I think I missunderstanding the behaviour of async await in c#.
I have two methods that return a Task defined like
public async Task Name()
{
await AsyncOperation()
}
Imagine AsyncOperation() like an PostAsync of HttpClient.
Now I call them inside some other methods
public asyn Task Method()
{
await Name1(() => { "bla bla"});
await Name2();
Console.WriteLine("Continue");
}
This works as expected to me. Waits until Name1() and Name2() finish and then continues.
Now I need to nest Name1() and Name2(). In fact Name1() is a Please Wait Window that recieve as lambda parameters a slow operation, while Name2() is a slow download of a file. I want the Plese Wait window appears while the file is downloaded.
So I try something like this:
public asyn Task Method()
{
await Name1( async ()=>
{
await Name2();
}
Console.WriteLine("Continue");
}
In this case the execution doesnt wait untile Name2() finished. Why this happen and await doesnt wait?
Update
This is the logic behind the method of please wait. It shows a Please Wait message using Mahapps Dialogs, executes the code that recieves by the lambda, and then close the please wait message.
public static async Task Name1(Action longOperation)
{
_progressController = await _metroWindow.ShowProgressAsync("Please wait...");
await Task.Run(() => longOperation());
await _progressController.CloseAsync();
}
The Name1 method takes a delegate and returns a Task<T> where T is the type returned by the delegate. In your case, the delegate returns Task, so we get Task<Task> as the result. Using await waits only for the completion of the outer task (which immediately returns the inner task) and the inner task is then ignored.
You can fix this by dropping the async and await in the lambda function.
Also, take a look at Asynchronous Gotchas in C#.
Related
In my WPF application, i am calling relaycommand
private void AutoRun(object parameter)
{
for(int i=0;i<10;i++)
{
MoveLotCommand.Execute(0);
}
}
which inturns call another relay command
private void MoveLot(object parameter)
{
//Some Code
var Task = StartLotProcessing(currentAssemblyPlant);
}
}
and this relay command will call another async function
async Task StartLotProcessing(int currentAssemblyPlant)
{
await Task.Delay(5000);
var nextAssemblyPlant = currentAssemblyPlant + 1;
//Process after await
}
The issue is, my code after 'await Task.Delay(5000)' does not executes until my AutoRun() function execution is completed.
I am trying to execute the await async code for each variable in my for loop.
Any help would be appreciated and sorry for my explanation.
You are not calling the method correctly : you are missing the await operator to indicate that you have to wait, otherwise you effectively schedule the task but do not wait for it. The task is scheduled in a synchronous fashion, hence when your method returns.
Change your call to this, and the issue should go away:
await StartLotProcessing(currentAssemblyPlant);
The problem appears to be that you are calling StartLotProcessing asynchronously but not waiting for the task to complete before moving on.
Ideally you would call use async/await the whole way up the stack:
await MoveLotCommand.Execute(0);
...
await StartLotProcessing(currentAssemblyPlant);
If you're unable to do that, then you should call
var task = StartLotProcessing(currentAssemblyPlant);
...
task.Wait();
I'm still getting up to speed with async & multi threading. I'm trying to monitor when the Task I Start is still running (to show in a UI). However it's indicating that it is RanToCompletion earlier than I want, when it hits an await, even when I consider its Status as still Running.
Here is the sample I'm doing. It all seems to be centred around the await's. When it hits an await, it is then marked as RanToCompletion.
I want to keep track of the main Task which starts it all, in a way which indicates to me that it is still running all the way to the end and only RanToCompletion when it is all done, including the repo call and the WhenAll.
How can I change this to get the feedback I want about the tskProdSeeding task status?
My Console application Main method calls this:
Task tskProdSeeding;
tskProdSeeding = Task.Factory.StartNew(SeedingProd, _cts.Token);
Which the runs this:
private async void SeedingProd(object state)
{
var token = (CancellationToken)state;
while (!token.IsCancellationRequested)
{
int totalSeeded = 0;
var codesToSeed = await _myRepository.All().ToListAsync(token);
await Task.WhenAll(Task.Run(async () =>
{
foreach (var code in codesToSeed)
{
if (!token.IsCancellationRequested)
{
try
{
int seedCountByCode = await _myManager.SeedDataFromLive(code);
totalSeeded += seedCountByCode;
}
catch (Exception ex)
{
_logger.InfoFormat(ex.ToString());
}
}
}
}, token));
Thread.Sleep(30000);
}
}
If you use async void the outer task can't tell when the task is finished, you need to use async Task instead.
Second, once you do switch to async Task, Task.Factory.StartNew can't handle functions that return a Task, you need to switch to Task.Run(
tskProdSeeding = Task.Run(() => SeedingProd(_cts.Token), _cts.Token);
Once you do both of those changes you will be able to await or do a .Wait() on tskProdSeeding and it will properly wait till all the work is done before continuing.
Please read "Async/Await - Best Practices in Asynchronous Programming" to learn more about not doing async void.
