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EditContext OnFieldChanged reporting wrong return type

I have the 3 methods below in a razor file
protected override async Task OnInitializedAsync()
{
EditContext = new EditContext(_projectModel);
EditContext.OnFieldChanged += EditContext_OnFieldChanged;
}
private async Task EditContext_OnFieldChanged(object sender, FieldChangedEventArgs e)
{
await SetOkDisabledStatus();
}
This method is an async method and I have to await it anywhere it is been called
private async Task SetOkDisabledStatus()
{
if (EditContext.Validate())
{
OkayDisabled = null;
await JsRuntime.InvokeVoidAsync("Animate");
}
else
{
OkayDisabled = "disabled";
}
}
I am using the EditContext for validation in a Blazor server application.
I have been getting the error message on this line below in the OnInitializedAsync() method and not sure how to proceed with it.
EditContext.OnFieldChanged += EditContext_OnFieldChanged;
Error Message:
Task MyProject.EditContext_OnFieldChanged(object, FieldChangedEventArgs)'
has the wrong return type.
Expected a method with void EditContext_OnFieldChanged(object?, FieldChangedEventArgs e)
Please note that I am using sonarqube to check all my code.

You can assign an async lambda to the event handler, like this:
EditContext.OnFieldChanged +=
async (sender,args) => await EditContext_OnFieldChanged(sender,args);
But, you should be aware that the EditContext/Form will not await your task. Anything you do in that async task will be out of sync with the editcontext.
You should probably include a cancellation token in your async code as well, so that multiple changes to a field do not fire multiple validation tasks at the same time.
Async validation is hard - make sure you test every possible scenario.

Generated Blazor eventhandlers (like #onclick="...") are flexible about return type and parameters but EditContext.OnFieldChanged is not, it has a fixed delegate type.
Make the following change:
//private async Task EditContext_OnFieldChanged(object sender, FieldChangedEventArgs e)
private async void EditContext_OnFieldChanged(object sender, FieldChangedEventArgs e)
{
await SetOkDisabledStatus();
StateHasChanged(); // make sure OkayDisabled takes effect
}
On another note, you can probably make OkayDisabled a boolean and use disabled="#OkayDisabled" where you need it.
Blazor makes the disabled attribute disappear when you assign it with false.
Alternative: keep the validation synchronous. That might prevent some problems as #Mister Magoo points out. And then let only the Animation run async.
private void EditContext_OnFieldChanged(object sender, FieldChangedEventArgs e)
{
SetOkDisabledStatus();
}
private void SetOkDisabledStatus()
{
if (EditContext.Validate())
{
OkayDisabled = null;
_ = JsRuntime.InvokeVoidAsync("Animate"); // no await, on purpose
}
else
{
OkayDisabled = "disabled";
}
}
StateHasChanged() should not be needed in this scenario.

Exiting from async infinite loops

I have initiated some async infinite loops in my WinForm application, but each time I am trying to break out of them, the program hangs up. I have read some similar topics where people suggested using CancellationTokens, but I am not able to adapt them to my needs. Here is the relevant part of my code.
static bool processStop = false;
static bool processStopped = false;
//Called once
private async void ProcessData()
{
while (!processStop)
{
await Task.Run
(
() =>
{
//Do stuff and call regular not async methods
}
);
}
processStopped = true;
}
//Button click handler to exit WinForm
btnExit.Click += (senders, args) =>
{
processStop = true;
//Programm hangs up here
while (!processStopped);
FormMain.Close();
}
Edited the code
The variables are static.
The Close method is the default Close() method for Forms.

The problem is that the call to Task.Run continues on the main thread. processStop = true; and while (!processStopped); execute synchronously one after the other. This doesn't let the ProcessData method continue its execution and a deadlock occures.
I see a couple of solutions:
Use ConfigureAwait(false) with Task.Run:
private async void ProcessData()
{
while (!processStop)
{
await Task.Run
(
() =>
{
//Do stuff and call regular not async methods
}
).ConfigureAwait(false);
}
processStopped = true;
}
This will cause the ProcessData to continue on a thread pool and you already use a thread pool by calling Task.Run, so it is not a great solution
Wrap the whole process in Task.Run:
static volatile bool processStop = false;
static volatile bool processStopped = false;
//Called once
private async void ProcessData()
{
await Task.Run(() =>
{
while (!processStop)
{
...
}
processStopped = true;
});
}
This would require changing the form of the method passed to work with the loop in it.
Make ProcessData a synchronous method to process CPU-intensive tasks and call it properly. CancellationToken would be the preferred way to cancel the task:
private void ProcessData(CancellationToken token)
{
while(!token.IsCancellationRequested)
{
// do work
}
}
And call it with this:
Task processingTask;
CancellationTokenSource cts;
void StartProcessing()
{
cts = new CancellationTokenSource();
processingTask = Task.Run(() => ProcessData(cts.Token), cts.Token);
}
btnExit.Click += async (senders, args) =>
{
cts.Cancel();
try
{
await processingTask;
}
finally
{
FormMain.Close();
}
}

