Develop Reference

Accessing controls in Form from a derived BackgroundWorker class results in cross-thread error

I'm working on writing a class which is derived from the System.ComponentModel.BackgroundWorker class. The reason I am doing so in my project is that I need a lot of information to be returned in different types of status update events, depending on which event is raised. When attempting to update any of the controls the main form from any of my update events, I am getting the following error:
System.InvalidOperationException: 'Cross-thread operation not valid:
Control '' accessed from a thread other than the thread it was created
on.'
The first control that I am attempting to update is a ToolStripStatusLabel, which does not have an .Invoke() method. I have created minimally verifiable example below. To recreate the error, simply create a new Windows Forms App (.NET Framework) project targeted to .NET 4.8 and copy paste the following code into the Form1.cs file:
using System;
using System.ComponentModel;
using System.Windows.Forms;
namespace WindowsFormsApp1
{
public partial class Form1 : Form
{
private StatusStrip statusStrip1;
private ToolStripStatusLabel toolStripStatusLabel1;
private ToolStripProgressBar toolStripProgressBar1;
private Button button1;
private MyBGW myBGW;
public Form1()
{
InitializeComponent();
this.statusStrip1 = new StatusStrip();
this.toolStripStatusLabel1 = new ToolStripStatusLabel() { Text = "Starting Text" };
this.toolStripProgressBar1 = new ToolStripProgressBar();
this.button1 = new Button();
this.myBGW = new MyBGW();
this.statusStrip1.Items.AddRange(new System.Windows.Forms.ToolStripItem[] {this.toolStripStatusLabel1, this.toolStripProgressBar1});
this.Controls.Add(this.statusStrip1);
this.Controls.Add(this.button1);
this.button1.Click += Button1_Click;
this.myBGW.OnMyBGW_StatusChanged += MyBGW_OnMyBGW_StatusChanged;
}
private void Button1_Click(object sender, EventArgs e) { myBGW.RunWorkerAsync(); }
private void MyBGW_OnMyBGW_StatusChanged(object sender, MyBGW.MyBGW_StatusChanged_EventArgs e)
{
// The following two lines will throw the cross-threading exception
this.toolStripStatusLabel1.Text = e.StatusText;
if (e.PBarStyle != MyBGW.pBarStyles.NoChange) { this.toolStripProgressBar1.Style = (ProgressBarStyle)e.PBarStyle; }
}
}
public class MyBGW : BackgroundWorker
{
public enum pBarStyles { Block = 0, Continuous = 1, Marquee = 2, NoChange = -1 }
public delegate void MyBGW_StatusChanged_EventHandler(object sender, MyBGW_StatusChanged_EventArgs e);
public event MyBGW_StatusChanged_EventHandler OnMyBGW_StatusChanged;
public class MyBGW_StatusChanged_EventArgs : EventArgs
{
public string StatusText;
public pBarStyles PBarStyle;
public MyBGW_StatusChanged_EventArgs(string statusText, pBarStyles pBarStyle)
{
this.StatusText = statusText; this.PBarStyle = pBarStyle;
}
}
public new void RunWorkerAsync() { base.RunWorkerAsync(); }
private void myBGW_DoWork(object sender, DoWorkEventArgs e)
{
OnMyBGW_StatusChanged(this, new MyBGW_StatusChanged_EventArgs(DateTime.Now.ToString(), pBarStyles.Marquee));
System.Threading.Thread.Sleep(10000);
OnMyBGW_StatusChanged(this, new MyBGW_StatusChanged_EventArgs("Done", pBarStyles.Continuous));
}
public MyBGW() { base.DoWork += new DoWorkEventHandler(this.myBGW_DoWork); }
}
}
My best guess is that I am raising or consuming the event incorrectly which is causing the code to still be run on the worker thread instead of the main/UI thread, but I'm coming up short in my research on what I'm missing.
EDIT: this question is not related to Cross-thread operation not valid: Control accessed from a thread other than the thread it was created on as it is not directly relying on a BackgroundWorker but is rather attempting to add additional events to a derived class, of which the addition of those events are causing the Cross-Thread exception. Also, the answer does not apply as the control attempting to be updated does not have the .Invoke method as the solution to that question stated.
The problem for this question is in relation to how the event was being raised, which was incorrectly, causing the consumption of that event to be on the wrong thread and raising the cross-thread exception.

