The MSDN article: How to: Make Thread-Safe Calls to Windows Forms Controls
says we should use async delegate to make the call. But why does the async delegate make the call safe?
Windows controls use the Component Object Model (COM) single-threaded apartment (STA) model because those underlying controls are apartment-threaded. Furthermore, many of the controls use the message pump for many operations. This model says that all function calls to each control must be on the same thread that created the control. Invoke (and BeginInvoke and EndInvoke) marshals method calls to the proper thread.
From Bill Wagner's More Effective C#. Item 16. Understand Cross-Thread Calls in Windows Forms and WPF
You'll call control.BeginInvoke() or control.Invoke() and that method will take care of inserting your delegate in the GUI dispatch thread safely, so a bit later on your delegate will be processed and executed in the GUI thread and not the thread you'r in
The bottom line is this: You shouldn't update the UI Controls from a thread other than the one on which the control was created (UI / Main Thread). Otherwise you may see some unpredictable behavior.
If you need to update the UI from a worker thread (other than the main thread) you need to switch back to the UI Thread before updating the UI.
The article suggests using
IsInvokeRequired (which returns true if the current thread is not the one in which UI was created.) and Invoke(delegate) which runs the delegate on the correct/UI thread. This is useful when you want to update the UI in between the async process. e.g. update progress on the UI.
BackgroundWorker which executes registered handlers to its DoWork event async. on a worker thread and runs the registered handlers to its RunWorkerCompleted event on the calling thread. This is ideal if you want to update the UI with something after the async task has completed. e.g. post a Done indication on the UI
Because, windows forms controls are designed in that way, so they can be accessed only from within he thread, which owes them. And the async delegate, when used correctly, makes the call safe.
The actual answer to the given question is contained in the second paragraph of the given MSDN article =)
Access to Windows Forms controls is not inherently thread safe. If you have two or more threads manipulating the state of a control, it is possible to force the control into an inconsistent state. Other thread-related bugs are possible, such as race conditions and deadlocks. It is important to make sure that access to your controls is performed in a thread-safe way.
You should check whether you can access control immediately, without indirection ( checking the InvokeRequired property ), if you can't, you should access it asynchronously ( very simplified explanation: the system will wait until it can safely access a control )
Related
I'm building a WPF application. I'm doing some async communication with the server side, and I use event aggregation with Prism on the client. Both these things results in new threads to be spawned which are not the UI thread. If I attempt to do "WPF operations" on these callback and event handler threads the world will fall apart, which it now has started doing.
First I met problems trying to create some WPF objects in the callback from server. I was told that the thread needed to run in STA mode. Now I'm trying to update some UI data in a Prism event handler, and I'm told that:
The caller cannot access this thread because a different thread owns it.
So; what's the key to getting things right in WPF? I've read up on the WPF Dispatcher in this MSDN post. I'm starting to get it, but I'm no wizard yet.
Is the key to always use Dispatcher.Invoke when I need to run something which I'm not sure will be called on the UI thread?
Does it matter if it actually was called on the UI thread, and I do Dispatcher.Invoke anyway?
Dispatcher.Invoke = synchronously. Dispathcher.BeginInvoke = async?
Will Dispatcher.Invoke request the UI thread, and then stop to wait for it? Is it bad practice and risk of less responsive programs?
How do I get the dispatcher anyway? Will Dispatcher.CurrentDispatcher always give me the dispatcher representing the UI thread?
Will there exist more than one Dispatcher, or is "Dispatcher" basically the same as the UI thread for the application?
And what's the deal with the BackgroundWorker? When do I use this instead? I assume this is always async?
Will everything that runs on the UI thread (by being Invoked) be run in STA apartment mode? I.e. if I have something that requires to be run in STA mode - will Dispatcher.Invoke be sufficient?
Anyone wanna clearify things for me? Any related recommendations, etc? Thanks!
Going over each of your questions, one by one:
Not quite; you should only invoke onto the UI thread when necessary. See #2.
Yes, it does matter. You should not just automatically Invoke everything. The key is to only invoke onto the UI thread if necessary. To do this, you can use the Dispatcher.CheckAccess method.
That is correct.
Also correct, and yes, you do run the risk of less responsive programs. Most of the time, you are not going to be looking at a severe performance hit (we're talking about milliseconds for a context switch), but you should only Invoke if necessary. That being said, at some points it is unavoidable, so no, I would not say it is bad practice at all. It is just one solution to a problem that you will encounter every now and then.
In every case I have seen, I have made due with Dispatcher.CurrentDispatcher. For complex scenarios, this may not be sufficient, but I (personally) have not seen them.
Not entirely correct, but this line of thinking will not do any harm. Let me put it this way: the Dispatcher can be used to gain access to the UI thread for the application. But it is not in and of itself the UI thread.
BackgroundWorker is generally used when you have a time-consuming operation and want to maintain a responsive UI while running that operation in the background. Normally you do not use BackgroundWorker instead of Invoke, rather, you use BackgroundWorker in conjunction with Invoke. That is, if you need to update some UI object in your BackgroundWorker, you can Invoke onto the UI thread, perform the update, and then return to the original operation.