If you want to spin a bunch of tasks without blocking you can do this:
using System;
using System.Diagnostics;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;
namespace WindowsFormsApp1
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
//Called once
private async Task ProcessData()
{
int count = 0;
while (true)
{
await Task.Run
(
() =>
{
this.Invoke(new Action(() => {
label2.Text = (count++).ToString();
label1.Text = DateTime.Now.ToString(); }));
Thread.Sleep(100);
}
);
}
Debugger.Break(); //you will never see this hit at all
}
private void button1_Click(object sender, EventArgs e)
{
this.Close();
}
private async void button2_Click(object sender, EventArgs e)
{
await ProcessData();
}
}
}

Custom Busy overlay on WPF WizardControl using async await

I need load a overlay on a WPF wizardcontrol. I am using a busyIndicator tool from the wpf extended tooklit.
The code for async await works but the gui thread locks . I am trying add a please wait message when the await calls the function
private async void Button1_Click(object sender, RoutedEventArgs e)
{
BusyIndicator.IsBusy = true;
BusyIndicator.IsEnabled = true;
BusyIndicator.BusyContent = "Please wait while Site is provisioned";
await Task.Run(() =>
{
LongRunningFunction();
});
BusyIndicator.IsBusy=false;
}
The XAML for the BusyIndicator is as below.
<xctk:BusyIndicator x:Name="BusyIndicator" IsBusy="False" BusyContent="Please Wait">
</xctk:BusyIndicator>
The LonRunningFunction is a Webservice call which does not update the UI only returns a Bool value
public static bool LongRunningFunction(string URL)
{
bool IsPresent = CallWebservice()
return IsPresent;
}
Issue
1) The BusyIndicator does not seem to fire before the async call instead it seems to be fire when the LongRunning task completes
2) What is the correct process to call a gui overlay when async and await is used.

This is the way I tackled the problem with asynchronous calls.
Context:
Here I am using MvvM to show you good practice when working with WPF
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Data;
using System.Windows.Input;
using System.Windows.Threading;
class VM
{
Dispatcher _dispatcher = Dispatcher.CurrentDispatcher;
//Time consuming operation
private void LongTask()
{
Thread.Sleep(5000);
//in here if you need to send something to the UI thread like an event use it like so:
_dispatcher.Invoke(new Action(() =>
{
//some code here to invoke an event
if (ComponentsLoaded != null)
ComponentsLoaded(this, new EventArgs { });
}));
}
private ICommand _command;
//This is the command to be used instead of click event handler
public ICommand Command
{
get { return _command; }
private set { _command = value; }
}
//method associated with ICommand
void commandMethod(object parameter)
{
Busy = true;
ThreadPool.QueueUserWorkItem(new WaitCallback(multiThreadTask));
Busy = false;
}
//the task to be started on another thread
void multiThreadTask(object parameter)
{
LongTask();
}
public event EventHandler ComponentsLoaded;
}
This is what I use when working with multiple threads in WPF.
You can still use this in the code-behind just instantiate the Dispatcher and you're good to go.
If you need any more info just let us know. HTH

Is it possible to have async methods as callbacks to eventhandlers in c#?

