I have a class SimpleTask which looks like this:
public class SimpleTask<T>
{
private readonly Action<Exception> _errorAction;
private readonly Func<T> _produce;
private readonly Action<T> _then;
public SimpleTask(Func<T> produce, Action<T> then, Action<Exception> errorAction)
{
_then = then ?? throw new ArgumentNullException(nameof(then));
_errorAction = errorAction ?? throw new ArgumentNullException(nameof(errorAction));
_produce = produce ?? throw new ArgumentNullException(nameof(produce));
}
public void Run()
{
using (var backgroundWorker = new BackgroundWorker())
{
var item = default(T);
backgroundWorker.DoWork += (_, e) => item = _produce();
backgroundWorker.RunWorkerCompleted += (_, e) =>
{
if (e.Error != null)
{
_errorAction(e.Error);
return;
}
_then(item);
};
backgroundWorker.RunWorkerAsync();
}
}
}
I would like to use a Task instead of a BackgroundWorker but I end up with something like this:
public class SimpleTask<T>
{
private readonly Action<Exception> _errorAction;
private readonly Func<T> _produce;
private readonly Action<T> _then;
public SimpleTask(Func<T> produce, Action<T> then, Action<Exception> errorAction)
{
_then = then ?? throw new ArgumentNullException(nameof(then));
_errorAction = errorAction ?? throw new ArgumentNullException(nameof(errorAction));
_produce = produce ?? throw new ArgumentNullException(nameof(produce));
}
public void Run()
{
try
{
var synchronizationContext = TaskScheduler.FromCurrentSynchronizationContext();
Task.Run(_produce,CancellationToken.None).ContinueWith(t =>
{
if (t.IsFaulted)
{
_errorAction(t.Exception);
}
else if (t.IsCompleted)
{
_then(t.Result);
}
}, CancellationToken.None,TaskContinuationOptions.ExecuteSynchronously,synchronizationContext);
}
catch (Exception ex)
{
_errorAction(ex);
}
}
}
Which is not the same after all. In my unit tests I have to add:
[SetUp]
public void TestSetUp()
{
SynchronizationContext.SetSynchronizationContext(new SynchronizationContext());
}
I wonder if I should keep using the BackgroundWorker and not pollute my code with task/async structures? What is best practice?
You are reinventing the wheel, because the new async functionality handles most of this for you.
I assume you want to:
Run in a background task a compute bound method that returns a result
Handle any exception thrown by that method
Access the returned value if no exception occurred
Here's an example of how to do this using await.
In this example, the compute-bound method is int computeBoundFunction(). The code assumes you have a Windows Forms form with a button called button1 and a multiline TextBox called textBox1:
async void button1_Click(object sender, EventArgs e)
{
textBox1.AppendText("Starting task\r\n");
try
{
int result = await Task.Run(computeBoundFunction);
// Instead of your "then" action, just call the code here.
// In this example, I'm just appending to a multiline text box.
// This runs on the UI thread.
textBox1.AppendText("Task returned " + result);
}
// Instead of your "errorAction" action, handle exceptions here.
// Note that this runs on the UI thread, so you can update controls safely at this point.
catch (Exception exception)
{
textBox1.AppendText("Exception: " + exception.Message);
}
}
int computeBoundFunction()
{
Thread.Sleep(2000); // Emulate workload.
return 42;
// Comment out the return above and uncomment the line below to test the exception handling:
//throw new InvalidOperationException("Test exception");
}
Note that normally you would never use async void instead of async Task, but this rule is relaxed for event handlers such as button1_Click() in this example.
Related
Gist of it has probably been asked before, but I'm completely lost so I'm looking for some personal guidance. Been trying to make a stock tracker app for funsies using WinForms and the Yahoo API. Trying to get it so you can input a tracker symbol and it will make a new Label that will keep updating itself every so often. However, it keeps giving me error messages about "Cross-thread operation not valid". I've tried to do some googling, but yeah, completely lost. Here is most of the code, hope you guys can make some sense of it.