Please read "StartNew is Dangerous" to learn more about why you should not be using StartNew the way you are using it.
P.S. In SeedingProd you should switch it to use await Task.Delay(30000); insetad of Thread.Sleep(30000);, you will then not tie up a thread while it waits. If you do this you likely could drop the
tskProdSeeding = Task.Run(() => SeedingProd(_cts.Token), _cts.Token);
and just make it
tskProdSeeding = SeedingProd(_cts.Token);
because the function no-longer has a blocking call inside of it.
I'm not convinced that you need a second thread (Task.Run or StartNew) at all. It looks like the bulk of the work is I/O-bound and if you're doing it asynchronously and using Task.Delay instead of Thread.Sleep, then there is no thread consumed by those operations and your UI shouldn't freeze. The first thing anyone new to async needs to understand is that it's not the same thing as multithreading. The latter is all about consuming more threads, the former is all about consuming fewer. Focus on eliminating the blocking and you shouldn't need a second thread.
As others have noted, SeedingProd needs to return a Task, not void, so you can observe its completion. I believe your method can be reduced to this:
private async Task SeedingProd(CancellationToken token)
{
while (!token.IsCancellationRequested)
{
int totalSeeded = 0;
var codesToSeed = await _myRepository.All().ToListAsync(token);
foreach (var code in codesToSeed)
{
if (token.IsCancellationRequested)
return;
try
{
int seedCountByCode = await _myManager.SeedDataFromLive(code);
totalSeeded += seedCountByCode;
}
catch (Exception ex)
{
_logger.InfoFormat(ex.ToString());
}
}
await Task.Dealy(30000);
}
}
Then simply call the method, without awaiting it, and you'll have your task.
Task mainTask = SeedingProd(token);
When you specify async on a method, it compiles into a state machine with a Task, so SeedingProd does not run synchronously, but acts as a Task even if returns void. So when you call Task.Factory.StartNew(SeedingProd) you start a task that kick off another task - that's why the first one finishes immediately before the second one. All you have to do is add the Task return parameter instead of void:
private async Task SeedingProdAsync(CancellationToken ct)
{
...
}
and call it as simply as this:
Task tskProdSeeding = SeedingProdAsync(_cts.Token);
I am not sure whether this is possible, so here me out:
I have a sequence of action to perform multiple
async Task MethodA(...)
{
// some code
// a call to specific Async IO bound method
// some code
}
there are also MethodB(), MethodC(), etc, and all of the have exactly the same code, except for the call to specific Async IO bound method. I am trying to find a way to pass a task pointer to a method, so that we can execute it later in a Method().
What i am currently doing is this:
private async Task Method(Func<Task<Entity>> func)
{
// some code
var a = await task.Run(func);
// some code
}
var task = async () => await CallToIOBoundTask(params);
Method(task);
This code, however, pulls a new thread each time, which is not required for IO bound task, and should be avoided.
So, is there a way to refactor the code so that no ThreadPool thread is used? A goal is to have a code like this:
private async Task Method(Task<Entity> task)
{
// some code
var a = await task;
// some code
}
It is also important to mention that different IO calls have different method signatures. Also, a task can start to execute only in Method() body, and not before.
Of course, simply invoke the func, get back a task, and await it:
async Task Method(Func<Task<Entity>> func)
{
// some code
var a = await func();
// some code
}
Also, when you're sending that lambda expression, since all it's doing is calling an async method which in itself returns a task, it doesn't need to be async in itself:
Method(() => CallToIOBoundTask(params));
That's fine as long as all these calls return Task<Entity>. If not, you can only use Task (which means starting the operation and awaiting its completion) and you won't be able to use the result.
Lets say I have this code:
public async void Init(){
init1();
init2();
init3();
}
First question is are they running asynchronously or not? The methods themselves are public void not async.
Second question is how do I make sure they're running async and to know when they're finished? Like this..
public async void Init(){
init1();
init2();
init3();
await ... all finished
}
Third, when I call Init() elsewhere, can it not be wrapped by an async method? Like this code below, or does it have to be async?
public async void Init(){
init1();
init2();
init3();
}
public void doIt(){
Init();
}
I have an async intro that should help.
First question is are they running asynchronously or not?
No, and you didn't really need to ask this on Stack Overflow because the compiler itself will give you a warning that explicitly states your method will run synchronously.
Second question is how do I make sure they're running async and to know when they're finished?
First, your methods need to be properly asynchronous:
async Task init1Async();
async Task init2Async();
async Task init3Async();
Then you can make the caller invoke them concurrently and then asynchronously wait for them all to complete:
async Task InitAsync() {
var task1 = init1Async();
var task2 = init2Async();
var task3 = init3Async();
await Task.WhenAll(task1, task2, task3);
}
Third, when I call Init() elsewhere, can it not be wrapped by an async method?
Once your InitAsync method is properly defined as returning a Task, it can be naturally consumed using await:
async Task doItAsync() {
await InitAsync();
}
Note that this does require the caller to be async.
