If I call RunSynchronously() on a Task in C#, will this lead to asynchronous calls further down the rabbit hole to be run synchronously as well?
Let's say I have a method named UpdateAsync(). Inside this method another asynchronous call is made to DoSomethingAsync() and inside this again we find DoSomethingElseAsync(), will calling RunSynchronously() on 'UpdateAsync()' lead to RunSynchronously() also indirectly being called on DoSomethingAsync()?
The reason for my question:
I have a situation where I "need" to call an asynchronous method (UpdateAsync()) inside a catch-block and wonder if calling this with RunSynchronously() is safe. The documentation is quite clear on the fact that you can't await inside a catch-block. (Strictly speaking I could use a boolean inside the catch-block and call UpdateAsync() after the try-catch, but that feels rather dirty). Sorry about the dual question, but as you probably understand I don't quite know how to phrase it and do not have a really good understanding of this field.
(Edit:
I don't know how to find out if a method was called asynchronously. How would you write a unit test for this? Is it possible to log it somehow?)
I "need" to call an asynchronous method and wonder if calling this with RunSynchronously() is safe
There are two kinds of Tasks: code-based* Tasks (you can create those by using the Task constructor or Task.Factory.StartNew()) and promise-style Tasks (you can create them manually by using TaskCompletionSource or by writing an async method).
And the only Tasks that you can start by calling Start() or RunSynchronously() are unstarted code-based Tasks. Since async methods return promise-style Tasks (or possibly already started code-based Tasks), calling RunSynchronously() on them is not valid and will result in an exception.
So, to actually answer your question: what you're asking isn't possible, so it doesn't make sense to ask whether it's safe.
* This is not an official name, I don't know if there is one.
It's hard to predict without code how it will execute nested async methods.
You can log on each async method Thread.CurrentThread.ManagedThreadId property and compare id with other thread IDs that you have. When they vary, then your async methods are in multithreaded executtion
Or try to use Concurrency Visualizer from Visual Studio, Analyze menu. With Task class instances and even with C#5 async syntax there is no way to get to know that you are executing in parallel or another thread.
I think I'll be contradicting #svick's answer, but I feel the OP question is valid, because an async method's "promised" (to use Svick's terminology) Task can be obtained, but not started, thus allowing to do Task.RunSynchronously().
static void Main(string[] args)
{
...
//obtain a Task that's not started
var t = new Task<int>((ob) => GetIntAsync((string)ob).Result, someString);
...
}
static async Task<int> GetIntAsync(string callerThreadId)
{
...
Having said that, the answer is: No, the RunSynchronously() affects the task you run this on. If the call chain later contains more async calls, they run asynchronously.
I have a little console app that is modeling this, is someone is interested in seeing it, but the concept is pretty simple to reproduce - just chain enough asynchronous calls and toggle between running the earliest one synchronously and asynchronously to see the different behavior.
Related
I have an ASPX page which I cannot convert to async but which uses some async methods in a synchronous context. The way it invokes them is like so:
public void MySyncMethod()
{
var myTask = Task.Run(() => _myField.DoSomethingAsync());
myTask.Wait();
//use myTask.Result
}
Is there any difference between doing that and the following as far as async/await and/or blocking goes?
public void MySyncMethod()
{
var myTask = _myField.DoSomethingAsync(); //just get the Task direct, no Task.Run
myTask.Wait();
//use myTask.Result
}
I assume a previous developer added the Task.Run for a reason. But I am having issues which accessing things in HttpContext as the work is being run on a different thread.
Is there a reason to use Task.Run here?
Is there any difference between doing that and the following as far as
async/await and/or blocking goes?
Yes the first block of code uses a thread pool thread then waits for this to return, so your using two threads not one. They both block.
I assume a previous developer added the Task.Run for a reason.
Yes, blocking (directly) on async code from an ASP.Net context is a bad idea and can cause deadlocks. So you second block of code is more efficent (in thread usage) but suffers from serious deadlock issues.