using System;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;
using YahooFinanceApi;
namespace stockpoging4
{
public partial class Form1 : Form
{
public Form1()
{
System.Globalization.CultureInfo.DefaultThreadCurrentUICulture = System.Globalization.CultureInfo.GetCultureInfo("en-US");
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
using (Prompt prompt = new Prompt("Enter the ticker symbol", "Add ticker"))
{
string result = prompt.Result;
result = result.ToUpper();
if (!string.IsNullOrEmpty(result))
{
do_Things(result);
}
}
}
public async Task<string> getStockPrices(string symbol)
{
try
{
var securities = await Yahoo.Symbols(symbol).Fields(Field.RegularMarketPrice).QueryAsync();
var aapl = securities[symbol];
var price = aapl[Field.RegularMarketPrice];
return symbol + " $" + price;
}
catch
{
return "404";
}
}
public async void do_Things(string result)
{
string price;
Label label = null;
if (label == null)
{
price = await getStockPrices(result);
label = new Label() { Name = result, Text = result + " $" + price };
flowLayoutPanel2.Controls.Add(label);
}
else
{
Thread testThread = new Thread(async delegate ()
{
uiLockingTask();
price = await getStockPrices(result);
label.Text = result + " $" + price;
label.Update();
});
}
System.Timers.Timer timer = new System.Timers.Timer(10000);
timer.Start();
timer.Elapsed += do_Things(results);
}
private void uiLockingTask() {
Thread.Sleep(5000);
}
}
}
Let me point out several things in your implementation.
You subscribe to timer.Elapsed after timer.Start that might be invalid in case of a short-timer interval
The event handler is called in background that's why you continuously get "Cross-thread operation not valid". UI components should be dispatched correctly from background threads, for example, by calling flowLayoutPanel2.BeginInvoke(new Action(() => flowLayoutPanel2.Controls.Add(label))); and label.BeginInvoke(new Action(label.Update)). This change already would fix your exception.
Despite the fact that I would implement this functionality in a different way, here I post slightly changed code that just does exactly what you need with some tweaks.
public partial class Form1 : Form
{
Task _runningTask;
CancellationTokenSource _cancellationToken;
public Form1()
{
System.Globalization.CultureInfo.DefaultThreadCurrentUICulture = System.Globalization.CultureInfo.GetCultureInfo("en-US");
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
using (Prompt prompt = new Prompt("Enter the ticker symbol", "Add ticker"))
{
string result = prompt.Result;
result = result.ToUpper();
if (!string.IsNullOrEmpty(result))
{
do_Things(result);
_cancellationToken = new CancellationTokenSource();
_runningTask = StartTimer(() => do_Things(result), _cancellationToken);
}
}
}
private void onCancelClick()
{
_cancellationToken.Cancel();
}
public async Task<string> getStockPrices(string symbol)
{
try
{
var securities = await Yahoo.Symbols(symbol).Fields(Field.RegularMarketPrice).QueryAsync();
var aapl = securities[symbol];
var price = aapl[Field.RegularMarketPrice];
return symbol + " $" + price;
}
catch
{
return "404";
}
}
private async Task StartTimer(Action action, CancellationTokenSource cancellationTokenSource)
{
try
{
while (!cancellationTokenSource.IsCancellationRequested)
{
await Task.Delay(1000, cancellationTokenSource.Token);
action();
}
}
catch (OperationCanceledException) { }
}
public async void do_Things(string result)
{
var price = await getStockPrices(result);
var label = new Label() { Name = result, Text = result + " $" + price };
flowLayoutPanel2.BeginInvoke(new Action(() => flowLayoutPanel2.Controls.Add(label)));
}
}
A much easier way is using async these days.
Here is a class which triggers an Action every interval:
public class UITimer : IDisposable
{
private readonly CancellationTokenSource _cancellationTokenSource = new CancellationTokenSource();
// use a private function which returns a task
private async Task Innerloop(TimeSpan interval, Action<UITimer> action)
{
try
{
while (!_cancellationTokenSource.IsCancellationRequested)
{
await Task.Delay(interval, _cancellationTokenSource.Token);
action(this);
}
}
catch (OperationCanceledException) { }
}
// the constructor calls the private StartTimer, (the first part will run synchroniously, until the away delay)
public UITimer(TimeSpan interval, Action<UITimer> action) =>
_ = Innerloop(interval, action);
// make sure the while loop will stop.
public void Dispose() =>
_cancellationTokenSource?.Cancel();
}
If you work with dotnet 3.0 or higher, you can use the IAsyncDisposable. With this you're able to await the DisposeAsync method, so you can await the _timerTask to be finished.
And I created a new form with this as code behind:
public partial class Form1 : Form
{
private readonly UITimer _uiTimer;
private int _counter;
public Form1()
{
InitializeComponent();
// setup the time and pass the callback action
_uiTimer = new UITimer(TimeSpan.FromSeconds(1), Update);
}
// the orgin timer is passed as parameter.
private void Update(UITimer timer)
{
// do your thing on the UI thread.
