How to run and interact with an async Task from a WPF gui
Long story short:
private async void ButtonClickAsync(object sender, RoutedEventArgs e){ // modify UI object in UI thread txt.Text = "started"; // run a method in another thread await HeavyMethodAsync(txt); // <<method execution is finished here>> // modify UI object in UI thread txt.Text = "done";}// This is a thread-safe method. You can run it in any threadinternal async Task HeavyMethodAsync(TextBox textBox){ while (stillWorking) { textBox.Dispatcher.Invoke(() => { // UI operation goes inside of Invoke textBox.Text += "."; // Note that: // Dispatcher.Invoke() blocks the UI thread anyway // but without it you can't modify UI objects from another thread }); // CPU-bound or I/O-bound operation goes outside of Invoke // await won't block UI thread, unless it's run in a synchronous context await Task.Delay(51); }}
Result:started....................done
You need to know about (1) how to write async
code (2) how to run UI operations in another thread and (3) how to cancel a task.
I'm not getting into (3) cancellation mechanism in this post. Just know that you can create a CancellationTokenSource
, which gives you a CancellationToken
which you can pass into any method. You cancel the source, all tokens will know.
async
and await
:
Basics of async
and await
You can only
await
in anasync
method.You can only
await
an awaitable object (i.e.Task
,ValueTask
,Task<T>
,IAsyncEnumerable<T>
, etc.) These objects wrap around the return type of anasync
method andawait
keyword unwraps them. (see Wrapping and Unwrapping section)Asynchronous method names should always end with
Async
to increase readability and to prevent mistakes. (see Patterns section)// Synchronous method:TResult MethodName(params) { }// Asynchronous method:async Task<TResult> MethodNameAsync(params) { }
The magic of async
and await
The
async-await
syntactic feature, uses a state-machine to let the compiler give up and take back the control over theawaited Task
in anasync
method.The execution waits at
await
for the task to finish and returns back its results, without blocking the main thread.Task.Run
queues aTask
in the thread pool. (Unless the it's a pure operation.)i.e. Theasync
method does not run in another thread.async
andawait
by themselves don't have anything to do with thread creation.
So
By putting async
in the method signature you tell the compiler to use state-machine to call this method (no threading so far). Then by running a Task
you (re)use a thread to call the method inside the task. And by await
ing the task you prevent the execution flow to move past the await
line without blocking UI thread.
The event handler looks like the code below.
Two possible cases for presense of async in the signature of ExecuteLongProcedure
(case 1 and 2) and MyButton_ClickAsync
(case A and B) are explained:
private async void MyButton_ClickAsync(object sender, RoutedEventArgs e){ //queue a task to run on threadpool // 1. if ExecuteLongProcedure is a normal method and returns void Task task = Task.Run(()=> ExecuteLongProcedure(this, intParam1, intParam2, intParam3) ); // or // 2. if ExecuteLongProcedure is an async method and returns Task Task task = ExecuteLongProcedureAsync(this, intParam1, intParam2, intParam3); // either way ExecuteLongProcedure is running asynchronously here // the method will exit if you don't wait for the Task to finish // A. wait without blocking the main thread // -> requires MyButton_ClickAsync to be async await task; // or // B. wait and block the thread (NOT RECOMMENDED AT ALL) // -> does not require MyButton_ClickAsync to be async task.Wait();}
Async method return types:
This is very important to know. Suppose you have the following declaration:
private async ReturnType MethodAsync() { ... }
If
ReturnType
isvoid
you can'tawait
it- If you try writing
await MethodAsync();
, you will get a compile error saying cannot await void. - You can only fire and forget i.e. just call the method normally:
MethodAsync();
and then go on with your life. - The
MethodAsync
execution will be synchronous, however since it hasasync
it will allow you to take advantage of the magic, i.e. you can writeawait task
within the method to control the flow of execution. - This is how WPF handles your button click event handler, obviously because your event handler returns
void
.
- If you try writing
If
ReturnType
isTask
thenawait MethodAsync();
returnsvoid
If
ReturnType
isTask<T>
thenawait MethodAsync();
returns a value of typeT
The return type of an async method must be
void
,Task
,Task<T>
, a task-like type,IAsyncEnumerable<T>
, orIAsyncEnumerator<T>
You might want to scroll down to WPF GUI section if you don't want to learn more!
Wrapping and Unrwapping:
Wrapping:
async
methods wrap their return values in a Task
.
