I'm having a "invoke" issue that I can't resolve. I'll try to be as thorough in my description as possible, but I'm new to this so bear with me and let me know if you need more information.
I've got a background thread running that when prompted will disable a bunch of check boxes on a form created on the main thread. In order to do this I need to safely cross-thread using an invoke and a delegate but I must be doing it incorrectly. Bottom line, when I check this in the debugger i see that it runs through the ACTION portion of the code twice if InvokeRequired. I can get around this by bracketing ACTION with an else, and although it won't run through the else twice it does still try to go through the method again.
delegate void ManualCurtainShuttoffHandler();
public void ManualCurtainShutoff()
{
if (InvokeRequired)
{
Invoke(new ManualCurtainShuttoffHandler(ManualCurtainShutoff));
}
// ACTION: Disable check boxes
}
I'd just like to know why it runs through the method two times. Let me know if you need any more information and I'd be happy to share it with you.
Just because you call Invoke, it doesn't stop execution of the current method. A quick and simple solution is to simply return after calling Invoke:
delegate void ManualCurtainShuttoffHandler();
public void ManualCurtainShutoff()
{
if (InvokeRequired)
{
Invoke(new ManualCurtainShuttoffHandler(ManualCurtainShutoff));
return;
}
// ACTION: Disable check boxes
}
This will skip the rest of the execution of ManualCurtainShutoff that's running on the background thread while still promoting a new execution of the method on the main thread.
Invoke will cause your function to be called again in a different thread (that's its purpose). You should add a return after the call to Invoke. The idea is that then your function will be called again (that's what you want), and that time InvokeRequired will be false, so your action will take place.
edit: dang, by the time I finish writing I've been beaten to the punch. Oh well!
Related
I have a multi-threaded UI application that starts numerous background threads. A lot of these threads execute code that looks as follows:
public void Update(){
if(Dispatcher.HasShutdownStarted()) return;
Dispatcher.Invoke(()=>{...});
...
}
Then I sometimes may have a thread execute the following code
pubic void Shutdown(){
if(Dispatcher.HasShutdownStarted()) return;
Dispatcher.InvokeShutdown();
}
The problem is that sometimes one thread executes Dispatcher.InvokeShutdown() AFTER another thread executed Dispatcher.HasShutdwonStarted() but before it got to Dispatcher.Invoke(()=>{...}). Which means, that there will be a thread trying to execute a lambda on the Dispatcher once the Dispatcher has begun to shut down. And that's when I get exceptions. What is the best solution to this?
The problem you face is that the HasShutdownStarted is checked, before the code inside the Invoke is executed (because it's queued on the dispatcher)
I think a better way is to check it inside the invoke, this way you don't need any locks.
public void Update(){
Dispatcher.Invoke(()=>
{
if(Dispatcher.HasShutdownStarted()) return;
...
});
}
With the help of others I managed to come up with the following solution to my problem and thought I'd share it. Calling Dispatcher.Invoke(...) after Dispatcher.InvokeShutdown() will always lead to a TaskCancelationException being thrown (as far as I can tell). Thus, checking Dispatcher.HasShutdownStarted inside of the Invoke method will not work.
What I did was create an application global CancellationToken by creating a static CancellationTokenSource. I now invoke the Dispatcher as follows:
Dispatcher.Invoke(()=>{...}, DispatcherPriority.Send, GlobalMembers.CancellationTokenSource.Token);
Then, when I wish to invoke shutdown on my dispatcher, I do the following:
GlobalMembers.CancellationTokenSource.Cancel();
Dispatcher.InvokeShutdown();
If by any chance I try to run Dispatcher.Invoke(()=>{...}, DispatcherPriority.Send, GlobalMembers.CancellationTokenSource.Token) after cancelling the global token and after invoking Dispatcher.InvokeShutdown(), nothing happens as the token is already cancelled and thus the action is not run.
public void RefreshData()
{
// this is called on UI thread
List<ds> dataSource;
GetDsDelegate caller = GetDs;
caller.BeginInvoke(out dataSource, RefreshCallback, null);
}
private void RefreshCallback(IAsyncResult ar)
{
// this is called on worker thread
try
{
var result = (AsyncResult)ar;
var caller = (GetDsDelegate)result.AsyncDelegate;
List<ds> dataSource;
var success = caller.EndInvoke(out dataSource, ar);
if (success)
{
BeginInvoke(new Action<List<ds>>(SetGridDataSource), dataSource);
}
}
catch
{
// NOTE: It's possible for this form to close after RefreshData is called
// but before GetDs returns thus the SetGridDataSource method no longer exists.