_counter++;
label1.Text= _counter.ToString();
}
private void Form1_FormClosed(object sender, FormClosedEventArgs e)
{
// make sure the time (whileloop) is stopped.
_uiTimer.Dispose();
}
}
The advantage is, that the callback runs on the UI thread but doesn't block it. The await Task.Delay(..) is using a Timer in the background, but posts the rest of the method/statemachine on the UI thread (because the UI thread has a SynchronizaionContext)
Easy but does the trick ;-)
I am generating more than 100 messages per second and sending these messages in separate thread. When connection is down, I want to catch the exception in caller. Since all my messages are sent asynchronous, I am not able to catch the exceptions.
Here is the DispatcherTimer code which calls the dispatcherTimer_Tick method
dispatcherTimer = new DispatcherTimer();
dispatcherTimer.Interval = new TimeSpan(0, 0, 0, 1, 0);
dispatcherTimer.Tick += new EventHandler(dispatcherTimer_Tick);
private void dispatcherTimer_Tick(object sender, EventArgs e)
{
try
{
item = "some generated message";
Task.Run(() => SendMessage(item));
}
catch (Exception)
{
}
}
Here is the SendMessage code. I have made change by reading Based on: Async/Await - Best Practices in Asynchronous Programming, BUT it's not working
private async static Task SendMessage(string message)
{
try
{
(MQTT.RunAsync(message.ToString(), topic)).Wait();
}
catch (Exception Ex)
{
// Exceptions are not getting cought here
}
}
Definition of MQTT.RunAsync
public static async Task RunAsync(string message)
{
var mqttClient = factory.CreateMqttClient()
try
{
await mqttClient.ConnectAsync(options);
}
catch (Exception exception)
{
}
}
And
Task<MqttClientConnectResult> ConnectAsync(IMqttClientOptions options)
Updated Question
My RunAsync is first trying to connect and if success then it sends the message. so I can't write return while connection check.
public Task RunAsync(string message, string topicName)
{
this.mqttClient.ConnectAsync(this.options);
mqttClient.SubscribeAsync(new TopicFilterBuilder().WithTopic(this._topicname).WithExactlyOnceQoS().Build());
var applicationMessage = new MqttApplicationMessageBuilder().WithTopic(this._topicname)
.WithPayload(message).WithAtLeastOnceQoS().Build();
if (stopSending == false)
{
return mqttClient.PublishAsync(applicationMessage);
}
return null;
}
Event handlers are an exception where async void is allowed
private async void dispatcherTimer_Tick(object sender, EventArgs e) {
try {
item = "some generated message";
await SendMessage(item);
} catch (Exception ex) {
//...handle exception
}
}
Plus you appear to be consuming the exception any way as it is already being caught further down the stack.
Try to keep the code async all the way through and not mix blocking calls like .Wait() or .Result
private static Task SendMessage(string message) {
return MQTT.RunAsync(message, topic);
}
public static async Task RunAsync(string message, string topicName) {
await this.mqttClient.ConnectAsync(this.options);
var topicFilter = new TopicFilterBuilder().WithTopic(this._topicname)
.WithExactlyOnceQoS().Build();
await mqttClient.SubscribeAsync(topicFilter);
var applicationMessage = new MqttApplicationMessageBuilder().WithTopic(this._topicname)
.WithPayload(message).WithAtLeastOnceQoS().Build();
if (stopSending == false) {
await mqttClient.PublishAsync(applicationMessage);
}
}
I need the following logic implemented:
a thread to which you can subscribe/unsubscribe methods at runtime.
It's fine for all these methods to have an header such as (Object sender, EventArgs e) and to return void.
These methods scope must be the scope of the class where they are defined lexically.
There's no guarantee about the order of execution
I've come up with the following implementation, which seems to do exactly what I need: basically I start an internal thread which triggers an event each x milliseconds. You can subscribe/unsubscribe delegates to this event through appropriate methods.