E.g., this method wraps a Task
around an int
and returns it:
// async Task<int>private async Task<int> GetOneAsync(){ int val = await CalculateStuffAsync(); return val;// returns an integer}
Unwrapping:
To retrieve or unwrap the value which is wrapped inside a Task<>
:
- asynchronous option:
await
- synchronous option:
task.Result
ortask.GetAwaiter().GetResult()
ortask.WaitAndUnwrapException()
or read How to call asynchronous method from synchronous method in C#?
e.g. await
unwraps the int
out of the Task
:
Task<int> task = GetOneAsync();int number = await task;//int <- Task<int>
Different ways to wrap and unwrap:
private Task<int> GetNumber(){ Task<int> task; task = Task.FromResult(1); // the correct way to wrap a quasi-atomic operation, the method GetNumber is not async task = Task.Run(() => 1); // not the best way to wrap a number return task;}private async Task<int> GetNumberAsync(){ int number = await Task.Run(GetNumber); // unwrap int from Task<int> // bad practices: // int number = Task.Run(GetNumber).GetAwaiter().GetResult(); // sync over async // int number = Task.Run(GetNumber).Result; // sync over async // int number = Task.Run(GetNumber).Wait(); // sync over async return number; // wrap int in Task<int>}
Still confused? Read async return types on MSDN.
To unwrap a task result, Always try to use
await
instead of.Result
otherwise there will be no asynchronous benefit but only asynchronous disadvantages. The latter is called "sync over async".
Note:
await
is a asynchronous and is different from task.Wait()
which is synchronous. But they both do the same thing which is waiting for the task to finish.
await
is a asynchronous and is different from task.Result
which is synchronous. But they both do the same thing which is waiting for the task to finish and unwrapping and returning back the results.
To have a wrapped value, you can always use Task.FromResult(1)
instead of creating a new thread by using Task.Run(() => 1)
.
Task.Run
is newer (.NetFX4.5) and simpler version of Task.Factory.StartNew
WPF GUI:
This is where I explain how to run UI operations in another thread.
Blocking:
First thing you need to know about WPF async event handlers is that the Dispatcher
will provide a synchronization context. Explained here
CPU-bound or IO-bound operations such as Sleep
and task.Wait()
will block and consume the thread even if they are called in a method with async
keyword. but await Task.Delay()
tells the state-machine to stop the flow of execution on the thread so it does not consume it; meaning that the thread resources can be used elsewhere:
private async void Button_Click(object sender, RoutedEventArgs e){ Thread.Sleep(1000);//stops, blocks and consumes threadpool resources await Task.Delay(1000);//stops without consuming threadpool resources Task.Run(() => Thread.Sleep(1000));//does not stop but consumes threadpool resources await Task.Run(() => Thread.Sleep(1000));//literally the WORST thing to do}
Thread Safety:
If you have to access GUI asynchronously (inside ExecuteLongProcedure
method), invoke any operation which involves modification to any non-thread-safe object. For instance, any WPF GUI object must be invoked using a Dispatcher
object which is associated with the GUI thread:
void UpdateWindow(string text){ //safe call Dispatcher.Invoke(() => { txt.Text += text; });}
However, If a task is started as a result of a property changed callback from the ViewModel, there is no need to use Dispatcher.Invoke
because the callback is actually executed from the UI thread.
Accessing collections on non-UI Threads
WPF enables you to access and modify data collections on threads other than the one that created the collection. This enables you to use a background thread to receive data from an external source, such as a database, and display the data on the UI thread. By using another thread to modify the collection, your user interface remains responsive to user interaction.
Value changes fired by INotifyPropertyChanged are automatically marshalled back onto the dispatcher.
How to enable cross-thread access
Remember, async
method itself runs on the main thread. So this is valid:
private async void MyButton_ClickAsync(object sender, RoutedEventArgs e){ txt.Text = "starting"; // UI Thread await Task.Run(()=> ExecuteLongProcedure1()); txt.Text = "waiting"; // UI Thread await Task.Run(()=> ExecuteLongProcedure2()); txt.Text = "finished"; // UI Thread}
Another way to invoke UI operations from UI thread is to use SynchronizationContext
as described here. SynchronizationContext
is a stronger abstraction than Dispatcher
and it's cross-platform.
var uiContext = SynchronizationContext.Current;while (stillWorking){ uiContext.Post(o => { textBox.Text += "."; }, null); await Task.Delay(51);}
Patterns:
Fire and forget pattern:
For obvious reasons this is how your WPF GUI event handlers such as Button_ClickAsync
are called.
void Do(){ // CPU-Bound or IO-Bound operations}async void DoAsync() // returns void{ await Task.Run(Do);}void FireAndForget() // not blocks, not waits{ DoAsync();}
Fire and observe:
Task-returning methods are better since unhandled exceptions trigger the TaskScheduler.UnobservedTaskException
.
void Do(){ // CPU-Bound or IO-Bound operations}async Task DoAsync() // returns Task{ await Task.Run(Do);}void FireAndWait() // not blocks, not waits{ Task.Run(DoAsync);}
Fire and wait synchronously while wasting thread resources:
This is known as Sync over async, it is a synchronous operation but it uses more than one thread which may cause starvation. This happens when you call Wait()
or try to read results directly from task.Result
before the task is finished.