My design is illustrated by below example. Having a while true loop doing something and notifying by an event that it has done something to all subscribers. My application should not continue its execution before its done notifying all subscribers, where this works as long as someone do not put a async void on the callback.
If someone put a async void on the callback to await some task, then my loop can continue before the callback is completed. What other designs can I do to avoid this situation.
Its 3th party plugins that register themeself and subscribe to the event, so I have no control over if they put a async void. Understandable I cant do Task callbacks for the EventHandler, so what alternatives do I have with .net 4.5.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
namespace ConsoleApplication4
{
public class Test
{
public event EventHandler Event;
public void DoneSomething()
{
if (Event != null)
Event(this,EventArgs.Empty);
}
}
class Program
{
static void Main(string[] args)
{
var test = new Test();
test.Event += test_Event;
test.Event +=test_Event2;
while(true)
{
test.DoneSomething();
Thread.Sleep(1000);
}
}
private static void test_Event2(object sender, EventArgs e)
{
Console.WriteLine("delegate 2");
}
static async void test_Event(object sender, EventArgs e)
{
Console.WriteLine("Del1gate 1");
await Task.Delay(5000);
Console.WriteLine("5000 ms later");
}
}
}

If someone put a async void on the callback to await some task, then my loop can continue before the callback is completed. What other designs can I do to avoid this situation.
There is really no way to avoid this. Even if you were to somehow "know" that the subscriber wasn't implemented via async/await, you still couldn't guarantee that the caller didn't build some form of asynchronous "operation" in place.
For example, a completely normal void method could put all of its work into a Task.Run call.
My application should not continue its execution before its done notifying all subscribers
Your current version does follow this contract. You're notifying the subscribers synchronously - if a subscriber does something asynchronously in response to that notification, that is something outside of your control.
Understandable I cant do Task callbacks for the EventHandler, so what alternatives do I have with .net 4.5.
Note that this is actually possible. For example, you can rewrite your above as:
public class Program
{
public static void Main()
{
var test = new Test();
test.Event += test_Event;
test.Event +=test_Event2;
test.DoneSomethingAsync().Wait();
}
}
public delegate Task CustomEvent(object sender, EventArgs e);
private static Task test_Event2(object sender, EventArgs e)
{
Console.WriteLine("delegate 2");
return Task.FromResult(false);
}
static async Task test_Event(object sender, EventArgs e)
{
Console.WriteLine("Del1gate 1");
await Task.Delay(5000);
Console.WriteLine("5000 ms later");
}
public class Test
{
public event CustomEvent Event;
public async Task DoneSomethingAsync()
{
var handler = this.Event;
if (handler != null)
{
var tasks = handler.GetInvocationList().Cast<CustomEvent>().Select(s => s(this, EventArgs.Empty));
await Task.WhenAll(tasks);
}
}
}
You can also rewrite this using event add/remove, as suggested by svick:
public class Test
{
private List<CustomEvent> events = new List<CustomEvent>();
public event CustomEvent Event
{
add { lock(events) events.Add(value); }
remove { lock(events) events.Remove(value); }
}
public async Task DoneSomething()
{
List<CustomEvent> handlers;
lock(events)
handlers = this.events.ToList(); // Cache this
var tasks = handlers.Select(s => s(this, EventArgs.Empty));
await Task.WhenAll(tasks);
}
}

My application should not continue its execution before its done notifying all subscribers, where this works as long as someone do not put a async void on the callback.
I have a blog entry on designing for async event handlers. It is possible to use Task-returning delegates or to wrap an existing SynchronizationContext within your own (which would allow you to detect and wait for async void handlers).
However, I recommend you use "deferrals", which are objects designed specifically to solve this problem for Windows Store applications. A simple DeferralManager is available in my AsyncEx library.
Your event args can define a GetDeferral method as such:
public class MyEventArgs : EventArgs
{
private readonly DeferralManager deferrals = new DeferralManager();
... // Your own constructors and properties.
public IDisposable GetDeferral()
{
return deferrals.GetDeferral();
}
internal Task WaitForDeferralsAsync()
{
return deferrals.SignalAndWaitAsync();
}
}
And you can raise an event and (asynchronously) wait for all asynchronous handlers to complete like this:
private Task RaiseMyEventAsync()
{
var handler = MyEvent;
if (handler == null)
return Task.FromResult<object>(null); // or TaskConstants.Completed
var args = new MyEventArgs(...);
handler(args);
return args.WaitForDeferralsAsync();
}
The benefit of the "deferral" pattern is that it is well-established in the Windows Store APIs, so it's likely to be recognized by end users.