// Not catching this error causes the entire application to terminate.
}
private void SetGridDataSource(List<ds> dataSource)
{
// this is called on UI thread
dataGrid.DataSource = dataSource;
}
RefreshData, RefreshCallback, and SetGridDataSource are all methods of a windows Form class. Calling RefreshData invokes the external method GetDs using the GetDsDelegate delegate. When GetDs completes, it calls RefreshCallback (now on a separate thread). Finally, SetGridDataSource is invoked to complete the update.
All this works fine unless GetDs is delayed and the form closes. Then when GetDs completes and calls RefreshCallback, SetGridDataSource no longer exists.
Is there a better way of handling this condition other then the shown try/catch block? I’d prefer to prevent the error rather than ignore it. Is there a better pattern to use?
EDIT
As I look that the error, it’s obvious to change if (success) to if (success && IsHandleCreated) to prevent it, but it still seems like I’m doing something wrong, or at least awkward. I could also replace the second BeginInvoke with just Invoke so EndInvoke is unnecessary. I like the idea of moving the logic away from the form, but I don't see how the result would change. I would think a BackgroundWorker would also have the same issue; that being the callback is no longer accessible. I suppose an event could be raised with the result, but that seems a little abstract. Could you please elaborate a little more or provide an example.
The method does exist. The fact the form was closed does not change the class or its methods. If you post the exact exception you're getting, it can help someone offer a more accurate help.
I'm guessing you got an ObjectDisposedException when you tried to do something on the form or one of its control after it has been closed.
If that's the case you just have to enhance your logic and check if the form has been closed or disposed before calling SetGridDataSource.
That said, there seem to be some issues with your design.
You're calling SetGridDataSource from the thread pool, but it probably should called from the UI thread.
Your'e not chaining the asynchronous calls correctly. And specifically, you're not calling EndInvoke on the second BeginInvoke.
A nice way that can help you get the threading right is to use something like the BackgroundWorker instead of chaining BeginInvoke calls.
Consider moving the business logic (including the threading and asynchronity logic) away from the UI layer and the form.
It's a very bad idea to do a catch without an exception type, and handle any thrown exception by ignoring it without rethrowing.
By the way, instead of doing this:
BeginInvoke(new Action<List<ds>>(SetGridDataSource), dataSource);
you can do the following, which I think is more readable:
BeginInvoke(SetGridDataSource, dataSource);
I am working on a project of my company in which they used Dispatcher.Invoke() in many places.If I am using BeginInvoke instead of Invoke then the Synchronisation between threads working fine but in case of Invoke the application is freezing and even not entering the execution to the delegate method also. Does anybody have any idea why it is happening like this?
Any answer will be appreciated.
Sample Code for Invoke used in the Project:
Dispatcher.Invoke(DispatcherPriority.Send,
new DelegateMethod(MethodtoExecute));
private delegate void DelegateMethod();
void MethodtoExecute()
{
try
{
}
catch (Exception /*ex*/)
{
}
finally
{
}
}
Dispatcher.Invoke executes synchronously on the same thread as your application, so whatever you invoke is able to block the main application thread. Dispatcher.BeginInvoke executes asynchronously, so it doesn't tie up the main application thread while executing.
Since you are using DispatcherPriority.Send, which is the highest dispatcher priority level, whatever you are invoking gets run before anything else, including rendering the screen or listening for events. I'd recommend switching that to DispatcherPriority.Background, which runs at a lower priority than Render and Input. See this page for a list of the DispatcherPriority levels and their execution order
I'd highly recommend you look at the answer posted here
I get the following exception thrown:
Invoke or BeginInvoke cannot be called on a control until the window handle has been created.
This is my code:
if (InvokeRequired)
{
BeginInvoke(new UpdateTextFieldDelegate(WriteToForm), finished, numCount);
}
else
Invoke(new UpdateTextFieldDelegate(WriteToForm), finished, numCount);
I found pages about this topic on this site but I don't know what is wrong.
The difference between Invoke and BeginInvoke is that the former is synchronous (waits for completion) while the later is asynchronous (sort of fire-and-forget). However, both work by posting a message to the UI message loop which will cause the delegate to be executed when it gets to that message.