The BackgroundWorker.DoWork event handler is supposed to do background work, and it's not intended for interacting with the UI. This handler is invoked on a ThreadPool thread, and interacting with UI components from any thread other than the UI thread is not allowed. The BackgroundWorker class offers two events that are raised on the UI thread¹, the ProgressChanged and the RunWorkerCompleted. You could take advantage of this, by invoking your StatusChanged event on the ProgressChanged event handler (or overriding the OnProgressChanged method), and passing your StatusChangedEventArgs as an argument of the ReportProgress method:
public class MyBGW : BackgroundWorker
{
public enum BarStyles { Block = 0, Continuous = 1, Marquee = 2, NoChange = -1 }
public delegate void StatusChangedEventHandler(object sender,
StatusChangedEventArgs e);
public event StatusChangedEventHandler StatusChanged;
public MyBGW() { this.WorkerReportsProgress = true; }
public class StatusChangedEventArgs : EventArgs
{
public string StatusText;
public BarStyles PBarStyle;
public StatusChangedEventArgs(string statusText, BarStyles pBarStyle)
{
this.StatusText = statusText; this.PBarStyle = pBarStyle;
}
}
protected override void OnDoWork(DoWorkEventArgs e)
{
this.ReportProgress(-1,
new StatusChangedEventArgs(DateTime.Now.ToString(), BarStyles.Marquee));
base.OnDoWork(e);
this.ReportProgress(-1,
new StatusChangedEventArgs("Done", BarStyles.Continuous));
}
protected override void OnProgressChanged(ProgressChangedEventArgs e)
{
if (e.ProgressPercentage == -1 && e.UserState is StatusChangedEventArgs args)
StatusChanged?.Invoke(this, args);
else
base.OnProgressChanged(e);
}
}
¹ To be precise, the ProgressChanged and RunWorkerCompleted events are raised on the SynchronizationContext.Current which is captured when the BackgroundWorker.RunWorkerAsync is invoked.

Because toolStripStatusLabel1 And toolStripProgressBar1 runs inside a thread other than the main thread, it needs to be Invoke. And since ToolStripStatusLabel And ToolStripProgressBar itself does not have an Invoke method, we use its parent Invoke method.
change MyBGW_OnMyBGW_StatusChanged to :
private void MyBGW_OnMyBGW_StatusChanged(object sender, MyBGW.MyBGW_StatusChanged_EventArgs e)
{
InvokeIfRequired(this, ()=>
{
this.toolStripStatusLabel1.Text = e.StatusText;
});
if (e.PBarStyle != MyBGW.pBarStyles.NoChange)
{
InvokeIfRequired(this, () =>
{
this.toolStripProgressBar1.Style = (ProgressBarStyle)e.PBarStyle;
});
}
}
add InvokeIfRequired method
public void InvokeIfRequired(Control control, MethodInvoker action)
{
if (control.InvokeRequired)
control.Invoke(action);
else
action();
}

