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
I know I may be downvoted but apparently I just don't understand async-await enough and the questions/answer I found, and the articles I found, didn't help me to find an answer for this question:
How can I make "2" to be printed out? Or actually, WHY doesn't 2 gets printed out, both in await t and in t.Wait() ?:
static Task t;
public static async void Main()
{
Console.WriteLine("Hello World");
t = GenerateTask();
await t;
//t.Wait();
Console.WriteLine("Finished");
}
public static Task GenerateTask()
{
var res = new Task(async () =>
{
Console.WriteLine("1");
await Task.Delay(10000);
Console.WriteLine("2");
});
res.Start();
return res;
}
Edit: I'm creating a task and returning it cause in real-life I need to await on this task later on, from a different method.
Edit2: await Task.Delay is just a placeholder for a real-life await on a different function.
Printing '2'
The 2 is actually printed, 10 seconds after 1 is printed. You can observe this if you add Console.ReadLine(); after printing 'Finished'.
The output is
Hello World
1
Finished
2
What is happening?
When you await t (which is res in GenerateTask method) you are awaiting the created Task and not the task that res created.
How to fix (fancy way)
You will need to await both the outer task and inner task. To be able to await the inner task you need to expose it. To expose it you need to change the type of the task from Task to Task<Task> and the return type from Task to Task<Task>.
It could look something like this:
public static async Task Main()
{
Console.WriteLine("Hello World");
var outerTask = GenerateTask();
var innerTask = await outerTask; // what you have
await innerTask; // extra await
Console.WriteLine("Finished");
Console.ReadLine();
}
public static Task<Task> GenerateTask() // returns Task<Task>, not Task
{
var res = new Task<Task>(async () => // creates Task<Task>, not Task
{
Console.WriteLine("1");
await Task.Delay(TimeSpan.FromSeconds(10));
Console.WriteLine("2");
});
res.Start();
return res;
}
The output now is:
Hello World
1
2
Finished
How to fix (easy way)
The outer task is not needed.
public static async Task Main()
{
Console.WriteLine("Hello World");
var t = GenerateTask();
await t;
Console.WriteLine("Finished");
Console.ReadLine();
}
public static async Task GenerateTask()
{
Console.WriteLine("1");
await Task.Delay(TimeSpan.FromSeconds(10));
Console.WriteLine("2");
}
It looks like it's because the constructor to new Task only takes some form of an Action (So the Task never gets returned even though it's async). So essentially what you're doing is an Async void with your delegate. Your await Task.Delay(10000) is returning and the action is considered 'done'.
You can see this if you change the await Task.Delay(10000) to Task.Delay(10000).Wait() and remove the async from the delegate.
On another note though, I've never personally seen or used new Task before. Task.Run() is a much more standard way to do it, and it'll allow for the await to be used. Also means you don't have to call Start() yourself.
Also you might already know this but, in this specific case you don't need a new task at all. You can just do this:
public static async Task GenerateTask()
{
Console.WriteLine("1");
await Task.Delay(10000);
Console.WriteLine("2");
}
Regarding your edits
Replacing your GenerateTask with what I wrote should do what you want. The async/await will turn your method into a Task that has started execution. This is exactly what you are trying to do so I'm not quite sure what you are asking with your edits.
The task returned from GenerateTask can be awaited whenever you want, or not awaited at all. You should almost never need to do new Task(). The only reason I can think is if you wanted to delay execution of the task until later, but there would be better ways around it rather than calling new Task().
If you use the way I showed in your real-life situation, let me know what doesn't work about it and I'll be happy to help.
You should use Task.Run() rather than creating a Task directly:
public static Task GenerateTask()
{
return Task.Run(async () =>
{
Console.WriteLine("1");
await Task.Delay(10000);
Console.WriteLine("2");
});
}
Task.Start() doesn't work because it doesn't understand async delegates, and the returned task just represents the beginning of the task.
Note that you can't fix this by using Task.Factory.StartNew() either, for the same reason.
See Stephen Cleary's blog post on this issue, from which I quote:
[Task.Factory.StartNew()] Does not understand async delegates. This is actually the same as
point 1 in the reasons why you would want to use StartNew. The problem
is that when you pass an async delegate to StartNew, it’s natural to
assume that the returned task represents that delegate. However, since
StartNew does not understand async delegates, what that task actually
represents is just the beginning of that delegate. This is one of the
first pitfalls that coders encounter when using StartNew in async
code.
These comments also apply to the Task constructor, which also doesn't understand async delegates.
However, it's important to note that if you are already awaiting in the code and you don't need to parallelise some compute-bound code, you don't need to create a new task at all - just using the code in your Task.Run() on its own will do.
While messing around with SignalR I found a behaviour that confuse me.
Calling StartCountDown from a client then make a call to Join behaves like
wait 10 seconds
Call clients CountDownStarted
Then call PlayerJoined
What I expected.