The InvokeRequired property determines whether you need to Invoke at all or if it is already on the correct thread, not whether you want synchronous or asynchronous calling. If InvokeRequired is false you are (in theory) already running on the UI thread and can simply perform synchronous actions directly (or still BeginInvoke if you need to fire them off asynchronously). This also means you can't use Invoke if InvokeRequired is false, because there's no way for the message loop on the current thread to continue. So that's one big problem with your code above, but not necessarily the error you're reporting. You can actually use BeginInvoke in either case, if you watch out for recursive invocation, and so on.
However, you can't use either one without a window handle. If the Form/Control has been instantiated but not initialized (ie. before it is first shown) it may not have a handle yet. And the handle gets cleared by Dispose(), such as after the Form is closed. In either case InvokeRequired will return false because it is not possible to invoke without a handle. You can check IsDisposed, and there is also a property IsHandleCreated which more specifically tests if the handle exists. Usually, if IsDisposed is true (or if IsHandleCreated is false) you want to punt into a special case such as simply dropping the action as not applicable.
So, the code you want is probably more like:
if (IsHandleCreated)
{
// Always asynchronous, even on the UI thread already. (Don't let it loop back here!)
BeginInvoke(new UpdateTextFieldDelegate(WriteToForm), finished, numCount);
return; // Fired-off asynchronously; let the current thread continue.
// WriteToForm will be called on the UI thread at some point in the near future.
}
else
{
// Handle the error case, or do nothing.
}
Or maybe:
if (IsHandleCreated)
{
// Always synchronous. (But you must watch out for cross-threading deadlocks!)
if (InvokeRequired)
Invoke(new UpdateTextFieldDelegate(WriteToForm), finished, numCount);
else
WriteToForm(finished, numCount); // Call the method (or delegate) directly.
// Execution continues from here only once WriteToForm has completed and returned.
}
else
{
// Handle the error case, or do nothing.
}
This will typically happen in multithreaded scenarios where some external source (maybe a NetworkStream) pushes data to a form before the form has properly initialized.
The message can also appear after a Form is disposed.
You can check IsHandleCreated to see if a form is already created, but you need to put everything in proper error handling as the Invoke statement can throw an exception if you try to update your form while your application is closing.
here is my answer
Let's say you want to write"Hello World" to a text box.
Then IF you use "Ishandlecreated" then your operation will not happen if handlers are not yet created. So You must force itself to CreateHandlers if not yet created.
Here is my code
if (!IsHandleCreated)
this.CreateControl();
this.Invoke((MethodInvoker)delegate
{
cmbEmail.Text = null;
});
Assuming the form is not disposed but not yet fully initialized just put var X = this.Handle; before that if statement... by this the instance of the respective form is meant.
see http://msdn.microsoft.com/en-us/library/system.windows.forms.control.handle.aspx .
If you're going to use a control from another thread before showing the control or doing other things with the control, consider forcing the creation of its handle within the constructor. This is done using the CreateHandle function. In a multi-threaded project, where the "controller" logic isn't in a WinForm, this function is instrumental for avoiding these kinds of errors.
What about this :
public static bool SafeInvoke( this Control control, MethodInvoker method )
{
if( control != null && ! control.IsDisposed && control.IsHandleCreated && control.FindForm().IsHandleCreated )
{
if( control.InvokeRequired )
{
control.Invoke( method );
}
else
{
method();
}
return true;
}
return false;
}
Use it like that :
this.label.SafeInvoke(new MethodInvoker( () => { this.label.Text = yourText; }));
You are probably calling this in the constructor of the form, at that point the underlying system window handle does not exist yet.
Add this before you call your invoke method:
while (!this.IsHandleCreated)
System.Threading.Thread.Sleep(100)
This solution worked for me.
In WinForms, how do I force an immediate UI update from UI thread?
What I'm doing is roughly:
label.Text = "Please Wait..."
try
{
SomewhatLongRunningOperation();
}
catch(Exception e)
{
label.Text = "Error: " + e.Message;
return;
}
label.Text = "Success!";
Label text does not get set to "Please Wait..." before the operation.
I solved this using another thread for the operation, but it gets hairy and I'd like to simplify the code.
At first I wondered why the OP hadn't already marked one of the responses as the answer, but after trying it myself and still have it not work, I dug a little deeper and found there's much more to this issue then I'd first supposed.