As mjwills has stated in the comments of the question, I was not raising the event properly, which was causing the event to be consumed on the same worker thread. After looking at the link for the .NET source code of the BackgroundWorker class, I can see that there is a bit of code, AsyncOperation.Post() that has the method protected virtual void OnStatusChangedin the code below raised in the main thread rather than the worker thread.
public class MyBGW : BackgroundWorker
{
public enum pBarStyles { Block = 0, Continuous = 1, Marquee = 2, NoChange = -1 }
private static readonly object statusChangedKey = new object();
private AsyncOperation asyncOperation = null;
public MyBGW() { base.DoWork += new DoWorkEventHandler(this.myBGW_DoWork); }
public delegate void StatusChanged_EventHandler(object sender, StatusChanged_EventArgs e);
public event StatusChanged_EventHandler StatusChanged
{
add { this.Events.AddHandler(statusChangedKey, value); }
remove { this.Events.RemoveHandler(statusChangedKey, value); }
}
protected virtual void OnStatusChanged(StatusChanged_EventArgs e) { ((StatusChanged_EventHandler)Events[statusChangedKey])?.Invoke(this, e); }
private void StatusReporter(object arg) { OnStatusChanged((StatusChanged_EventArgs)arg); }
public void UpdateStatus(StatusChanged_EventArgs e) { asyncOperation.Post(new System.Threading.SendOrPostCallback(StatusReporter), e); }
public class StatusChanged_EventArgs : EventArgs
{
public string StatusText;
public pBarStyles PBarStyle;
public StatusChanged_EventArgs(string statusText, pBarStyles pBarStyle)
{
this.StatusText = statusText; this.PBarStyle = pBarStyle;
}
}
public new void RunWorkerAsync() { asyncOperation = AsyncOperationManager.CreateOperation(null); base.RunWorkerAsync(); }
private void myBGW_DoWork(object sender, DoWorkEventArgs e)
{
UpdateStatus(new StatusChanged_EventArgs(DateTime.Now.ToString(), pBarStyles.Marquee));
System.Threading.Thread.Sleep(3000);
UpdateStatus(new StatusChanged_EventArgs("Done", pBarStyles.Continuous));
}
}
I don't fully understand the how and why, but it works. Hopefully someone can comment below with a better explanation.

Application.Current returning null before shutdown

I have a Page inside a NavigationWindow in WPF. When I press the close button (the X) I need to execute some code when the Page closes. Right now in my code-behind I've got this:
public partial class CompactLayout : Page
{
private readonly CompactLayoutViewModel _viewModel;
public CompactLayout()
{
InitializeComponent();
DataContext = _viewModel = new CompactLayoutViewModel();
Dispatcher.ShutdownStarted += OnShutdownStarted;
//instead of Dispatcher.ShutdownStarted I've also tried: this.Unloaded += OnShutdownStarted;
}
private void OnShutdownStarted(object sender, EventArgs e)
{
_viewModel.releaseServerViewModel();
}
}
In my ViewModel constructor I have an Event Handler called ImageUpdate (ClientProxy is the class where I raise the event):
ClientProxy.ImageUpdate += async (sender, args) => await ImageUpdateAsync(args.image,args.fps);
Declaration of ImageUpdateAsync in the ViewModel:
private Task ImageUpdateAsync(byte[] img, int fps)
{
Application.Current.Dispatcher.Invoke(new Action(() =>
{
try
{
// things occurr
}
catch (Exception ex)
{
// things occurr v2
}
}));
return Task.CompletedTask;
}
I get the error System.Windows.Application.Current.get returned null from the call Application.Current.Dispatcher.Invoke at ImageUpdateAsync before _viewModel.releaseServerViewModel is called. If instead of Dispatcher.ShutdownStarted I use this.Unloaded += OnShutdownStarted the error ocurrs even before OnShutdownStarted executes.
How can I fix this code so the Page unloads before this error can happen?

class ViewModel
{
private readonly Dispatcher _dispatcher = Application.Current.Dispatcher;
private Task ImageUpdateAsync(byte[] img, int fps)
{
_dispatcher.Invoke(() => {/*...*/});
return Task.CompletedTask;
}
}