Call start CountDown, return
immediately call PlayerJoined
After 10 seconds complete CountDownStarted.
public class AHub : Hub
{
public async Task Join(string player)
{
await Clients.All.PlayerJoined(player);
}
public async Task StartCountDown()
{
await Task.Delay(10000);
await Clients.All.CountDownStarted();
}
}
This is from a SignalR Hub
This is a common misconception about the async and await pattern. Awaiting something does actually await the completion of the task.
If you want to run the task unobserved (or colloquially known as fire and forget), you could do thus
// task gets started hot and unobserved, remove the warning with a discard
_ = StartCountDownAsync();
Note : An exception that's raised in a method that returns a Task or Task<TResult> is stored in the returned task. If you don't await the task or explicitly check for exceptions, the exception is lost. If you await the task, its exception is rethrown.
As a best practice, you should always await the call.
Though, you have other options. Which is to start a task, complete other tasks, and then await the completion of the original
Given
public async Task SomeTask1() { }
public async Task SomeTask2() { }
public async Task SlowApiAsync() { }
You might want
var slowApiTask = SlowApiAsync();
await SomeTask1();
await SomeTask2();
await slowApiTask;
Or if you want to run all the tasks concurrently (and yet await them all)
var slowApiTask = SlowApiAsync();
var task1 = SomeTask1();
var task2 = SomeTask2();
await Task.WhenAll(slowApiTask,task1,task2)
In my current project, I have a piece of code that, after simplifying it down to where I'm having issues, looks something like this:
private async Task RunAsync(CancellationToken cancel)
{
bool finished = false;
while (!cancel.IsCancellationRequested && !finished)
finished = await FakeTask();
}
private Task<bool> FakeTask()
{
return Task.FromResult(false);
}
If I use this code without awaiting, I end up blocking anyway:
// example 1
var task = RunAsync(cancel); // Code blocks here...
... // Other code that could run while RunAsync is doing its thing, but is forced to wait
await task;
// example 2
var task = RunAsync(cancelSource.Token); // Code blocks here...
cancelSource.Cancel(); // Never called
In the actual project, I'm not actually using FakeTask, and there usually will be some Task.Delay I'm awaiting in there, so the code most of the time doesn't actually block, or only for a limited amount of iterations.
In unit testing, however, I'm using a mock object that does pretty much do what FakeTask does, so when I want to see if RunAsync responds to its CancellationToken getting cancelled the way I expect it to, I'm stuck.
I have found I can fix this issue by adding for example await Task.Delay(1) at the top of RunAsync, to force it to truly run asynchronous, but this feels a bit hacky. Are there better alternatives?
You have an incorrect mental picture of what await does. The meaning of await is:
Check to see if the awaitable object is complete. If it is, fetch its result and continue executing the coroutine.
If it is not complete, sign up the remainder of the current method as the continuation of the awaitable and suspend the coroutine by returning control to the caller. (Note that this makes it a semicoroutine.)
In your program, the "fake" awaitable is always complete, so there is never a suspension of the coroutine.
Are there better alternatives?
If your control flow logic requires you to suspend the coroutine then use Task.Yield.
Task.FromResult actually runs synchronously, as would await Task.Delay(0). If you want to actually simulate asynchronous code, call Task.Yield(). That creates an awaitable task that asynchronously yields back to the current context when awaited.
As #SLaks said, your code will run synchronously. One thing is running async code, and another thing is running parallel code.
If you need to run your code in parallel you can use Task.Run.
class Program
{
static async Task Main(string[] args)
{
var tcs = new CancellationTokenSource();
var task = Task.Run(() => RunAsync("1", tcs.Token));
var task2 = Task.Run(() => RunAsync("2", tcs.Token));
await Task.Delay(1000);
tcs.Cancel();
Console.ReadLine();
}
private static async Task RunAsync(string source, CancellationToken cancel)
{
bool finished = false;
while (!cancel.IsCancellationRequested && !finished)
finished = await FakeTask(source);
}
private static Task<bool> FakeTask(string source)
{
Console.WriteLine(source);
return Task.FromResult(false);
}
}
C#'s async methods execute synchronously up to the point where they have to wait for a result.
In your example there is no such point where the method has to wait for a result, so the loop keeps running forever and thereby blocking the caller.
Inserting an await Task.Yield() to simulate some real async work should help.
Lets say I have a method defined as follows:
public async Task CreateUser()
{
await GetUserDetails();
GetUserOrder();
}
private void GetUserDetails() {
private void GetUserOrder() {
Does the method GetUserDetails(); and GetUserOrder() have to be async as well to avoid UI blocking ?
I cannot await the GetUserDetails() method since it is not async. How can I achieve this in c# ? I want to ensure all these methods are invoked step by step.
The relevant question is in a comment:
How can I ensure all my methods are invoked completely sequentially?
The fact that you're asking the question indicates that you don't understand what "await" is. Await is the sequencing operation on a asynchronous workflows. An await means this workflow will not proceed until the awaited task is complete. It's an asynchronous wait, hence the name await.
Consider this question: in a synchronous workflow, what is the sequencing operation?
No, really, give it some thought.
.
.
.