Yes. The UI thread of a WPF application, by definition, must be running in a single-threaded apartment.
There's a lot to be said about BackgroundWorker, I'm sure many questions are already devoted to it, so I won't go into too much depth. If you're curious, check out the MSDN page for BackgroundWorker class.
BackgroundWorker can access UI controls directly, but Thread's cannot, Why?
Isn't BackgroundWorker a thread? If it isn't, what is it? Also, why is direct accessing restricted? Is restricting direct access Microsoft's choosen way for doing something or must it be done this way?
According to MSDN, "The BackgroundWorker class allows you to run an operation on a separate, dedicated thread."
BackgroundWorker acutally uses Form.Invoke() to switch control to the main UI thread to report progress. In the fact BackgroundWorker is just a helper class that you could easily write yourself, it uses a second thread to do the work but then it switches to the main UI thread to access GUI.
Also from MSDN: "You can listen for events that report the progress of your operation and signal when your operation is completed." This means that the background operation runs on a separate thread and you can't access GUI from that thread. But you can access GUI controls from progress event because it is raised on the main UI thread. Whenever you call BackgroundWorker.ReportProgress() on the worker thread, it raises BackgroundWorker.ProgressChanged event on the main thread.
Also, you ask why GUI cannot be accessed from other threads. It is because whole Windows GUI subsystem is not thread safe. As far as I know this is pretty normal in other platforms (outside Windows) as well, so it definitely isn't Microsoft specific. It is probably from historical reasons and probably has something to do with code efficiency. (A thread safe GUI would be slower.) But I can't provide any strong information source as a reference.
A background worker will throw a cross-thread exception just as a new thread would. A backgroundworker is just a fancy way to start a new thread.
If you are using the ProgressChanged event then you do not need to invoke it because there is some .NET magic behind the scenes doing it for you.
More information on background workers:
http://msdn.microsoft.com/en-us/library/system.componentmodel.backgroundworker(v=vs.100).aspx
I am a beginner with WPF, in my application I need to perform a series of Initialization steps, these take 10-15 seconds to complete during which my UI becomes unresponsive.
I was using yesterday the background worker but it didn't update my window, in fact it was frozen. Not sure, but maybe it didn't work because this control is only for Windows Forms.
UPDATE:
If not too much trouble, can you post me an example to use the alternative? For my case, the program will get some values from a database in a blucle.
Dispatcher.
The Dispatcher maintains a prioritized queue of work items for a specific thread. This might help you for updating your UI. If you have a lot of UI related initializations even this won't be able to help you much.
Dispatcher is not always an alternative to BackgroundWorker actually. The best practice is to select the more appropriate one as per your requirement. For example if you want something to execute without queuing BackgroundWorker is the solution. On the other hand if queuing is not a problem then Dispatcher is an alternative. For example, Dispatcher is using in Spell checkers and syntax highlighting functionality.
WPF Thread Model
All WPF applications start out with two important threads, one for
rendering and one for managing the user interface. The rendering
thread is a hidden thread that runs in the background, so the only
thread that you ordinarily deal with is the UI thread. WPF requires
that most of its objects be tied to the UI thread. This is known as
thread affinity, meaning you can only use a WPF object on the thread
on which it was created. Using it on other threads will cause a
runtime exception to be thrown. Note that the WPF threading model
interoperates well with Win32®-based APIs. This means that WPF can
host or be hosted by any HWND-based API (Windows Forms, Visual Basic®,
MFC, or even Win32).
The thread affinity is handled by the Dispatcher
class, a prioritized message loop for WPF applications. Typically your
WPF projects have a single Dispatcher object (and therefore a single
UI thread) that all user interface work is channeled through.
NOTE :
The main difference between the Dispatcher and other threading methods
is that the Dispatcher is not actually multi-threaded. The Dispatcher
governs the controls, which need a single thread to function properly;
the BeginInvoke method of the Dispatcher queues events for later
execution (depending on priority etc.), but still on the same thread.
See this thread for more information.
You could also queue items up with the thread pool and run the tasks like that, but be careful, if your tasks need to update the UI when they are finished you will have to marshal the data back to the UI thread.
One could use asynchronous delegates.
http://msdn.microsoft.com/en-us/library/ms228963.aspx
Just make sure if you are doing any UI related updates use:
Dispatcher.CheckAccess()
Here a simple example:
private void HandleUIButtons()
{
if (!btnSplit.Dispatcher.CheckAccess())
{
//if here - we are on a different non-UI thread
btnSplit.Dispatcher.BeginInvoke(new Action(HandleUIButtons));
}
else
{
btnSplit.IsEnabled = true; //this is ultimately run on the UI-thread
}
}
Taken from here:
http://blog.clauskonrad.net/2009/03/wpf-invokerequired-dispatchercheckacces.html
I am a bit confused about how GUI can be used in multi-threaded applications.
I hear there is a thing called the UI thread. Which I assume is my main executing thread at the startup of the application.