Before sticking to it I'd like to know if there may be subtle issues following this approach.
public class Orchestrator
{
private Thread _mainThread;
private event MethodDelegate _mainEvent;
public delegate void MethodDelegate (Object sender, EventArgs e);
private bool _stop = false;
private short _ms = 100;
public short PollingInterval { get { return _ms; }
set
{
_ms = value;
}
}
public Orchestrator()
{
_mainThread = new Thread(new ThreadStart(_execute));
}
public void Start()
{
_stop = false;
_mainThread.Start();
}
public void Stop()
{
_stop = true;
}
public void Clear()
{
_mainEvent = null;
}
public void Push(MethodDelegate method)
{
_mainEvent += method;
}
public void Pop(MethodDelegate method)
{
_mainEvent -= method;
}
private void _execute()
{
while(!_stop)
{
if (_mainEvent != null)
try
{
_mainEvent(this, new EventArgs());
}
catch (Exception e)
{
Console.WriteLine(e.ToString());
}
Thread.Sleep(_ms);
}
}
}
That's basically fine. You need to make _stop volatile. In C# the event accessor methods are thread-safe so that works fine.
The exception handling is very questionable. Do you really want to spam errors to the console? Define an event OnError and report errors to the consumer of your class.
You could use a timer or await Task.Delay to save a thread. This would make sense if there are a lot of such class instances at the same time. If there is just one this is likely not worth the effort.
You have a race condition which could cause a NullReferenceException, in:
while(!_stop)
{
if (_mainEvent != null)
try
{
_mainEvent(this, new EventArgs());
}
catch (Exception e)
{
Console.WriteLine(e.ToString());
}
Thread.Sleep(_ms);
}
Some other thread could unsubscribe from the event or call Clear() inbetween if (_mainEvent != null) and the call of _mainEvent.
To avoid this, you should copy _mainEvent into a local variable and check that against null, and use that instead:
var mainEvent = _mainEvent;
if (mainEvent != null)
try
{
mainEvent(this, new EventArgs());
In any case, I think you should be using a Timer for this rather than rolling-your-own.
im new here and always find helpful threads here, but not in this case. I'm having an issue with my program using threads. In special, my thread is doing some image pattern in the backround and everything seems to be fine.
It also works an undefined time (sometimes 15 s, sometimes minutes) without any exception or freeze or whatever. But then my GUI freezes, not my whole GUI, just the GUI parts updated from the thread. Two other pictureboxes are working fine(streaming a video), but the rest isnt working.
Trying to stop the thread where it is, works, but starting it from there makes my program collapse.
There arn't eny exceptions thrown. Every GUI element is updated via Invoke() if necessary. I only work on a copy of the Picture to avoid any lockmodes or anything else. Also i try to let the UI doing what it needs to do (DoEvents())
Some ideas?
Code:
namespace My.Name.Space
{
public class MyThread : MyThreadBase
{
public MyThread ( getting object s from the form for updating UI elements)
{
//referencing objects
Stopwatch.Start();
}
//example Method for UI updating
private void UpdateRobot1Box(int angle, int x, int y)
{
if (_rob1.InvokeRequired)
{
_rob1.Invoke(new Action(() => _rob1.Clear()));
_rob1.Invoke(new Action(() => _rob1.Text = angle.ToString() + "°, X:" + x.ToString() + ", Y:" + y.ToString()));
}
else
{
_rob1.Clear();
_rob1.Text = angle.ToString() + "°, X:" + x.ToString() + ", Y:" + y.ToString();
}
}
protected override void Loop(CancellationToken token)
{
while(!token.IsCancellationRequested)
{
if( PictureBox != null && Stopwatch.ElapsedMilliseconds >= tick)
{
//DoWork
Application.DoEvents();
}
else
{
Thread.Sleep(1);
}
}
}
}
}
}
Edit 1:
MyThreadBase:
namespace My.Name.Space
{
public abstract class MyThreadBase : DisposableBase//just some simple gc stuff
{
private CancellationTokenSource _cancellationTokenSource;
public bool IsAlive
{
get { return _cancellationTokenSource != null; }
}
public event Action<Object, Exception> UnhandledException;
public void Start()
{
if (_cancellationTokenSource != null)
return;
lock (this)
{
if (_cancellationTokenSource != null)
return;
_cancellationTokenSource = new CancellationTokenSource();
var thread = new Thread(RunLoop) {Name = GetType().Name};
thread.Start();
}
}
public void Stop()
{
if (_cancellationTokenSource == null)
return;
lock (this)
{
if (_cancellationTokenSource == null)
return;
_cancellationTokenSource.Cancel();
}
}
public void Join()
{
while (IsAlive) Thread.Sleep(1);
}
private void RunLoop()
{
try
{
CancellationToken token = _cancellationTokenSource.Token;
Loop(token);
}
catch (OperationCanceledException)
{
throw;
}
catch (Exception exception)
{
OnException(exception);
}
finally
{
lock (this)
{
CancellationTokenSource cancellationTokenSource = _cancellationTokenSource;
_cancellationTokenSource = null;
cancellationTokenSource.Dispose();
}
}
}
protected abstract void Loop(CancellationToken token);
protected virtual void OnException(Exception exception)
{
Trace.TraceError("{0} - Exception: {1}", GetType(), exception.Message);
Trace.TraceError(exception.StackTrace);
OnUnhandledException(exception);
}
protected virtual void OnUnhandledException(Exception exception)
{
if (UnhandledException != null)
UnhandledException(this, exception);
}
protected override void DisposeOverride()
{
Stop();
}
}
The UpdateRobot1Box is called in a switch-case construct within the thread. I got a little for-squence where I go through my list of own created objects to decide what to write in my textbox.