It is ;. When you say
fResult = foo();
bResult = bar();
qResult = qux();
that means that foo has to finish completely before bar can begin. That is not true for asynchronous workflows. If we have
fTask = fooAsync();
bTask = barAsync();
qTask = quxAsync();
Then the asynchronous operations can complete in any order. If we say
await fooAsync();
await barAsync();
await quxAsync();
Then barAsync will not start until fooAsync's task completes. await sequences the asynchronous workflow. The difference is that the thread can continue to do other unrelated work while asynchronously waiting for foo to complete, which is not true in a synchronous workflow; in a synchronous workflow the thread is already busy computing the foo result, so it can't do other work.
yes if you want to wait than you have to write await for that methods also. because after first await your code agian will be synchronous ..and if UI thread than it will run on it.
1.you code will be , so by this code you code become asynchronous for GetUserORder also. you just have to wrap method in Task construct and return
public async Task CreateUser()
{
await GetUserDetails();
await Task.Factory.SartNew(()=> GetUserOrder());
}
2.or you can do this also
public async Task CreateUser()
{
await Task.Factory.SartNew(()=>{
GetUserDetails();
GetUserOrder(); });
}
3.or you can do like this also, in below code will not wait for getuserorder method and excute await one method
public async Task CreateUser()
{
Task.Factory.SartNew(()=> GetUserOrder()).ContinueWith((t)=> Console.WriteLine("Completed");
await GetUserDetails();
}
4.or last one variation, here you start GetUserOrder first and dont wait for it than you call GetUserDetails in async fashion , and if you want to work on GetUserOrder method want to wait just use Wait method.
public async Task CreateUser()
{
var task = Task.Factory.SartNew(()=> GetUserOrder());
await GetUserDetails();
if(!task.IsCompleted)
task.Wait();
}
in your case you can go for 3 and if you want to wait go for 4th one.
As you asked me in comment what is difference between Task.Run and statnew method -: for that you can check this SO question : Regarding usage of Task.Start() , Task.Run() and Task.Factory.StartNew()
You should put await only in front of async methods. To run a synchronous one that you don't want to wait, you can use a new thread from the tread pool:
new Thread(() => DoSomething()).Start();
or
Task.Factory.SartNew(()=> DoSomething());
Here is the help page: https://msdn.microsoft.com/en-us/library/dd321439(v=vs.110).aspx
Otherwise, your call to GetUserDetails will have to finish before you execute the next line.
I have a simple async and await example I'm trying to work through and the execution is not returning to the caller as I expect. Here is the top level method:
protected async void MyDDL_SelectedIndexChanged(object sender, EventArgs e)
{
Task longRunningTask = LongRunningOperationAsync();
DoOtherStuff1();
DoOtherStuff2();
DoOtherStuff3();
await longRunningTask;
}
Here is the LongRunningOperationAsync method which does not work as expected and runs synchronously:
private async Task LongRunningOperationAsync()
{
var myValues = await GetStuffViaLongRunningTask();
//Code to work with myValues here...
}
Here is the definition of GetStuffViaLongRunningTask
private async Task<IList<MyClass>> GetStuffViaLongRunningTask()
{
//...Calls to get and build up IList<MyClass>
return results;
}
The problem is the above code does not return to the caller and begin running the DoOtherStuff1(); method as I would expect. However, instead of calling my own method and replacing it with a call to await Task.Delay(10000); like all the simple examples show, the code works as expected:
private async Task LongRunningOperationAsync()
{
//Returns to caller as expected:
await Task.Delay(10000);
}
The caller using the code above has longRunningTask with a WaitingForActivation as its status instead of RanToCompletion, showing it is still processing.
You might say my GetStuffViaLongRunningTask() method runs so quickly and I just can't see the results. However it always takes between 3-7 seconds to run and you can tell when debugging that the call is blocking and synchronous.
What am I doing incorrectly here, so that my call to LongRunningOperationAsync() is not working asynchronously when reaching the await word to call LongRunningOperationAsync within that method?
Assuming that //...Calls to get and build up IList<MyClass> is synchronous CPU bound work, the issue there is that GetStuffViaLongRunningTask won't return until it either ends, or hits its first await call. You should be getting a compiler warning on that method as it's an async method that doesn't have an await in it.
Instead, the method simply shouldn't be async, to clearly indicate to it's callers that it's synchronous work. Just adjust the signature to:
private IList<MyClass> GetStuffViaLongRunningTask()
Then when calling it use Task.Run to ensure that the long running CPU bound work is done in another thread:
private async Task LongRunningOperationAsync()
{
var myValues = await Task.Run(() => GetStuffViaLongRunningTask());
//Code to work with myValues here...
}
//...Calls to get and build up IList<MyClass>
You need to show us which calls are being made. If you want to use async/await with this structure then you need to make an async call.
If your GetStuffViaLongRunningTask function is not doing async calls then you can start a new task like the following:
private Task<IList<MyClass>> GetStuffViaLongRunningTask()
{
return Task.Factory.StartNew(() =>
{
//...Calls to get and build up IList<MyClass>
// You can make synchronous calls here
return list;
});
}