Task.Delay() not behaving as expected
Solution 1
Task.Delay
returns a Task
object on which you have to wait. Otherwise the following code will be executed immediately. Therfore you have to declare your method as async
. Then you can await Task.Delay
public async Task ChattyWriter()
{
int count = 0;
while (true)
{
var message = String.Format("Chatty Writer number {0}", count);
Trace.WriteLine(message);
count++;
await Task.Delay(1000);
...
}
}
You have to interrupt your calling thread somehow. The unit test would terminate and the background thread as well. But if you call this method from the UI, the UI will not get blocked.
By calling Wait
on an asynchronous method you will end up with a deadlock. See here for more information.
Solution 2
Task.Delay(1000);
creates an async task that will complete after a second, but the execution of the current method will continue in parallel.
In order to wait for the task to complete first, you could replace
Task.Delay(1000);
with
Task.Delay(1000).Wait();
Solution 3
Task.Delay() actually creates a task, so you just need to wait for it.
Try this
public static void ChattyWriter()
{
int count = 0;
while (true)
{
var message = String.Format("Chatty Writer number {0}", count);
Console.WriteLine(message);
count++;
var t = Task.Delay(1000);
t.Wait();
if (count >= 20)
{
break;
}
}
}
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trailmax
tech.trailmax.info - My blog. I write about C# and ASP.Net MVC (and sometimes about other random stuff) hrnet.trailmax.info My other blog. There I write about a technology I have been exposed for longer than I wanted. And now I know too much about HR.Net. This is part of my consulting service.
Updated on December 07, 2020Comments
-
trailmax about 3 years
Task.Delay() not behaving as expected or rather I'm not understanding what it is supposed to do. I'm trying to get my head around
Task
s in C# and how to replaceThread
s in my implementation.What I'm trying to do is something like this:
- While true
- print a line
- wait for a second,
- If condition is satisfied, exit loop, otherwise keep looping
I've got that implemented with Threads fine, but all cool kids say I should be using Task, and don't touch Thread.
So for the code I have this (Ignore
[Test]
- this just a convenient way to try out things)[Test] public void ChattyTask() { var chattyTask = new Task(ChattyWriter); chattyTask.Start(); chattyTask.Wait(); } public void ChattyWriter() { int count = 0; while (true) { var message = String.Format("Chatty Writer number {0}", count); Trace.WriteLine(message); count++; Task.Delay(1000); if (count >= 20) { break; } } }
When I run this, the test finishes in milliseconds, not in 20 seconds as I would expect. If I replace
Task.Delay()
withThread.Sleep()
, everything works as expected and I get the printout once a second. I have tried addingasync
andawait
intoChattyWriter()
, but not only it did not add a 1 second delay, it only printed only one line instead of 20.What am I doing wrong?
Probably It'll help to describe what I'm doing: my project works with external API (RESTful) and after I request some tasks to happen, I need to poll the API to check if the task has been completed. External tasks can be long running: 1-15 minutes. So I need some delay between checking for completion. And there could be many different concurrent processes executed with multiple external tasks. I understand, that if I use
Thread.Sleep()
when polling, other processes on the sameThread
will be blocked for no good reason. - While true
-
trailmax over 10 yearsI did try this, and it gave me only first line printed out and completed right after that.
-
slfan over 10 yearsThat's probably because your main thread (the test routine) can go on running and you don't see the rest, because the program terminates. Would you start the thread from a UI button, the UI would still be responsive.
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slfan over 10 yearsThe primary reason why Task.Delay returns a Task is, that you can call await on it. Your answer is the same as @w0lf's
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trailmax over 10 yearsYou are absolutely correct on this one! I've placed
Thread.Sleep(20000)
in the test routine and got what was expecting.