I have something doing background and I want to show a messagebox if something wrong happens.
First I tried
var _timer = new System.Threading.Timer((o) =>
{
if(!DoCheck()){
Messagebox.Show("The message");
}
});
Nothing wrong happens.
And I have another job to be done in background, and it's invoked by button click, like
private void button3_Click(object sender, EventArgs e)
{
var task = new Task(() =>
{
DoWork();
Messagebox.Show("Done");
});
_task.Start();
}
A System.Reflection.TargetInvocationException is thrown when the MessageBox is shown.
I have also tried this.Invoke, it raised an exception, too.
My question is:
Is the first case safe?
How to make the second case work?
No. You should preferably be using System.Windows.Forms.Timer in a WinForms application. The documentation specifically calls this out:
This Windows timer is designed for a single-threaded environment where UI threads are used to perform processing. It requires that the user code have a UI message pump available and always operate from the same thread, or marshal the call onto another thread.
Furthermore, it depends on what your DoCheck method is doing. We will need to see the code of that method.
Use the BeginInvoke method:
var form = this;
var task = new Task(() =>
{
DoWork();
form.BeginInvoke(() =>
{
MessageBox.Show("Done");
});
});
I'm working on a WPF (MVVM) app.
Using 2 buttons, one to load data from db and another one to delete a selected item.
When clicking the Load button, a LoadCommand is fired and calls the StartLoadingThread
private void StartLoadingThread()
{
ShowLoadProcessing(); // show some text on top of screen ("Loading in progress...")
ThreadStart ts = delegate
{
LoadMyitems();
System.Windows.Application.Current.Dispatcher.BeginInvoke(DispatcherPriority.Normal, (EventHandler)
delegate
{
HideLoadProcessing(); // hide the text "Loading in progress..."
}, null, null);
};
ts.BeginInvoke(ts.EndInvoke, null);
}
Works fine, now when I select an item and click the Delete button, the DeleteItemCommand is fired and calls the StartDeletingThread
private void StartDeletingThread()
{
ShowDeleteProcessing(); // Show on top of screen "Deleting in progress..."
ThreadStart ts = delegate
{
DeleteSelectedItem();
System.Windows.Application.Current.Dispatcher.BeginInvoke(DispatcherPriority.Normal, (EventHandler)
delegate
{
HideDeletingProcessing();
}, null, null);
};
ts.BeginInvoke(ts.EndInvoke, null);
}
When StartDeletingThread is started, I'm getting the following exception:
{"The calling thread must be STA, because many UI components require this."}
I think you want something like this, though I am a bit newbie in WPF, you will have to replace this.BeginInvoke with something what is doing same in WPF (Dispatcher). Also code may not compile (add Action type conversion for Thread?), but idea is to simply start thread (you invoke it, for some reasons, why?) and in that thread invoke UI operation after deleting.
private void StartDeletingThread()
{
ShowDeleteProcessing(); // Show on top of screen "Deleting in progress..."
new Thread(() =>
{
DeleteSelectedItem();
this.BeginInvoke(() => HideDeletingProcessing());
}.Start();
}
Try invoking StartDeletingThread using the application dispatcher like this:
Application.Current.Dispatcher.BeginInvoke(DispatcherPriority.Normal,new
Action(()=>StartDeletingThread())
private void StartDeletingThread()
{
ShowDeleteProcessing(); // Show on top of screen "Deleting in progress..."
Task.Run(() =>
{
DeleteSelectedItem();
Application.Current.Dispatcher.BeginInvoke((Action)HideDeletingProcessing);
});
}
probably will blow up at DeleteSelectedItem();. I know we all love to show progress bars for actions that take less than a second, but why bother when it leads to questions on StackOverflow?
private void StartDeletingThread()
{
DeleteSelectedItem();
}
Done and done. I seriously doubt it takes a long time to delete the selected item.
If, in some rare case it does... then you need to find out in DeleteSelectedItem where the UI is getting touched, and use the application's dispatcher to do the touching.