A better understanding can be gained by reading from a similar question: Why won't control update/refresh mid-process
Lastly, for the record, I was able to get my label to update by doing the following:
private void SetStatus(string status)
{
lblStatus.Text = status;
lblStatus.Invalidate();
lblStatus.Update();
lblStatus.Refresh();
Application.DoEvents();
}
Though from what I understand this is far from an elegant and correct approach to doing it. It's a hack that may or may not work depending upon how busy the thread is.
Call Application.DoEvents() after setting the label, but you should do all the work in a separate thread instead, so the user may close the window.
Call label.Invalidate and then label.Update() - usually the update only happens after you exit the current function but calling Update forces it to update at that specific place in code.
From MSDN:
The Invalidate method governs what gets painted or repainted. The Update method governs when the painting or repainting occurs. If you use the Invalidate and Update methods together rather than calling Refresh, what gets repainted depends on which overload of Invalidate you use. The Update method just forces the control to be painted immediately, but the Invalidate method governs what gets painted when you call the Update method.
If you only need to update a couple controls, .update() is sufficient.
btnMyButton.BackColor=Color.Green; // it eventually turned green, after a delay
btnMyButton.Update(); // after I added this, it turned green quickly
I've just stumbled over the same problem and found some interesting information and I wanted to put in my two cents and add it here.
First of all, as others have already mentioned, long-running operations should be done by a thread, which can be a background worker, an explicit thread, a thread from the threadpool or (since .Net 4.0) a task: Stackoverflow 570537: update-label-while-processing-in-windows-forms, so that the UI keeps responsive.
But for short tasks there is no real need for threading although it doesn't hurt of course.
I have created a winform with one button and one label to analyze this problem:
System::Void button1_Click(System::Object^ sender, System::EventArgs^ e)
{
label1->Text = "Start 1";
label1->Update();
System::Threading::Thread::Sleep(5000); // do other work
}
My analysis was stepping over the code (using F10) and seeing what happened. And after reading this article Multithreading in WinForms I have found something interesting. The article says at the bottom of the first page, that the UI thread can not repaint the UI until the currently executed function finishes and the window is marked by Windows as "not responding" instead after a while. I have also noticed that on my test application from above while stepping through it, but only in certain cases.
(For the following test it is important to not have Visual Studio set to fullscreen, you must be able to see your little application window at the same time next to it, You must not have to switch between the Visual Studio window for debugging and your application window to see what happens. Start the application, set a breakpoint at label1->Text ..., put the application window beside the VS window and place the mouse cursor over the VS window.)
When I click once on VS after app start (to put the focues there and enable stepping) and step through it WITHOUT moving the mouse, the new text is set and the label is updated in the update() function. This means, the UI is repainted obviously.
When I step over the first line, then move the mouse around a lot and click somewhere, then step further, the new text is likely set and the update() function is called, but the UI is not updated/repainted and the old text remains there until the button1_click() function finishes. Instead of repainting, the window is marked as "not responsive"! It also doesn't help to add this->Update(); to update the whole form.
Adding Application::DoEvents(); gives the UI a chance to update/repaint. Anyway you have to take care that the user can not press buttons or perform other operations on the UI that are not permitted!! Therefore: Try to avoid DoEvents()!, better use threading (which I think is quite simple in .Net).
But (#Jagd, Apr 2 '10 at 19:25) you can omit .refresh() and .invalidate().
My explanations is as following: AFAIK winform still uses the WINAPI function. Also MSDN article about System.Windows.Forms Control.Update method refers to WINAPI function WM_PAINT. The MSDN article about WM_PAINT states in its first sentence that the WM_PAINT command is only sent by the system when the message queue is empty. But as the message queue is already filled in the 2nd case, it is not send and thus the label and the application form are not repainted.
<>joke> Conclusion: so you just have to keep the user from using the mouse ;-) <>/joke>
you can try this
using System.Windows.Forms; // u need this to include.
MethodInvoker updateIt = delegate
{
this.label1.Text = "Started...";
};
this.label1.BeginInvoke(updateIt);
See if it works.
After updating the UI, start a task to perform with the long running operation:
label.Text = "Please Wait...";
Task<string> task = Task<string>.Factory.StartNew(() =>
{
try
{
SomewhatLongRunningOperation();
return "Success!";
}
catch (Exception e)
{
return "Error: " + e.Message;
}
});
Task UITask = task.ContinueWith((ret) =>
{
label.Text = ret.Result;
}, TaskScheduler.FromCurrentSynchronizationContext());
This works in .NET 3.5 and later.