Passing events between threads run from separate class C#

I'm struggling to pass data between a thread started in a separate class from my main form. I believe (I could be wrong) that I should use an event. The problem I have is my subscribers are always null as I call the BluetoothScan class and start the thread before the event is subscribed to:
BluetoothScan bluetoothScan = new BluetoothScan(this);
bluetoothScan.BluetoothDeviceDiscovered += OnBluetoothDeviceDiscovered;
How do I subscribe to the event before starting the thread?
I have my Main Form:
using System;
using System.Windows.Forms;
//https://learn.microsoft.com/en-us/dotnet/api/system.windows.forms.control.invoke?view=net-5.0#System_Windows_Forms_Control_Invoke_System_Delegate_System_Object___
namespace YieldMonitor
{
public partial class MainForm : Form
{
public MainForm()
{
InitializeComponent();
}
private void MainForm_Load(object sender, EventArgs e)
{
}
private void BtnConnectBT_Click(object sender, EventArgs e)
{
//Start looking for the yield monitor device.
BluetoothScan bluetoothScan = new BluetoothScan(this);
bluetoothScan.BluetoothDeviceDiscovered += OnBluetoothDeviceDiscovered;
}
static void OnBluetoothDeviceDiscovered(object sender, EventArgs e)
{
System.Diagnostics.Debug.WriteLine("Message recieved from event");
}
}
}
My class that looks for bluetooth devices and if the right one is found should fire the event:
using InTheHand.Net.Sockets;
using System;
using System.Linq;
namespace YieldMonitor
{
class BluetoothScan
{
public event EventHandler BluetoothDeviceDiscovered;
public BluetoothScan(MainForm mainForm)
{
System.Diagnostics.Debug.WriteLine("Starting BluetoothScan Class");
Run();
}
public void Run()
{
System.Diagnostics.Debug.WriteLine("Running BluetoothScan Class");
string myDeviceName;
ulong myDeviceAddress;
BluetoothClient btClient = new BluetoothClient();
BluetoothDeviceInfo[] btDevices = btClient.DiscoverDevices().ToArray();
foreach (BluetoothDeviceInfo d in btDevices)
{
System.Diagnostics.Debug.WriteLine(d.DeviceName);
System.Diagnostics.Debug.WriteLine(d.DeviceAddress);
//have we found the device we are looking for?
if (d.DeviceName == "DSD TECH HC-05")
{
myDeviceName = d.DeviceName;
myDeviceAddress = d.DeviceAddress;
//Send out found adapter to the next stage
OnBluetoothScanned(EventArgs.Empty);
break;
}
}
}
protected virtual void OnBluetoothScanned(EventArgs e)
{
System.Diagnostics.Debug.WriteLine("Running OnBlueToothScanned");
EventHandler handler = BluetoothDeviceDiscovered;
if (handler != null)// we have a subscriber to our event
{
System.Diagnostics.Debug.WriteLine("BluetoothScanned is Not empty");
handler(this, e);
}
else
{
System.Diagnostics.Debug.WriteLine("BluetoothScanned is Empty");
}
}
}
}
EDIT
I've found some nice solutions using Tasks where I need to update a label once a task is completed ie.
bool myDevicePaired = false;
var eventDevicePaired = new Progress<bool>(boDevicePaired => myDevicePaired = boDevicePaired);
await Task.Factory.StartNew(() => BluetoothPair.Run(myDeviceAddress, eventDevicePaired), TaskCreationOptions.LongRunning);
//Register the device is paired with the UI
if (myDevicePaired)
{
BtnConnectBT.Text = "Disconnect?";
}
Which is working well for Tasks that have an end that I am waiting for example waiting for a bluetooth device to connect.
But I'm beginning to pull my hair out with System.InvalidOperationException: 'Cross-thread operation not valid: Control 'tbInfo' accessed from a thread other than the thread it was created on.' error when trying to update a form text box.
Example:
in my MainForm Class:
I create what I've called an Event Reciever...
private void BluetoothSocketEventReciever(object sender, EventArgs e)
{
Debug.WriteLine("Event!!!"); //writes data to debug fine
tbInfo.AppendText("Event!!!!"); //causing error
}
I create a task to read from the device...
private void ReadDataFromDevice(UInt64 myDeviceAddress)
{
BluetoothSocket bluetoothSocket = new BluetoothSocket(myDeviceAddress);
bluetoothSocket.BluetoothDataRecieved += BluetoothSocketEventReciever;
Task.Factory.StartNew(() => bluetoothSocket.Run(), TaskCreationOptions.LongRunning);
}
In my BluetoothSocket class I have an endless while loop which will be reading data from a socket (hopefully) At the moment its just creating an empty EventArgs to trigger the Event every second:
namespace YieldMonitor
{
class BluetoothSocket
{
ulong myDeviceAddress;
public event EventHandler BluetoothDataRecieved;
public BluetoothSocket (ulong deviceAddress)
{
myDeviceAddress = deviceAddress;
}
public void Run()
{
System.Diagnostics.Debug.WriteLine("Were in BluetoothSocket ... Address: " + myDeviceAddress);
while (true)
{
Thread.Sleep(1000);
Debug.WriteLine("In BluetoothSocket - Address = " + myDeviceAddress);
OnBluetoothDataRecieved(EventArgs.Empty);
}
}
protected virtual void OnBluetoothDataRecieved(EventArgs e)
{
EventHandler handler = BluetoothDataRecieved;
if (handler != null)
{
handler(this, e);
} else
{
//No subscribers
}
}
}
}
I'm sure I'm missing something simple here but how can I pass the data from the endless loop to the text box on the main form?
EDIT
Think I've just sorted it.
private void BluetoothSocketEventReciever(object sender, EventArgs e)
{
Debug.WriteLine("Event!!!");
tbInfo.Invoke((Action)delegate
{
tbInfo.AppendText("Event!!!");
});
//tbInfo.AppendText("Event!!!!");
}
Is this the correct way to do it?