Create a method in main form class to perform the UI update actions:
private void AsyncFormUpdate(Action action)
{
if (this.InvokeRequired)
{
this.Invoke(action, null);
}
else
{
action();
}
}
Having that in proper place you are sure that InvokeRequired acts properly, well, and the code is better encapsulated.
Next be simple. Use the asynchronously invoked delegate with feedback to report angle and coordinates changes to UI, where you'll actually call the AsyncFormUpdate method.
A good example is shown here:
http://www.csharp-examples.net/asynchronous-method-progress/
There they update the progress, where as you'll update the angle and X/Y coordinates.
In it's simplicity what I am trying to do is handle "Doing Something" by firing off a process on a seperate thread to do what I need to do and waiting for an event to be raised to say "I have finished doing what I need to do". In the EventArgs though I will have a property for any errors which may be encountered during the process. Here is a simplified example of my situation.
public class MessageHandler
{
private AutoResetEvent MessageHasSent = new AutoResetEvent(false);
public void SendMessage()
{
MessageSender ms = new MessageSender();
ms.MessageSent += new EventHandler<MessageSentEventArgs>(MessageHandler_MessageSent);
Thread t = new Thread(ms.Send());
t.Start();
MessageHasSent.WaitOne();
//Do some check here
//Same again but for "Message recieved"
}
void MessageHandler_MessageSent(object sender, MessageSentEventArgs e)
{
if (e.Errors.Count != 0)
{
//What can I do here to return to the next step after waitone?
}
else
MessageHasSent.Set();
}
}
public class MessageSender
{
public event EventHandler<MessageSentEventArgs> MessageSent;
public void Send()
{
//Do some method which could potentiallialy return a List<Error>
MessageSent(this, new MessageSentEventArgs() { Errors = new List<Error>() });
}
}
public class Error { }
public class MessageSentEventArgs : EventArgs
{
public List<Error> Errors;
}
Essentially once the event has been raised from Send the code will continute, however I want some way of the event giving feedback, potentially using the MessageHasSent. I have tried different methods, I thought if I called Close instead of Set it would perhaps allow me to access something such as IsClosed. You could throw an exception or set a flag outside of the scope of the event to check but I feel like this is dirty.
Any suggestions?
Using the TPL isn't applicable in my case as I am using .NET 3.5.
Since it seems that this entire section of code is already running in a background thread, and you're doing nothing more than starting up a new thread just so that you can wait for it to finish, you'd be better off just calling Send directly, rather than asynchronously.
You don't need to fire off an event when you're completed.
You don't need to signal the main thread when it needs to continue.
You don't need to log the exceptions in a List, you can just throw them and catch them in SendMessage with a try/catch block.
This will do what you want:
public class MessageHandler
{
private AutoResetEvent MessageHasSent = new AutoResetEvent(false);
private bool IsSuccess = false;
public void SendMessage()
{
MessageSender ms = new MessageSender();
ms.MessageSent += new EventHandler<MessageSentEventArgs>(MessageHandler_MessageSent);
Thread t = new Thread(ms.Send());
t.Start();
MessageHasSent.WaitOne();
if(IsSuccess)
//wohooo
else
//oh crap
//Same again but for "Message recieved"
}
void MessageHandler_MessageSent(object sender, MessageSentEventArgs e)
{
IsSuccess = e.Errors.Count == 0;
MessageHasSent.Set();
}
}
public class MessageSender
{
public event EventHandler<MessageSentEventArgs> MessageSent;
public void Send()
{
//Do some method which could potentiallialy return a List<Error>
MessageSent(this, new MessageSentEventArgs() { Errors = new List<Error>() });
}
}
public class Error { }
public class MessageSentEventArgs : EventArgs
{
public List<Error> Errors;
}