(Side note, here's how you'd safely multithread in 4.5, using async/await... safe, as long as you understand the repercussions of async void, that is)
private async void StartDeletingThread()
{
// we're in the UI thread
ShowDeleteProcessing();
await DeleteSelectedItem();
// back in the UI thread
HideDeletingProcessing();
}
private Task DeleteSelectedItem()
{
// doing the work on a Task thread
return Task.Run(() => DeleteSelectedItem = null);
}
So I currently have this code below, which has a background worker call showdialog(). However, I thought that the UI cannot be updated on a background thread, so how does the dialog display? Does the dialog actually get opened on the UI thread? what happens?
public partial class ProgressDialog : Window
{
BackgroundWorker _worker;
public BackgroundWorker Worker
{
get { return _worker; }
}
public void RunWorkerThread(object argument, Func<object> workHandler)
{
//store reference to callback handler and launch worker thread
workerCallback = workHandler;
_worker.RunWorkerAsync(argument);
//display modal dialog (blocks caller)
//never returns null, but is a nullable boolean to match the dialogresult property
ShowDialog();
}
I have gotten suggestions that I just run the code and check, but how do i check whether the show dialog window was opened on a new thread or on the background thread itself? Not sure how I would check that.
Anyway this was just a post to try to help my understanding of what is actually happening in my code.
Anyway finally understood more of the comments, so I think I understand everything that is going on. Most of my real problems weren't caused by this dialog anyway, they were caused by updating observable collections from a non-ui thread while controls were bound to them.
Technically you are not changing a property on your Main thread just creating a instance of another object.
But it could help if you elaborate a bit more on your method ShowDialog().
I had also problem with calling ShowDialog() from non-UI thread. And my answer is that it depends on the thread which calls the ShowDialog(). If you set the ApartamentState property for this thread before its start then everything will work as called from the UI thread. I have finally ended up with such a code:
private async void button1_Click(object sender, EventArgs e)
{
var foo = new Foo();
// MessageBox.Show(foo.DirPath) - this works as a charm but
// if, it is called from non UI thread needs special handling as below.
await Task.Run(() => MessageBox.Show(foo.DirPath));
}
public class Foo
{
private string dirPath;
public string DirPath
{
get
{
if (dirPath == null)
{
var t = new Thread(() =>
{
using (var dirDialog = new FolderBrowserDialog())
{
if (dirDialog.ShowDialog() == DialogResult.OK)
dirPath = dirDialog.SelectedPath;
}
}
);
t.IsBackground = true;
t.SetApartmentState(ApartmentState.STA);
t.Start();
t.Join();
}
return dirPath;
}
set
{
dirPath = value;
}
}
}
I dont know for sure but i thought that the showDialog doesnt create the object only showing it. So when u say ShowDialog it only tells to show. So it will run on the UI thread instead of the backgroundworker
(dont know for sure)
I am currently writing my first program on C# and I am extremely new to the language (used to only work with C so far). I have done a lot of research, but all answers were too general and I simply couldn't get it t work.
So here my (very common) problem:
I have a WPF application which takes inputs from a few textboxes filled by the user and then uses that to do a lot of calculations with them. They should take around 2-3 minutes, so I would like to update a progress bar and a textblock telling me what the current status is.
Also I need to store the UI inputs from the user and give them to the thread, so I have a third class, which I use to create an object and would like to pass this object to the background thread.
Obviously I would run the calculations in another thread, so the UI doesn't freeze, but I don't know how to update the UI, since all the calculation methods are part of another class.
After a lot of reasearch I think the best method to go with would be using dispatchers and TPL and not a backgroundworker, but honestly I am not sure how they work and after around 20 hours of trial and error with other answers, I decided to ask a question myself.