The correct solution here is to make public void MySyncMethod() async itself (public async Task MySyncMethod()). Both these solutions have drawbacks and the only real way out is to make the whole call stack async. If you can do this, do it.
If you can't call an async method from another async method then Task.Run is the way to go. See How to call asynchronous method from synchronous method in C#? for more details.
If you want HttpContext inside your thread have a read though Using HttpContext in Async Task I would definitely favour:
Make every thing async
Or the Read the values from the context then pass them
Options of those answers and keep in mind
First off, you're not creating a copy of the object, you're just
copying the reference to the object.HttpContext isn't a
struct.....etc
The internal workings of asynchronous code based on async/await is fundamentally different than tasks started by Task.Run. async/await tasks are promise based and depend on the caller cooperating with returning the execution back to the asynchronous method when appropriate. Tasks started by Task.Run however are usually started on a parallel thread taken from the thread pool and do not depend on the caller's cooperation to continue execution when appropriate.
This constellation leads to the problem that you can not treat a promise based task the same as the other tasks, since the promise based task might wait for the callers cooperation to return the execution, which might never occur since the other task is executed independently and might wait for the caller. The result is a deadlock.
The solution is a specific Task.Run overload that will create a proxy for an existing task-based method that allows proper execution of a promise based task. It is safe to call Wait on this proxy. That's why the other developer used this construct. He could have also simplified the call and avoided an anonymous method like this:
var myTask = Task.Run(_myField.DoSomethingAsync);
Task.Run is used to run code asynchronously.
Be clear that it returns Task and needs to be awaited. Here's an example:
Task myTask = Task.Run(() => DoSomething());
await myTask;
I've been reading about asynchronous methods, specifically in C# with the new async/await keywords, and despite much reading and perusing this forum, I still am convinced that async requires multithreading. Please explain what I am misunderstanding!
I understand that you can write an async method without spawning a background thread. Super basic example:
async System.Threading.Tasks.Task<int> GetZeroAsync()
{
return 0;
}
But of course, this method is completely useless to be marked as async because, well, it isn't asynchronous. I also get a compiler warning about the method lacking an "await" operator, as expected. Okay, so what can we await? I can await something like Task.Run(), but that defeats the point, because I'm now using multithreading. Any other example I've found online tries to prove that you don't need multithreading by simply doing something like so:
async System.Threading.Tasks.Task<int> MyMethodAsync()
{
return await CallAnotherAsyncMethod();
}
Maybe I'm misunderstanding this, but all it proves to me is that I'm not the one who starts the multithreaded task, but I'm just calling another method that does. Since CallAnotherAsyncMethod() is also an async method, it must follow the exact same rules, right?. I can't have every async method just await another async sub-method forever, at some point it must stop unless you want infinite recursion.
The way I currently understand it, and I know this is wrong, is that async doesn't use multithreading, but it does require it, otherwise it's just a synchronous method lying to you.
So here's what might help. If async truly does not require multithreading, the following situation must be producible, I just can't find a way to do it. Can somebody create an example of a method that follows these rules:
Asynchronous.
Actually runs asynchronously.
Doesn't use any multithreading like calling Task.Run() or using a BackgroundWorker etc.
Doesn't call any other Async methods (unless you can also prove that this method follows these rules too).
Doesn't call any methods of the Task class like Task.Delay() (unless you can also prove that this method follows these rules too).
Any help or clarification would be really helpful! I feel like an idiot for not understanding this topic.
The easiest example of an async operation that does not use any kind of threads is waiting for a event to happen.
Create a UI app with your framework of choice and have two buttons, one called PrimeButton and one called RunButton
private TaskCompletionSource<object> _event = new TaskCompletionSource<object>();
//You are only allowed to do async void if you are writing a event handler!
public async void PrimeButton_OnClick(object sender, EventArgs e)
{
//I moved the code in to Example() so the async void would not be a distraction.
await Example();
}
public async Task Example()
{
await _event.Task;
MessageBox.Show("Run Clicked");
}
public async void RunButton_OnClick(object sender, EventArgs e)
{
_event.SetResult(null);
}
The await will wait till you click the 2nd button before it allows the code to continue and show the message box. No extra threads where involved at all here, all work was done using only the UI thread.