I also hear (though I am not 100% on this) that doing UI stuff on other (non UI) threads is a very bad idea.
So, if I create a separate thread and I want to call MyForm myForm = new MyForm(); myForm.ShowDialog(); in it, what changes do I need to make for that to be "safe"?
Also, I have had some people tell me that events are spun out on a different thread. (Though I am not sure I believe this.) If they are, then I am confused. I can open a dialog (ie myForm.ShowDialog() in an event and nothing truly horrible happens. (Maybe this depends on if the event delegate was called with Invoke or BeginInvoke?)
Here are a few bits of info that may help you out. What you're saying about working with UI on non UI threads isn't just a bad idea, you'll get an exception. Meaning, if you create a Form in the main thread, and then spawn off a background thread to do some processing and then want to update the Form in that background thread, it'll throw an exception. In your example though, where you create the Form in a background thread, you should be OK. I'd question your design, but it won't blow up AS LONG AS YOU ONLY TOUCH THE UI IN THAT SAME THREAD.
As for events, events handlers are executed on the same thread they were raised on. Meaning, if you have a Form on one thread that spawns off some work on another thread that raises events, but before doing so, you hook into this event on the Form thread, you need to be careful not to touch the UI directly in the event handlers, because those event handlers are being called on the background thread.
Finally, the way to correctly manipulate the UI from a background thread, is by calling Invoke and passing in a delegate that does the UI work you want. HTH
In WinForms you need to call UI-things on UI thread, you always can check on what thread you currents are getting InvokeRequired of UI-control.
void ApplyUiChanges()
{
if(this.InvokeRequired)
{
this.Invoke(new Action(ApplyUiChanges));
return;
}
// UI stuff here...
}
In WPF techinic is alike. But instead of using InvokeRequired you should ask CheckAccess() of DispatcherObject (All UI-controls derive from it)
void ApplyUiChanges()
{
if (!dispatcherObject.CheckAccess())
{
dispatcherObject.Dispatcher.Invoke(DispatcherPriority.Send, new Action(ApplyUiChanges));
return;
}
// UI stuff here...
}
Also you can take a look at Async CTP, which might be useful. But it's only CTP, not a release yet.
Another way to handle UI-thread communication is to use PostSharp. Write (or copy-paste) GuiThreadAttribute. After that, you'll be able to use such semantics:
[GuiThread]
void ApplyUiChanges()
{
// UI stuff here...
}
From what I've experienced, "UI thread" is a misnomer. There isn't one single thread that handles all of the UI for an application. To keep things simple, it's generally a good idea to have UI on one thread, but nothing stops you from spawning another thread, and creating new controls on that thread and showing them to the user. What's important is that control properties are only changed on the thread it was created on. As mentioned by another person, you can see if you are currently on that thread by looking at the Control.InvokeRequired property.
If you are on a thread that isn't the one you want a new form to run on and you don't have the luxury of being on the context of a control that is created on the thread you want, then you'll have to get a reference to the System.Threading.SynchronizationContext of the thread you want it to be on (I usually achieve this by storing a reference of System.Threading.SynchronizationContext.Current from the main thread in a static variable, but this can only be done after at least one control has been created on the thread). This object will allow you to run a delegate on its home thread.
I had to do this once in a Windows application that also hosted a WCF service, and UI needed to be launched from the service, but I wanted it on the same thread as the rest of the UI.
HTH,
Brian
In WinForms applications there is only a single thread which is the UI thread. You don't want to block this thread with long operations so that the UI is always responsive. Also you shouldn't update any UI elements from any thread other than the UI thread.
I always use a BackgroundWorker if I want to perform any lengthy operations from the UI. The major benefit of BackgroundWorker is that it can report progress and report that it is complete via ProgressChanged and RunWorkerCompleted. These 2 events occur in the UI thread thus you can update any UI element safely without the need to use InvokeRequired and Invoke.
how can i make a user control to run on its own thread ?
e.g. by following code in a user control , coz user control uses main app thread it make main thread to sleep
Thread.Sleep(TimeSpan.FromMinutes(2));
User controls need to run on the UI thread because that is a restriction in the Windows API. If you try and use Windows Forms controls from another thread you will get an exception.
You can run other code in another thread, but use the UI thread to update the controls. You can use BackgroundWorker for this. Or you can use the InvokeRequired and Invoke or BeginInvoke methods on the control instance to have it execute code on the UI thread.
You mention you want to use a mutex lock. A mutex is to avoid having multiple threads access a resource at the same time. If all your code is running in the same thread then you don't need a lock at all.
Objects don't really "run" themselves - methods are executed on a thread.
Now if you want a particular method to execute in a different thread, you need to either create a new thread, use the threadpool explicitly, or use something which uses the threadpool for you - such as BackgroundWorker.
What are you doing when you want to sleep for two minutes? Could you avoid sleeping by just setting a timer to fire (in the UI thread) in two minutes instead? If this is part of some long-running process, you should use BackgroundWorker or some other way of executing on a different thread, but with the control itself still handling updates and events on the UI thread.