Here a very simple structure of my program:
public partial class MainWindow : Window
{
public MainWindow()
{
Initialize Component();
}
private void startCalc(object sender, RoutedEventArgs e)
{
inputValues input = new inputValues();
calcClass calculations = new calcClass();
try
{
input.pota = Convert.ToDouble(aVar.Text);
input.potb = Convert.ToDouble(bVar.Text);
input.potc = Convert.ToDouble(cVar.Text);
input.potd = Convert.ToDouble(dVar.Text);
input.potf = Convert.ToDouble(fVar.Text);
input.potA = Convert.ToDouble(AVar.Text);
input.potB = Convert.ToDouble(BVar.Text);
input.initStart = Convert.ToDouble(initStart.Text);
input.initEnd = Convert.ToDouble(initEnd.Text);
input.inita = Convert.ToDouble(inita.Text);
input.initb = Convert.ToDouble(initb.Text);
input.initc = Convert.ToDouble(initb.Text);
}
catch
{
MessageBox.Show("Some input values are not of the expected Type.", "Wrong Input", MessageBoxButton.OK, MessageBoxImage.Error);
}
Thread calcthread = new Thread(new ParameterizedThreadStart(calculations.testMethod);
calcthread.Start(input);
}
public class inputValues
{
public double pota, potb, potc, potd, potf, potA, potB;
public double initStart, initEnd, inita, initb, initc;
}
public class calcClass
{
public void testmethod(inputValues input)
{
Thread.CurrentThread.Priority = ThreadPriority.Lowest;
int i;
//the input object will be used somehow, but that doesn't matter for my problem
for (i = 0; i < 1000; i++)
{
Thread.Sleep(10);
}
}
}
I would be very grateful if someone had a simple explanation how to update the UI from inside the testmethod. Since I am new to C# and object oriented programming, too complicated answers I will very likely not understand, I'll do my best though.
Also if someone has a better idea in general (maybe using backgroundworker or anything else) I am open to see it.
First you need to use Dispatcher.Invoke to change the UI from another thread and to do that from another class, you can use events.
Then you can register to that event(s) in the main class and Dispatch the changes to the UI and in the calculation class you throw the event when you want to notify the UI:
class MainWindow : Window
{
private void startCalc()
{
//your code
CalcClass calc = new CalcClass();
calc.ProgressUpdate += (s, e) => {
Dispatcher.Invoke((Action)delegate() { /* update UI */ });
};
Thread calcthread = new Thread(new ParameterizedThreadStart(calc.testMethod));
calcthread.Start(input);
}
}
class CalcClass
{
public event EventHandler ProgressUpdate;
public void testMethod(object input)
{
//part 1
if(ProgressUpdate != null)
ProgressUpdate(this, new YourEventArgs(status));
//part 2
}
}
UPDATE:
As it seems this is still an often visited question and answer I want to update this answer with how I would do it now (with .NET 4.5) - this is a little longer as I will show some different possibilities:
class MainWindow : Window
{
Task calcTask = null;
void buttonStartCalc_Clicked(object sender, EventArgs e) { StartCalc(); } // #1
async void buttonDoCalc_Clicked(object sender, EventArgs e) // #2
{
await CalcAsync(); // #2
}
void StartCalc()
{
var calc = PrepareCalc();
calcTask = Task.Run(() => calc.TestMethod(input)); // #3
}
Task CalcAsync()
{
var calc = PrepareCalc();
return Task.Run(() => calc.TestMethod(input)); // #4
}
CalcClass PrepareCalc()
{
//your code
var calc = new CalcClass();
calc.ProgressUpdate += (s, e) => Dispatcher.Invoke((Action)delegate()
{
// update UI
});
return calc;
}
}
class CalcClass
{
public event EventHandler<EventArgs<YourStatus>> ProgressUpdate; // #5
public TestMethod(InputValues input)
{
//part 1
ProgressUpdate.Raise(this, status); // #6 - status is of type YourStatus
// alternative version to the extension for C# 6+:
ProgressUpdate?.Invoke(this, new EventArgs<YourStatus>(status));
//part 2
}
}
static class EventExtensions
{
public static void Raise<T>(this EventHandler<EventArgs<T>> theEvent,
object sender, T args)
{
if (theEvent != null)
theEvent(sender, new EventArgs<T>(args));
}
}
#1) How to start the "synchronous" calculations and run them in the background
#2) How to start it "asynchronous" and "await it": Here the calculation is executed and completed before the method returns, but because of the async/await the UI is not blocked (BTW: such event handlers are the only valid usages of async void as the event handler must return void - use async Task in all other cases)
#3) Instead of a new Thread we now use a Task. To later be able to check its (successfull) completion we save it in the global calcTask member. In the background this also starts a new thread and runs the action there, but it is much easier to handle and has some other benefits.
