小开

最佳答案

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 thread
internal 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 an async 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 an async method and await keyword unwraps them. (see Wrapping and Unwrapping section)

Asynchronous method names should always end with Async to increase readability and to prevent mistakes.

// 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 the awaited Task in an async method.
The execution waits at await for the task to finish and returns back its results, without blocking the main thread.
Task.Run queues a Task in the thread pool. (Unless the it's a pure operation.) i.e. The async method does not run in another thread. async and await by themselves don't have anything to do with thread creation.

So

When you run a Task (e.g. Task.Run(action)) you (re)use a thread for that action. And you can put that task in an async method to control its flow. By putting async in the method signature you tell the compiler to use state-machine to control the flow of that method (this does not mean threading at all). And by awaiting the task you prevent the execution flow within that method from moving past the awaited statement without blocking UI thread. If you want to pass the flow onto the caller then the async method itself can become a Task so you'll be able to cascade the same pattern out into the caller and so forth:

async Task Caller() { await Method(); }
async Task Method() { await Inner(); }
async Task Inner() { await Task.Run(action); }

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:

Suppose you have the following declaration:

private async ReturnType MethodAsync() { ... }

If ReturnType is Task then await MethodAsync(); returns void
If ReturnType is Task<T> then await MethodAsync(); returns a value of type T

This is called Unwrapping, see the next section (Wrapping and Unrwapping).
If ReturnType is void you can't await 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 has async it will allow you to take advantage of the magic, i.e. you can write await 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.

The return type of an async method must be void, Task, Task<T>, a task-like type, IAsyncEnumerable<T>, or IAsyncEnumerator<T>

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 or task.GetAwaiter().GetResult() or task.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.

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;
}
}

小开

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 the async modifier which tells the compiler to enable threading for that method.
- Thread.Sleep in an async 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 an async method
Second method (ExecuteLongProcedureAsync) is wrapped within a task and returns a generic Task<original return type> object which can be instructed to be processed asynchronously by adding await 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.

小开

Here is an example using async/await, IProgress<T> and CancellationTokenSource. These are the modern C# and .Net Framework language features that you should be using. The other solutions are making my eyes bleed a bit.

Code Features

Count to 100 over a period of 10 seconds
Display progress on a progress bar
Long running work (a 'wait' period) performed without blocking the UI
User triggered cancellation
Incremental progress updates
Post operation status report

The view

<Window x:Class="ProgressExample.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
Title="MainWindow" SizeToContent="WidthAndHeight" Height="93.258" Width="316.945">
<StackPanel>
<Button x:Name="Button_Start" Click="Button_Click">Start</Button>
<ProgressBar x:Name="ProgressBar_Progress" Height="20"  Maximum="100"/>
<Button x:Name="Button_Cancel" IsEnabled="False" Click="Button_Cancel_Click">Cancel</Button>
</StackPanel>
</Window>

The code

    /// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
private CancellationTokenSource currentCancellationSource;


public MainWindow()
{
InitializeComponent();
}


private async void Button_Click(object sender, RoutedEventArgs e)
{
// Enable/disabled buttons so that only one counting task runs at a time.
this.Button_Start.IsEnabled = false;
this.Button_Cancel.IsEnabled = true;


try
{
// Set up the progress event handler - this instance automatically invokes to the UI for UI updates
// this.ProgressBar_Progress is the progress bar control
IProgress<int> progress = new Progress<int>(count => this.ProgressBar_Progress.Value = count);


currentCancellationSource = new CancellationTokenSource();
await CountToOneHundredAsync(progress, this.currentCancellationSource.Token);


// Operation was successful. Let the user know!
MessageBox.Show("Done counting!");
}
catch (OperationCanceledException)
{
// Operation was cancelled. Let the user know!
MessageBox.Show("Operation cancelled.");
}
finally
{
// Reset controls in a finally block so that they ALWAYS go
// back to the correct state once the counting ends,
// regardless of any exceptions
this.Button_Start.IsEnabled = true;
this.Button_Cancel.IsEnabled = false;
this.ProgressBar_Progress.Value = 0;


// Dispose of the cancellation source as it is no longer needed
this.currentCancellationSource.Dispose();
this.currentCancellationSource = null;
}
}


private async Task CountToOneHundredAsync(IProgress<int> progress, CancellationToken cancellationToken)
{
for (int i = 1; i <= 100; i++)
{
// This is where the 'work' is performed.
// Feel free to swap out Task.Delay for your own Task-returning code!
// You can even await many tasks here


// ConfigureAwait(false) tells the task that we dont need to come back to the UI after awaiting
// This is a good read on the subject - https://blog.stephencleary.com/2012/07/dont-block-on-async-code.html
await Task.Delay(100, cancellationToken).ConfigureAwait(false);


// If cancelled, an exception will be thrown by the call the task.Delay
// and will bubble up to the calling method because we used await!


// Report progress with the current number
progress.Report(i);
}
}


private void Button_Cancel_Click(object sender, RoutedEventArgs e)
{
// Cancel the cancellation token
this.currentCancellationSource.Cancel();
}
}

如何从 WPF GUI 运行和交互异步任务

`async` and `await`:

Basics of `async` and `await`

The magic of `async` and `await`

Async method return types:

Wrapping and Unrwapping:

Wrapping:

Unwrapping:

Different ways to wrap and unwrap:

Note:

WPF GUI:

Blocking:

Thread Safety:

Patterns:

Fire and forget pattern:

Fire and observe:

Fire and wait synchronously while wasting thread resources:

Is that all to it?

Task uses Thread? Are you sure?

Read more

Important:

Code Features

The view

The code

如何从 WPF GUI 运行和交互异步任务

async and await:

Basics of async and await

The magic of async and await

Async method return types:

Wrapping and Unrwapping:

Wrapping:

Unwrapping:

Different ways to wrap and unwrap:

Note:

WPF GUI:

Blocking:

Thread Safety:

Patterns:

Fire and forget pattern:

Fire and observe:

Fire and wait synchronously while wasting thread resources:

Is that all to it?

Task uses Thread? Are you sure?

Read more

Important:

Code Features

The view

The code

`async` and `await`:

Basics of `async` and `await`

The magic of `async` and `await`