
            return 1;
        }),
    Task.Factory.StartNew<int>(()=>
        {
            Thread.Sleep(3000);
            return 2;
        })
};

TimeSpan timeSpan = new TimeSpan(0, 0, 5);
var stopwatch = Stopwatch.StartNew();
Task.WaitAll(tasks, timeSpan);
Console.WriteLine("WaitAll took {0} seconds", stopwatch.Elapsed.TotalSeconds);

int[] results = new int[tasks.Length];
for (int i = 0; i < tasks.Length; i++)
{
    stopwatch = Stopwatch.StartNew();
    Console.WriteLine(tasks[i].Result);
    Console.WriteLine("Printing result took {0} seconds", stopwatch.Elapsed.TotalSeconds);
}

This yields console output of:

WaitAll took 4.9996961 seconds
1
Printing result took 5.0050012 seconds
2
Printing result took 0.0004338 seconds

score 3 · Accepted Answer · edited May 23 '17 at 11:52

3

Do not use Thread.Sleep in tasks. The Task scheduler does not guarantee one thread per tasks and sleeping may affect other tasks (see Task is ignoring Thread.Sleep). Use Task.Delay() instead.

edited May 23 '17 at 11:52

Community

1
1

answered Sep 07 '12 at 15:17

akton

14,148
3
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1

is this only available in 4.5? – TheJediCowboy Sep 07 '12 at 15:21
@CitadelCSAlum Unfortunately, Task.Delay() is only available in .Net 4.5. – akton Sep 07 '12 at 15:24
3

Switching to Task.Delay wouldn't actually do anything to fix the issue here, since the cause is accessing the Result property is blocking until the 10 second task completes. – James Manning Sep 07 '12 at 15:40

score 0 · Answer 3 · answered Sep 07 '12 at 15:21

0

try converting your time span to

TimeSpan span = new TimeSpan(0, 0, 0, 10, 0);
//TimeSpan(Days, hours, minutes, seconds, milliseconds);

answered Sep 07 '12 at 15:21

sikas

5,435
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That shouldn't be the problem. [The constructor of `TimeSpan` that takes three parameters](http://msdn.microsoft.com/en-us/library/bk8a3558.aspx) is hours, minutes, seconds. – svick Sep 07 '12 at 23:46
1

Not to mention, if I'm going to change how it's created, it'll be to use TimeSpan.FromSeconds(10) :) – James Manning Sep 08 '12 at 21:37

Task.WaitAll - Not Waiting?

3 Answers3