#4) Here we also start the action, but this time we return the task, so the "async event handler" can "await it". We could also create async Task CalcAsync() and then await Task.Run(() => calc.TestMethod(input)).ConfigureAwait(false); (FYI: the ConfigureAwait(false) is to avoid deadlocks, you should read up on this if you use async/await as it would be to much to explain here) which would result in the same workflow, but as the Task.Run is the only "awaitable operation" and is the last one we can simply return the task and save one context switch, which saves some execution time.
#5) Here I now use a "strongly typed generic event" so we can pass and receive our "status object" easily
#6) Here I use the extension defined below, which (aside from ease of use) solve the possible race condition in the old example. There it could have happened that the event got null after the if-check, but before the call if the event handler was removed in another thread at just that moment. This can't happen here, as the extensions gets a "copy" of the event delegate and in the same situation the handler is still registered inside the Raise method.
I am going to throw you a curve ball here. If I have said it once I have said it a hundred times. Marshaling operations like Invoke or BeginInvoke are not always the best methods for updating the UI with worker thread progress.
In this case it usually works better to have the worker thread publish its progress information to a shared data structure that the UI thread then polls at regular intervals. This has several advantages.
It breaks the tight coupling between the UI and worker thread that Invoke imposes.
The UI thread gets to dictate when the UI controls get updated...the way it should be anyway when you really think about it.
There is no risk of overrunning the UI message queue as would be the case if BeginInvoke were used from the worker thread.
The worker thread does not have to wait for a response from the UI thread as would be the case with Invoke.
You get more throughput on both the UI and worker threads.
Invoke and BeginInvoke are expensive operations.
So in your calcClass create a data structure that will hold the progress information.
public class calcClass
{
private double percentComplete = 0;
public double PercentComplete
{
get
{
// Do a thread-safe read here.
return Interlocked.CompareExchange(ref percentComplete, 0, 0);
}
}
public testMethod(object input)
{
int count = 1000;
for (int i = 0; i < count; i++)
{
Thread.Sleep(10);
double newvalue = ((double)i + 1) / (double)count;
Interlocked.Exchange(ref percentComplete, newvalue);
}
}
}
Then in your MainWindow class use a DispatcherTimer to periodically poll the progress information. Configure the DispatcherTimer to raise the Tick event on whatever interval is most appropriate for your situation.
public partial class MainWindow : Window
{
public void YourDispatcherTimer_Tick(object sender, EventArgs args)
{
YourProgressBar.Value = calculation.PercentComplete;
}
}
You're right that you should use the Dispatcher to update controls on the UI thread, and also right that long-running processes should not run on the UI thread. Even if you run the long-running process asynchronously on the UI thread, it can still cause performance issues.
It should be noted that Dispatcher.CurrentDispatcher will return the dispatcher for the current thread, not necessarily the UI thread. I think you can use Application.Current.Dispatcher to get a reference to the UI thread's dispatcher if that's available to you, but if not you'll have to pass the UI dispatcher in to your background thread.
Typically I use the Task Parallel Library for threading operations instead of a BackgroundWorker. I just find it easier to use.
For example,
Task.Factory.StartNew(() =>
SomeObject.RunLongProcess(someDataObject));
where
void RunLongProcess(SomeViewModel someDataObject)
{
for (int i = 0; i <= 1000; i++)
{
Thread.Sleep(10);
// Update every 10 executions
if (i % 10 == 0)
{
// Send message to UI thread
Application.Current.Dispatcher.BeginInvoke(
DispatcherPriority.Normal,
(Action)(() => someDataObject.ProgressValue = (i / 1000)));
}
}
}
Everything that interacts with the UI must be called in the UI thread (unless it is a frozen object). To do that, you can use the dispatcher.
var disp = /* Get the UI dispatcher, each WPF object has a dispatcher which you can query*/
disp.BeginInvoke(DispatcherPriority.Normal,
(Action)(() => /*Do your UI Stuff here*/));
I use BeginInvoke here, usually a backgroundworker doesn't need to wait that the UI updates. If you want to wait, you can use Invoke. But you should be careful not to call BeginInvoke to fast to often, this can get really nasty.