All a Task is, is a object that represents "something that will be finished at some point in the future". That something could be waiting for a background thread to complete that was started by a Task.Run or it could be waiting for a function to be called like the .SetResult( on the TaskCompletionSource<T>, or it could be waiting for some kind of disk or network IO to finish and be read in to a buffer by the OS (however internally this is usually implemented via a internal TaskCompletionSource<T> buried inside of the ReadAsync() function, so it is just a repeat of the last example with a wrapper around it)
When we are dealing with async tasks, the task consists of multiple method calls (possibly of other classes) which in turn consist of yet further method calls. We could call this a tree. Do all methods have to be of type async task for the original task to truly behave async?
Would the answer be true for any kind of background task of other similar languages, mostly Java as well?
(In C#, once I had one of my methods somewhere in this tree get into an infinite loop (talk about a long running task!), yet sadly, the original async task's isRunning property returned false.)
Do all methods have to be of type async task for the original task to truly behave async?
In order to behave asynchronously, an async method must have an await statement for an operation that is not already completed.
I think my async intro blog post would be helpful for you.
Would the answer be true for any kind of background task of other similar languages, mostly Java as well?
Yes.
sadly, the original async task's isRunning property returned false
I assume you mean that the TaskStatus was not TaskStatus.Running. This is normal and expected, since asynchronous tasks are Promise Tasks, not Delegate Tasks (see my blog for more info).
The short answer is no. Async means that work being done by another process can be handed off to that process and so not block the calling thread. All the better if those processes don’t reside on the same machine, since that work will occupy very little compute or IO at the caller. This is achieved by suspending method execution using the async/await mechanism, which promises to resume on some synchronisation context once completed. So, why the overview?
Because this means that we can and should judiciously choose those operations that should be async, and so apply the await keyword in an async method. Some won’t need it, since they are lightweight and adding async to them may even add compute time to them, due to the creation of the async state machine. So the operations in a method can often be a mixture of asynchronous and synchronous code.
And this in turn means that some methods would use the Task return type with async/await, and some will be regular, synchronous methods.
No, they don't have to. Simple example, in a Forms application this
button.Click += async (_, __) =>
{
await Task.Run(() => MyClass.SomeTimeconsumingMethod());
};
still lets you move the window during the execution of SomeTimeconsumingMethod, whereas this one
button.Click += (_, __) =>
{
MyClass.SomeTimeconsumingMethod();
};
doesn't let you do it.
The only requirement for using the await keyword is that the method you are awaiting returns a Task or Task<T>.
The exact implementation of this method (async or return someTask) has no influence on the correctness of the code, although it may influence the function.
Any normal code can run in an async method alongside your await calls. The only significant restriction is that out and ref are not allowed.
Do all methods have to be of type async task for the original task to truly behave async
It depends on what do you mean by "all" word.
Some APIs must not be asynchronous. Think about List<T>.Add, for example. Do you really want it to be asynchronous?
Some APIs should be asynchronous, but there's no asynchronous counterpart (e.g. SMO, or File static class).
Ideally you should do "async all the way", when talking about IO-bound operations. But IRL you can face with violation of this rule, when some part of your async method chain will execute IO-bound operation synchronously.