You can Pass the event handler as a parameter on the constructor
public event EventHandler BluetoothDeviceDiscovered;
public BluetoothScan(MainForm mainForm, EventHandler bluetoothDeviceDiscovered)
{
System.Diagnostics.Debug.WriteLine("Starting BluetoothScan Class");
BluetoothDeviceDiscovered += bluetoothDeviceDiscovered
Run();
}
Personally, i'm not so fun of calling method on constructor. It can be source of bugs or performance issues
Constructor
In class-based object-oriented programming, a constructor
(abbreviation: ctor) is a special type of subroutine called to create
an object. It prepares the new object for use, often accepting
arguments that the constructor uses to set required member variables.
You can pass eventhandler as parameter and call Run later

Could creating an event be a valid way to make a XF OnAppearing into an async method?

I recently saw this suggestion on how to make an app OnStart into an async OnStart:
protected override void OnStart()
{
this.started += onStarted; //Subscribe to event
started(this, EventArgs.Empty); //Raise event
}
protected async void onStarted(object sender, EventArgs args)
{
try
{
await // do things
}
catch (Exception ex)
{
var ignore = ex;
}
this.started -= onStarted;
}
Can anyone see any possible issues with this and if not then could something similar be applied to the OnAppearing and if so would there be any changes needed.