By the way, The BackgroundWorker class helps with this kind of taks. It allows Reporting changes, like a percentage and dispatches this automatically from the Background thread into the ui thread. For the most thread <> update ui tasks the BackgroundWorker is a great tool.
If this is a long calculation then I would go background worker. It has progress support. It also has support for cancel.
http://msdn.microsoft.com/en-us/library/cc221403(v=VS.95).aspx
Here I have a TextBox bound to contents.
private void backgroundWorker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
Debug.Write("backgroundWorker_RunWorkerCompleted");
if (e.Cancelled)
{
contents = "Cancelled get contents.";
NotifyPropertyChanged("Contents");
}
else if (e.Error != null)
{
contents = "An Error Occured in get contents";
NotifyPropertyChanged("Contents");
}
else
{
contents = (string)e.Result;
if (contentTabSelectd) NotifyPropertyChanged("Contents");
}
}
You are going to have to come back to your main thread (also called UI thread) in order to update the UI.
Any other thread trying to update your UI will just cause exceptions to be thrown all over the place.
So because you are in WPF, you can use the Dispatcher and more specifically a beginInvoke on this dispatcher. This will allow you to execute what needs done (typically Update the UI) in the UI thread.
You migh also want to "register" the UI in your business, by maintaining a reference to a control/form, so you can use its dispatcher.
Thank God, Microsoft got that figured out in WPF :)
Every Control, like a progress bar, button, form, etc. has a Dispatcher on it. You can give the Dispatcher an Action that needs to be performed, and it will automatically call it on the correct thread (an Action is like a function delegate).
You can find an example here.
Of course, you'll have to have the control accessible from other classes, e.g. by making it public and handing a reference to the Window to your other class, or maybe by passing a reference only to the progress bar.
Felt the need to add this better answer, as nothing except BackgroundWorker seemed to help me, and the answer dealing with that thus far was woefully incomplete. This is how you would update a XAML page called MainWindow that has an Image tag like this:
<Image Name="imgNtwkInd" Source="Images/network_on.jpg" Width="50" />
with a BackgroundWorker process to show if you are connected to the network or not:
using System.ComponentModel;
using System.Windows;
using System.Windows.Controls;
public partial class MainWindow : Window
{
private BackgroundWorker bw = new BackgroundWorker();
public MainWindow()
{
InitializeComponent();
// Set up background worker to allow progress reporting and cancellation
bw.WorkerReportsProgress = true;
bw.WorkerSupportsCancellation = true;
// This is your main work process that records progress
bw.DoWork += new DoWorkEventHandler(SomeClass.DoWork);
// This will update your page based on that progress
bw.ProgressChanged += new ProgressChangedEventHandler(bw_ProgressChanged);
// This starts your background worker and "DoWork()"
bw.RunWorkerAsync();
// When this page closes, this will run and cancel your background worker
this.Closing += new CancelEventHandler(Page_Unload);
}
private void bw_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
BitmapImage bImg = new BitmapImage();
bool connected = false;
string response = e.ProgressPercentage.ToString(); // will either be 1 or 0 for true/false -- this is the result recorded in DoWork()
if (response == "1")
connected = true;
// Do something with the result we got
if (!connected)
{
bImg.BeginInit();
bImg.UriSource = new Uri("Images/network_off.jpg", UriKind.Relative);
bImg.EndInit();
imgNtwkInd.Source = bImg;
}
else
{
bImg.BeginInit();
bImg.UriSource = new Uri("Images/network_on.jpg", UriKind.Relative);
bImg.EndInit();
imgNtwkInd.Source = bImg;
}
}
private void Page_Unload(object sender, CancelEventArgs e)
{
bw.CancelAsync(); // stops the background worker when unloading the page
}
}
public class SomeClass
{
public static bool connected = false;
public void DoWork(object sender, DoWorkEventArgs e)
{
BackgroundWorker bw = sender as BackgroundWorker;
int i = 0;
do
{
connected = CheckConn(); // do some task and get the result
if (bw.CancellationPending == true)
{
e.Cancel = true;
break;
}
else
{
Thread.Sleep(1000);
// Record your result here
if (connected)
bw.ReportProgress(1);
else
bw.ReportProgress(0);
}
}
while (i == 0);
}
private static bool CheckConn()
{
bool conn = false;
Ping png = new Ping();
string host = "SomeComputerNameHere";
try
{
PingReply pngReply = png.Send(host);
if (pngReply.Status == IPStatus.Success)
conn = true;
}
catch (PingException ex)
{
// write exception to log
}
return conn;
}
}
For more information: https://msdn.microsoft.com/en-us/library/cc221403(v=VS.95).aspx
I'm having trouble with the concept of threads and how to use them.