It's very tempting to want to "fix" this and force a UI update, but the best fix is to do this on a background thread and not tie up the UI thread, so that it can still respond to events.
Try calling label.Invalidate()
http://msdn.microsoft.com/en-us/library/system.windows.forms.control.invalidate(VS.80).aspx
Think I have the answer, distilled from the above and a little experimentation.
progressBar.Value = progressBar.Maximum - 1;
progressBar.Maximum = progressBar.Value;
I tried decrementing the value and the screen updated even in debug mode, but that would not work for setting progressBar.Value to progressBar.Maximum, because you cannot set the progress bar value above the maximum, so I first set the progressBar.Value to progressBar.Maximum -1, then set progressBar.Maxiumum to equal progressBar.Value. They say there is more than one way of killing a cat. Sometimes I'd like to kill Bill Gates or whoever it is now :o).
With this result, I did not even appear to need to Invalidate(), Refresh(), Update(), or do anything to the progress bar or its Panel container or the parent Form.
myControlName.Refresh() is a simple solution to update a control before moving on to a "SomewhatLongRunningOperation".
From: https://learn.microsoft.com/en-us/dotnet/api/system.windows.forms.control.update?view=windowsdesktop-6.0
There are two ways to repaint a form and its contents:
You can use one of the overloads of the Invalidate method with the Update method.
You can call the Refresh method, which forces the control to redraw itself and all its children. This is equivalent to setting the Invalidate method to true and using it with Update.
The Invalidate method governs what gets painted or repainted. The Update method governs when the painting or repainting occurs. If you use the Invalidate and Update methods together rather than calling Refresh, what gets repainted depends on which overload of Invalidate you use. The Update method just forces the control to be painted immediately, but the Invalidate method governs what gets painted when you call the Update method.
I had the same problem with property Enabled and I discovered a first chance exception raised because of it is not thread-safe.
I found solution about "How to update the GUI from another thread in C#?" here https://stackoverflow.com/a/661706/1529139 And it works !
When I want to update the UI in "real-time" (or based on updates to data or long running operations) I use a helper function to "simplify" the code a bit (here it may seem complex, but it scales upward very nicely). Below is an example of code I use to update my UI:
// a utility class that provides helper functions
// related to Windows Forms and related elements
public static class FormsHelperFunctions {
// This method takes a control and an action
// The action can simply be a method name, some form of delegate, or it could be a lambda function (see: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/lambda-expressions)
public static void InvokeIfNeeded(this Control control, Action action)
{
// control.InvokeRequired checks to see if the current thread is the UI thread,
// if the current thread is not the UI thread it returns True - as in Invoke IS required
if(control.InvokeRequired)
{
// we then ask the control to Invoke the action in the UI thread
control.Invoke(action);
}
// Otherwise, we don't need to Invoke
else
{
// so we can just call the action by adding the parenthesis and semicolon, just like how a method would be called.
action();
}
}
}
// An example user control
public class ExampleUserControl : UserControl {
/*
//
//*****
// declarations of label and other class variables, etc.
//*****
//
...
*/
// This method updates a label,
// executes a long-running operation,
// and finally updates the label with the resulting message.
public void ExampleUpdateLabel() {
// Update our label with the initial text
UpdateLabelText("Please Wait...");
// result will be what the label gets set to at the end of this method
// we set it to Success here to initialize it, knowing that we will only need to change it if an exception actually occurs.
string result = "Success";
try {
// run the long operation
SomewhatLongRunningOperation();
}
catch(Exception e)
{
// if an exception was caught, we want to update result accordingly
result = "Error: " + e.Message;
}
// Update our label with the result text
UpdateLabelText(result);
}
// This method takes a string and sets our label's text to that value
// (This could also be turned into a method that updates multiple labels based on variables, rather than one input string affecting one label)
private void UpdateLabelText(string value) {
// call our helper funtion on the current control
// here we use a lambda function (an anonymous method) to create an Action to pass into our method
// * The lambda function is like a Method that has no name, here our's just updates the label, but it could do anything else we needed
this.InvokeIfNeeded(() => {
// set the text of our label to the value
// (this is where we could set multiple other UI elements (labels, button text, etc) at the same time if we wanted to)
label.Text = value;
});
}
}