OnAppearing is simply a void method on Page class
Xamarin.Forms.Page
//...
protected virtual void OnAppearing()
{
}
//...
Source
that is called as part of the Page's life cycle.
//...
[EditorBrowsable(EditorBrowsableState.Never)]
public void SendAppearing()
{
if (_hasAppeared)
return;
_hasAppeared = true;
if (IsBusy)
{
if (IsPlatformEnabled)
MessagingCenter.Send(this, BusySetSignalName, true);
else
_pendingActions.Add(() => MessagingCenter.Send(this, BusySetSignalName, true));
}
OnAppearing(); //<---
Appearing?.Invoke(this, EventArgs.Empty); //NOTE HOW ACTUAL EVENT IS RAISED AFTER
var pageContainer = this as IPageContainer<Page>;
pageContainer?.CurrentPage?.SendAppearing();
FindApplication(this)?.OnPageAppearing(this);
}
//...
Source
They are not to be mistaken for event handlers, that are the one exception allowed to use async void.
Reference Async/Await - Best Practices in Asynchronous Programming
The approach shown in your OnStart example can also be applied to OnAppearing
For example
public partial class SomePage : ContentPage {
public SomelPage() {
InitializeComponent();
appearing += onAppearing;
}
protected override void OnAppearing() {
appearing(this, EventArgs.Empty);
appearing -= onAppearing;
}
event EventHandler appearing = delegate { };
private async void onAppearing(object sender, EventArgs args) {
try {
var locator = CrossGeolocator.Current;
var position = await locator.GetPositionAsync();
var places = await SomeService.getPlacesOfInterest(position.Latitude, position.Longitude);
placesListView.ItemsSource = places;
} catch( Exception ex) {
//handler error (Log?)
}
}
}
Or you could subscribe to the actual Appearing event directly
//...
public event EventHandler Appearing;
//...
Source
and forgo overriding the OnAppearing() method
public partial class SomePage : ContentPage {
public SomelPage() {
InitializeComponent();
Appearing += onAppearing;
}
private async void onAppearing(object sender, EventArgs args) {
try {
var locator = CrossGeolocator.Current;
var position = await locator.GetPositionAsync();
var places = await SomeService.getPlacesOfInterest(position.Latitude, position.Longitude);
placesListView.ItemsSource = places;
} catch( Exception ex) {
//handler error (Log?)
}
}
}

Update UI from another GPIOListener class

I'm developing an application with windows-10-t platform on raspberry-pi3. The application has several pages and listens GPIO ports asyncrhonously in the background. It collects data from GPIO and sends to the WCF-Service, after a bit the UI should be updated by the data coming from the WCFService. I've also tried using Tasks, Dispatcher.Invoke etc. but nothing worked properly. I can collect data coming from GPIO but cannot update UI. What am I doing wrong?
Here is the background GPIO listener class with static variables (I'm listening GPIO in other pages too.):
public sealed class GPIO{
private static MainPage mainpage;
public static event EventHandler ProgressUpdate;
public static void InitGPIO(MainPage sender)
{
mainpage = sender;
DataPin.DebounceTimeout = TimeSpan.FromMilliseconds(50);
DataPin.ValueChanged += DataPin_ValueChanged;
}
public static void DataPin_ValueChanged(GpioPin sender, GpioPinValueChangedEventArgs e)
{
if (e.Edge == GpioPinEdge.FallingEdge)
{
Task.Run(() => AddData(0));
}
}
public static async void AddData(int prm_Data)
{
// WCF-Service Operation
await Service.wsClient.GPIOValueAddition(prm_Data);
GPIO.ProgressUpdateOperation();
}
private static void ProgressUpdateOperation()
{
mainpage.GPIO_ProgressUpdate(typeof(GPIO), new EventArgs());
}
}
And here is the page that contains the UI to be updated:
public sealed partial class MainPage : Page
{
public MainPage()
{
GPIO.InitGPIO(this);
GPIO.ProgressUpdate += GPIO_ProgressUpdate;
}
public void GPIO_ProgressUpdate(object sender, EventArgs e)
{
// WCF-Service Operation
service_data = (int)Service.wsClient.GetDataFromServicetoUpdateUI(parameter).Result;
// UI-update
txtUpdate.Text = service_data.ToString();
}
}
EDIT: I forgot to add the exception. "The application called an interface that was marshalled for a different thread. (Exception from HRESULT: 0x8001010E (RPC_E_WRONG_THREAD))" exception is thrown at AddData function called in DataPin_Valuechanged.

I found the solution in here : https://stackoverflow.com/a/27698035/1093584
Here is the new update-UI function :
public void GPIO_ProgressUpdate(object sender, EventArgs e)
{
await Windows.ApplicationModel.Core.CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(CoreDispatcherPriority.Normal, async () =>
{
service_data = await Service.wsClient.GetDataFromServicetoUpdateUI(parameter);
// UI-update
txtUpdate.Text = service_data.ToString();
});
}