Accessing UI controls in Task.Run with async/await on WinForms

I have the following code in a WinForms application with one button and one label:
using System;
using System.IO;
using System.Threading.Tasks;
using System.Windows.Forms;
namespace WindowsFormsApplication1
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private async void button1_Click(object sender, EventArgs e)
{
await Run();
}
private async Task Run()
{
await Task.Run(async () => {
await File.AppendText("temp.dat").WriteAsync("a");
label1.Text = "test";
});
}
}
}
This is a simplified version of the real application I'm working on. I was under the impression that by using async/await in my Task.Run I could set the label1.Text property. However, when running this code I get the error that I'm not on the UI thread and I can't access the control.
Why can't I access the label control?

When you use Task.Run(), you're saing that you don't want the code to run on the current context, so that's exactly what happens.
But there is no need to use Task.Run() in your code. Correctly written async methods won't block the current thread, so you can use them from the UI thread directly. If you do that, await will make sure the method resumes back on the UI thread.
This means that if you write your code like this, it will work:
private async void button1_Click(object sender, EventArgs e)
{
await Run();
}
private async Task Run()
{
await File.AppendText("temp.dat").WriteAsync("a");
label1.Text = "test";
}

Try this:
replace
label1.Text = "test";
with
SetLabel1Text("test");
and add the following to your class:
private void SetLabel1Text(string text)
{
if (InvokeRequired)
{
Invoke((Action<string>)SetLabel1Text, text);
return;
}
label1.Text = text;
}
The InvokeRequired returns true if you are NOT on the UI thread. The Invoke() method takes the delegate and parameters, switches to the UI thread and then calls the method recursively. You return after the Invoke() call because the method has already been called recursively prior to the Invoke() returning. If you happen to be on the UI thread when the method is called, the InvokeRequired is false and the assignment is performed directly.

Try this
private async Task Run()
{
await Task.Run(async () => {
await File.AppendText("temp.dat").WriteAsync("a");
});
label1.Text = "test";
}
Or
private async Task Run()
{
await File.AppendText("temp.dat").WriteAsync("a");
label1.Text = "test";
}
Or
private async Task Run()
{
var task = Task.Run(async () => {
await File.AppendText("temp.dat").WriteAsync("a");
});
var continuation = task.ContinueWith(antecedent=> label1.Text = "test",TaskScheduler.FromCurrentSynchronizationContext());
await task;//I think await here is redundant
}
async/await doesn't guarantee that it will run in UI thread. await will capture the current SynchronizationContext and continues execution with the captured context once the task completed.
So in your case you have a nested await which is inside Task.Run hence second await will capture the context which is not going to be UiSynchronizationContext because it is being executed by WorkerThread from ThreadPool.
Does this answers your question?

Why do you use Task.Run? that start a new worker thread (cpu bound), and it causes your problem.
you should probably just do that:
private async Task Run()
{
await File.AppendText("temp.dat").WriteAsync("a");
label1.Text = "test";
}
await ensure you will continue on the same context except if you use .ConfigureAwait(false);

Because it's on a different thread and cross-thread calls aren't allowed.
You will need to pass on the "context" to the thread you are starting. See an example here: http://reedcopsey.com/2009/11/17/synchronizing-net-4-tasks-with-the-ui-thread/

I am going to give you my latest answer that I have given for async understanding.
The solution is as you know that when you are calling async method you need to run as a task.
Here is a quick console app code that you can use for your reference, it will make it easy for you to understand the concept.
using System;
using System.Threading;
using System.Threading.Tasks;
public class Program
{
public static void Main()
{
Console.WriteLine("Starting Send Mail Async Task");
Task task = new Task(SendMessage);
task.Start();
Console.WriteLine("Update Database");
UpdateDatabase();
while (true)
{
// dummy wait for background send mail.
if (task.Status == TaskStatus.RanToCompletion)
{
break;
}
}
}
public static async void SendMessage()
{
// Calls to TaskOfTResult_MethodAsync
Task<bool> returnedTaskTResult = MailSenderAsync();
bool result = await returnedTaskTResult;
if (result)
{
UpdateDatabase();
}
Console.WriteLine("Mail Sent!");
}
private static void UpdateDatabase()
{
for (var i = 1; i < 1000; i++) ;
Console.WriteLine("Database Updated!");
}
private static async Task<bool> MailSenderAsync()
{
Console.WriteLine("Send Mail Start.");
for (var i = 1; i < 1000000000; i++) ;
return true;
}
}
Here I am trying to initiate task called send mail. Interim I want to update database, while the background is performing send mail task.
Once the database update has happened, it is waiting for the send mail task to be completed. However, with this approach it is quite clear that I can run task at the background and still proceed with original (main) thread.