Also recommended reading: my Best Practices for Asynchronous Code MSDN article, particularly the sections on "avoid async void" and "async all the way".
Also One Thing you can do await on each on the methods to get the required result of each methods.
var task1 = await init1Async();
var task2 = await init2Async();
var task3 = await init3Async();
i go through a msdn sample code where a function is called when button is clicked and when routine is called then Await keyword is used and function has async keyword used.
private async void StartButton_Click(object sender, RoutedEventArgs e)
{
int contentLength = await AccessTheWebAsync();
resultsTextBox.Text +=
String.Format("\r\nLength of the downloaded string: {0}.\r\n", contentLength);
}
async Task<int> AccessTheWebAsync()
{
HttpClient client = new HttpClient();
Task<string> getStringTask = client.GetStringAsync("http://msdn.microsoft.com");
DoIndependentWork();
string urlContents = await getStringTask;
return urlContents.Length;
}
void DoIndependentWork()
{
resultsTextBox.Text += "Working . . . . . . .\r\n";
}
When AccessTheWebAsync is called then await keyword is used, what does it mean?
When this function AccessTheWebAsync() will be executing then DoIndependentWork() function is called and i guess here control will be waiting until this function DoIndependentWork() is finished. Am I right?
again there is another statement called
string urlContents = await getStringTask;
why they use here await. if we do not use await here then what would happen?
Please guide me to understand the code that how it is working.
I have an intro blog post here, and the MSDN docs are also extremely good.
You can think of await as "pausing" the method (without blocking the thread) until the awaitable operation completes. When this happens, it returns a task that is not completed to the calling method, so it also has the option to await.
Here's a brief description about async/await methods.
Async Methods:
Caller is not necessarily blocked for the full execution of async
method
Possible return types
void: “fire-and-forget”
Task: allows to await termination of async method
Task<T>: allows to await termination and get result of type T
No ref or out parameter for async methods
Must again contain an await => Otherwise compile warning
await for Tasks
Await termination of a TPL task
Return result of task (if task with
result type)
Must only occur in async methods => Otherwise compile error
An async method is partly synchronous and partly asynchronous
Caller synchronously executes the method until a blocking await
Thereafter, the method rest is asynchronously executed
async & await Mechanism
Efficient
public async Task<int> GetSiteLengthAsync(string url)
{
HttpClient client = new HttpClient(); <= Sync
Task<string> download1 = client.GetStringAsync(url); <= Sync
string site1 = await download1; <= Async (Another thread)
return site1.Length; <= Async (Another thread)
}
Not sure if that simplier for you to understand that in the following way, but this is how it helped myself:
As you can see, the AccessTheWebAsync returns Task<int> but not just int.
If you would have called it without "await", you would just get the Task<int> object as its result. And could do anything further you want (manually) with that task: for instance, to wait until it finishes theTask.Wait(); and obtain the result of int in theTask.Result.
But await does all that instead of you and returns just int: Task<int> => int.
This is it.
from MSDN:
the await operator is applied to a task in an asynchronous method to suspend the execution of the method until the awaited task completes. The task represents ongoing work.await does not block the thread on which it is executing. Instead, it causes the compiler to sign up the rest of the async method as a continuation on the awaited task. Control then returns to the caller of the async method. When the task completes, it invokes its continuation, and execution of the async method resumes where it left off.
So when the compiler encounter
int contentLength = await AccessTheWebAsync();
it waits till the AccessTheWebAsync() task is complted
please take a look at this example C# Async,Await
All await does is blocks the thread until the result of an async operation returns.
Edit: sorry when I said block I should have said suspend, since blocking would prevent execution from continuing!
Edit2: As Alex pointed out - I should have probably said "execution is suspended" rather than the thread. Basically "Stuff happens until await returns but the point is it appears you are writing and executing synchronous code"
Since async operations have the potential to be take a while (e.g. web service calls), they tend to use callbacks to return a result.
If you have to handle callbacks in your code it can get a little messy, especially when waiting on multiple async tasks that are dependant on each other. Async/await and Task simplify the code so that you can write async code literally in the order you would read it.
e.g. example standard async code with callbacks:
public int CallSomeServiceAndReturnItsValue()
{
int result = 0;
WebService.SomeAsyncServiceCall((svcResult) => { result = svcResult.Value; });
return result;
}
and if you have multiple calls that need to be chained:
public int CallSomeServiceAndReturnItsValue()
{
int result = 0;
WebService.GetSomeIdentifier((svcResult) =>
{
var identifier = svcResult.Value;
WebService.GetResult(identifier, (anotherResult) =>
{
result = anotherResult.Value;
}
}
);
return result;
}
As you can see, it starts getting messy, the code doesn't really read in an order that feels natural. The alternative is to use callback methods instead of anonymous methods but still, the callback methods are far away from the code that called them and things can feel disjointed
The other alternative is of course async/await which is much clearer
public int CallSomeServiceAndReturnItsValue()
{
int identifier = await WebService.GetSomeIdentifier();
return await WebService.GetResult(identifier);
}