I'm trying to code a fairly basic chat program (as part of a larger program) and it currently works like this:
The 'NetworkSession' class receives the input from the server on a separate thread in a loop. If it receives input that indicates it should open a new chat window it constructs a new WPF class (ChatWindow) and displays it.
Originally I got the error that "The calling thread must be STA, because many UI components require this.". So i set the thread to be STA but now of course the WPF form is unusable because its running on the same thread as the blocking loop.
So my question is how do I create a new instance of a WPF form from within another thread.
I've seen alot of discussion about this but it tends to deal with running a delegate from a form that has already been constructed.
Here is some code.
while (Connected) //this loop is running on its own thread
{
Resp = srReceiver.ReadLine();
if (Resp.StartsWith("PING")) SendToServer("PONG");
if (Resp.StartsWith("CHAT FROM"))
{
String[] split = Resp.Split(' ');
Console.WriteLine("Incoming Chat from {0}", split[2]);
bool found = false;
if (Chats.Count != 0)
{
foreach (ChatWindow cw in Chats)
{
if (cw.User == split[2])
{
found = true;
cw.AddLine(cw.User, split[3]); // a function that adds a line to the current chat
}
}
}
if (!found)
{
ChatWindow temp = new ChatWindow(split[2], split[3]);
Chats.Add(temp); //this is a collection with T = ChatWindow
temp.Show();
}
}
}
If you're constructing NetworkSession from your UI Thread, you can snag a reference to the current Dispatcher that can manipulate the UI later.
NetworkSession.cs
private Dispatcher _dispatcher;
public NetworkSession()
{
_dispatcher = Dispatcher.CurrentDispatcher;
}
//any thread can call this method
public void DoStuff()
{
Action action = () =>
{
ChatWindow temp = new ChatWindow(split[2], split[3]);
Chats.Add(temp);
temp.Show();
};
_dispatcher.BeginInvoke(action);
}
The code below, which I took from here worked for me:
public static void StartChatWindow()
{
Thread thread = new Thread(() =>
{
ChatWindow chatWindow = new ChatWindow();
chatWindow.Chat(); // Do your stuff here, may pass some parameters
chatWindow.Closed += (sender2, e2) =>
// Close the message pump when the window closed
chatWindow.Dispatcher.InvokeShutdown();
// Run the message pump
System.Windows.Threading.Dispatcher.Run();
});
thread.SetApartmentState(ApartmentState.STA);
thread.Start();
}
What you really need to do is construct the window/form on your main UI thread. You probably need to define a delegate that you can call from your network thread and that delegate should have a method attached that will call this.Dispatcher.BeginInvoke() -> inside which you will construct your window.
The this.Dispatcher.BeginInvoke() call is necessary to execute code on the UI thread, otherwise even with a delegate, code would be executed on the network thread.
Both the delegate and the method for creating a new chat window should probably be attached to the MainWindow...