I have the following code that connects to a SignalR Hub
private static async Task StartListening()
{
try
{
var hubConnection = new HubConnection("http://localhost:8080/");
IHubProxy hubProxy = hubConnection.CreateHubProxy("Broadcaster");
hubProxy.On<EventData>("notifyCardAccessEvent", eventData =>
{
Log.Info(string.Format("Incoming data: {0} {1}", eventData.Id, eventData.DateTime));
});
ServicePointManager.DefaultConnectionLimit = 10;
await hubConnection.Start();
Log.Info("Connected");
}
catch (Exception ex)
{
Log.Error(ex);
}
}
In my Form_Load method, I have this
StartListening();
However, Resharper prompts me to "consider applying the 'await' operator to the result of the call"
So I did this:
Log.Info("Connecting to SignalR hub...");
StartListening().Wait();
Log.Info("Connected!");
However, this causes my UI thread to hang and Connected! is never printed to the log file.
So my question is, when should I use Wait()? What are the instances and scenarios that I should use Wait(), and when should I not use Wait()?
await is not Wait. It is unclear what the code is that is calling StartListening(), but one option is to await it, as suggested:
await StartListening();
However, in some other cases it may be better to do nothing at all:
StartListening(); // drop the Task on the floor
or perhaps use ContinueWith for a manual continuation. Since the StartListening method catches any exceptions, there isn't anything wrong with just ignoring the returned Task - so what you had already. I would suggest calling it StartListeningAsync, though.
The reason for the deadlock is that if you use Wait, your UI thread blocks waiting on an asynchronous method to complete, but that asynchronous method is capturing the sync-context, which means in order to process each continuation it tries to get onto the UI thread - which is blocked... on it.
#MarcGravell has the correct answer; I'm just going to answer this other question:
So my question is, when should I use Wait()? What are the instances and scenarios that I should use Wait(), and when should I not use Wait()?
The confusion is coming from the fact that the Task type is used for two almost completely different things.
Task was originally introduced in .NET 4.0 as part of the Task Parallel Library. Normally, you would use Parallel LINQ or the Parallel class for parallel processing (which used the Task type underneath). However, in advanced scenarios, you could use the Task type directly. Task.Wait was used to wait for those independent tasks to complete.
When async/await were introduced in .NET 4.5, the existing Task type was almost good enough to be used as an abstract "future". So instead of inventing some new "future" type, they just slightly extended Task to work as a future.
This brings us to today, where Task can be used as either:
An item of work in a parallel computation.
An asynchronous future.
(There's a tiny bit of crossover: you can treat parallel work as asynchronous, and in rare situations like Console Main methods you do need to block on asynchronous tasks; but ignore those for the moment.)
This means that the API for Task is split along those lines. Members such as Start, Wait, Result, and ContinueWith belong pretty firmly on the parallel side. In the asynchronous world, await is more appropriate.
I have a small table at the bottom of my async intro that has some new (asynchronous) equivalents for the old (parallel) ways of doing things.
You seem to misunderstand the message from Resharper. Instead of applying the await operator, you called the Task.Wait() Method. They may seem similar, but they're working completely different.
This nice answer will provide more information about the differences: https://stackoverflow.com/a/13140963/3465395
Here is my minimum repro case:
public Form1()
{
Task.Delay(100).Wait(); // Works just fine
this.Await().Wait(); // Blocks indefinitely
}
private async Task Await()
{
await Task.Delay(100);
}
What is going on here? Why are these two behaving differently? What can I do to make the later one work?
My actual case is less trivial, and I can't "just use the first option".
You're seeing a classic deadlock situation that I describe on my blog and in an MSDN article. In short, after the await completes, the async method is attempting to resume on the UI thread, which you have blocked by calling Wait.
To fix it, you ideally want to use async all the way (i.e., never block on async code). Constructors pose a difficulty here (since they cannot be async); I explore several options on my blog. The correct option depends on your code base, but I recommend the async factory method if possible. The options are:
Async factory method.
Asynchronous lazy initialization.
Asynchronous initialization pattern.
If you absolutely cannot use one of the options that I describe on my blog, then you can work around this by using ConfigureAwait(false) in all your async methods, and then your Wait() will not deadlock. However, this will block the UI thread during those asynchronous method calls (which sort of defeats the purpose of them being async in the first place...)