(AVOID THIS PATTERN)
void Do(){ // CPU-Bound or IO-Bound operations}async Task DoAsync() // returns Task{ await Task.Run(Do);}void FireAndWait() // blocks, waits and uses 2 more threads. Yikes!{ var task = Task.Run(DoAsync); task.Wait();}
Is that all to it?
No. There is a lot more to learn about async
, its context and its continuation. This blogpost is especially recommended.
Task uses Thread? Are you sure?
Not necessarily. Read this answer to know more about the true face of async
.
Stephen Cleary has explained async-await
perfectly. He also explains in his other blog post when there is no thread involved.
Read more
how-to-call-asynchronous-method-from-synchronous-method
async await
- Behind the scenes
Make sure you know the difference between Asynchronous, Parallel and Concurrent.
You may also read a simple asynchronous file writer to know where you should concurrent.
Investigate concurrent namespace
Ultimately, read this e-book: Patterns_of_Parallel_Programming_CSharp
Your use of TaskCompletionSource<T>
is incorrect. TaskCompletionSource<T>
is a way to create TAP-compatible wrappers for asynchronous operations. In your ExecuteLongProcedureAsync
method, the sample code is all CPU-bound (i.e., inherently synchronous, not asynchronous).
So, it's much more natural to write ExecuteLongProcedure
as a synchronous method. It's also a good idea to use standard types for standard behaviors, in particular using IProgress<T>
for progress updates and CancellationToken
for cancellation:
internal void ExecuteLongProcedure(int param1, int param2, int param3, CancellationToken cancellationToken, IProgress<string> progress){ //Start doing work if (progress != null) progress.Report("Work Started"); while (true) { //Mid procedure progress report if (progress != null) progress.Report("Bath water n% thrown out"); cancellationToken.ThrowIfCancellationRequested(); } //Exit message if (progress != null) progress.Report("Done and Done");}
Now you have a more reusable type (no GUI dependencies) that uses the appropriate conventions. It can be used as such:
public partial class MainWindow : Window{ readonly otherClass _burnBabyBurn = new OtherClass(); CancellationTokenSource _stopWorkingCts = new CancellationTokenSource(); //A button method to start the long running method private async void Button_Click_3(object sender, RoutedEventArgs e) { var progress = new Progress<string>(data => UpdateWindow(data)); try { await Task.Run(() => _burnBabyBurn.ExecuteLongProcedure(intParam1, intParam2, intParam3, _stopWorkingCts.Token, progress)); } catch (OperationCanceledException) { // TODO: update the GUI to indicate the method was canceled. } } //A button Method to interrupt and stop the long running method private void StopButton_Click(object sender, RoutedEventArgs e) { _stopWorkingCts.Cancel(); } //A method to allow the worker method to call back and update the gui void UpdateWindow(string message) { TextBox1.Text = message; }}
This is a simplified version of the most popular answer here by Bijan. I simplified Bijan's answer to help me think through the problem using the nice formatting provided by Stack Overflow.
By carefully reading and editing Bijan's post I finally understood: How to wait for async method to complete?
In my case the chosen answer for that other post is what ultimately led me to solve my problem:
"Avoid async void
. Have your methods return Task
instead of void
. Then you can await
them."
My simplified version of Bijan's (excellent) answer follows:
1) This starts a task using async and await:
private async void Button_Click_3(object sender, RoutedEventArgs e){ // if ExecuteLongProcedureAsync has a return value var returnValue = await Task.Run(()=> ExecuteLongProcedureAsync(this, intParam1, intParam2, intParam3));}
2) This is the method to execute asynchronously:
bool stillWorking = true;internal void ExecuteLongProcedureAsync(MainWindow gui, int param1, int param2, int param3){ //Start doing work gui.UpdateWindow("Work Started"); while (stillWorking) { //put a dot in the window showing the progress gui.UpdateWindow("."); //the following line blocks main thread unless //ExecuteLongProcedureAsync is called with await keyword System.Threading.Thread.Sleep(50); } gui.UpdateWindow("Done and Done");}
3) Invoke the operation which involves a property from gui:
void UpdateWindow(string text){ //safe call Dispatcher.Invoke(() => { txt.Text += text; });}
Or,
void UpdateWindow(string text){ //simply txt.Text += text;}
Closing comments) In most cases you have two methods.
First method (
Button_Click_3
) calls the second method and has theasync
modifier which tells the compiler to enable threading for that method.Thread.Sleep
in anasync
method blocks the main thread. but awaiting a task does not.- Execution stops on current thread (second thread) on
await
statements until task is finished. - You can't use
await
outside anasync
method
Second method (
ExecuteLongProcedureAsync
) is wrapped within a task and returns a genericTask<original return type>
object which can be instructed to be processed asynchronously by addingawait
before it.- Everything in this method in executed asynchronously
Important:
Liero brought up an important issue. When you are Binding an element to a ViewModel property, the property changed callback is executed in UI thread. So there is no need to use Dispatcher.Invoke
. Value changes fired by INotifyPropertyChanged are automatically marshalled back onto the dispatcher.