I have a C# library that would like to have the ability to Send/Post work to the "main" ui thread (if one exists).
This library may be used by:
A winforms application
A native application (with UI)
A console application (with no UI)
In the library I'd like to capture something (A SynchronizationContext, a Dispatcher, a Task Scheduler, or something else) during initialization, that will allow me to (at a later time) Send/Post work to the main thread (if the main thread has that ability--i.e. it has a message pump). For example, the library would like to put up some Winforms UI on the main thread if and only if the main application has the ability for me to get to the main thread.
Things I've tried:
A SynchronizationContext:
Capturing this works fine for a Winforms application (a WindowsFormsSynchronizationContext will be installed as the Current SynchronizationContext. This also works fine for the console app--since I can detect that the Current SynchronizationContext is null (and thus, know that I don't have the ability to send/post work to the main thread). The problem here is the native UI application: It has the ability (i.e. it has a message pump), but the Current Synchronization context is null and thus I can't differentiate it from the Console app case. If I could differentiate, then I could simply install a WindowsFormsSynchronizationContext on the main thread, and I'm good to go.
A Dispatcher: Capturing this using Current creates a new SynchronizationContext. Thus, in all situations I will get back a Dispatcher. However, for a Console app, using Dispatcher.Invoke from a background thread will hang (as expected). I could use Dispatcher.FromThread (which doesn't create a Dispatcher for the thread if one doesn't exist). But the native UI application will return a null Dispatcher using this method, and so then I'm, again, stuck not being able to distinguish the UI application from the console application.
A TaskScheduler: I could use FromCurrentSynchronizationContext. This has the same problems as the SynchronizationContext. I.e. Before calling FromCurrentSyncronizationContext, I'd have to check if the Current SynchronizationContext is null (which will be the case for the Console app and the native ui application). So, again I can't distinguish the native ui application from the console application.
I, of course, could have the user of my library specify whether or not it is a UI application when they call my Initialize method, but I was hoping to avoid that complication for the user of the library if possible.
This is not in general possible, a library that's apt to be used in threads cannot make any assumptions about which particular thread is the UI thread. You can capture Synchronization.Current but that will only work correctly if your initialization method is called from the UI thread. That's not terribly unusual to work out well, like TaskScheduler.FromCurrentSynchronizationContext() tends to work by accident, but not a guarantee. You can add a check, if Thread.CurrentThread.GetApartmentState() doesn't return STA then the odds that you are not being called from the UI thread are very high. SynchronizationContext.Current will also often be null in that case, another way to check.
The (arguably) better ways are to just not worry about it and let the client code figure it out, it won't have any trouble marshaling the callback. Or to expose a property of type SynchronizationContext so that the client code can assign it. Or add it as a constructor argument. Throw an InvalidOperationException if you are ready to Post but find out that it is still null, that's an oversight that the client programmer only makes once.
I think you should make this an option to your Initialize method (or somehow allow your caller to request UI interaction), to me that just makes more sense. I don't know the specifics but these seems to me to be a "courteous" thing to do, let your caller decide if they want you to or want to support your UI. I would take it one step further and even as the caller to supply a synchronization context. But that's my opinion.
To answer your question, there are a few "hacks" you can use to determine if you're running in a console applicaiton. This SO question has some information on that: C#/.NET: Detect whether program is being run as a service or a console application
Change the library initialization to have a SyncronizationContext parameter. If the parameter is null then the library doesn't need to do anything special, if not null Post/Send GUI updates there.
I think this is exactly what AsyncOperationManager.CreateOperation() is for.“Implementing the Event-based Asynchronous Pattern” states:
The Event-based Asynchronous Pattern provides a standardized way to package a class that has asynchronous features. If implemented with helper classes like AsyncOperationManager, your class will work correctly under any application model, including ASP.NET, Console applications, and Windows Forms applications.
It is up to the caller to decide if they want to call your API on the UI thread or not. If they do, this will capture the context and events will go through the message pump in order. In a Console application you can get the same behavior if you install a SynchronizationContext such you get for free by using AsyncContext.Run() from the Nito.AsyncEx nuget package. No need for an extra property or to have to write the conditional code yourself. If no serializing synchronization context is available, AsyncOperation.Post() will use the fake synchronization context available to Console apps which just queue the event to the threadpool instead (meaning that the posts may not execute in order). Just remember to call AsyncOperation.OperationCompleted() or AsyncOperation.PostOperationCompleted() when done.
In the library I'd like to capture something (A SynchronizationContext, a Dispatcher, a Task Scheduler, or something else) during initialization
This is exactly what AsyncOperationManager.CreateOperation() does, and in an environment-agnostic way. But you should try to pair this with a call to OperationCompleted() which maybe would be more difficult given the API you want to expose. The easiest way to use AsyncOperation would be to start an operation when your library actually starts an operation instead of during initialization. Or by having the initialization routine return an IDisposable context object handle which would signal to the consumer that they need to manage its lifetime.
Related
I have read that async/await methods runs in the same thread as caller and I saw it in a WPF application but while testing a code in console application I see it is running in a different thread than caller.
Have I missed something?
Have I missed something?
Sort of. await doesn't know anything about threads. It causes the code after the await keyword (by default) to run in the same SynchronizationContext as the caller, if it exists.
In the case of a WPF Application, the current SynchronizationContext is setup to marshal the call back to the UI thread, which means it will end up on the UI thread. (The same is true in Windows Forms). In a Console Application, there is no SynchronizationContext, so it can't post back onto that context, which means you'll end up on a ThreadPool thread.
If you were to install a custom SynchronizationContext into SynchronizationContext.Current, the call would post there. This is not common in Console Applications, however, as it typically requires something like a "message loop" to exist to work properly.
I'm working on integrating a single-threaded API that does not have any multi-threaded support into a multi-threaded program. I would like to keep all APIinteraction on the main thread and do other stuff on other threads. However the program I am working with has a Producer-Consumer oriented threading design(which I can't modify).
Is there a way I can make threads switch to main thread when I want? Or some other way to get it working?
I apologize for not being able to express the problem clearer
You can use Control.Invoke on a worker thread to have it run some code on the main user interface thread.
Or maybe you could just synchronize all access to the single-threaded API using lock?
More details would be great, but those are some ideas to get you started.
EDIT: After reading your comment, the easiest & most light-weight way to do it would be to synchronize using lock, as previously mentioned. That way, only one thread calls the 3rd-party API at a time. Example:
static object APILock = new object(); // global variable
// Do this anytime you need to make calls on this non-thread-safe API:
lock (APILock) {
// call the API; only one thread will run code inside the lock at a time
}
This is generally the accepted way of calling non-thread-safe code.
You can try to use window messages that are (usually) handled by one "main" thread. Create a window, expose wrappers that mimic your API methods to clients, and send window messages internally. When you handle window messages, call your actual implementation.
That is how COM single-thread apartment model works, and it solves exactly the same problem. However, that is quite advanced solution.
Can't your code be refactored in order to make it thread-safe? That would be simpler, I think.
I know the BackgroundWorker should not be used in Windows Services but would anyone have a good online reference explaining why?
BackgroundWorker relies on a current SynchronizationContext being set in order to function. It's really intended and designed specifically for working with UI code.
It's typically better in a service to self-manage your threads, since there are no UI synchronization issues. Using the threading API (or .NET 4 Task API) is a much better option here.
Well, it's okayish to use a BGW in a service, it just doesn't do anything especially useful. Its reason for being is its ability to raise the ProgressChanged and RunWorkerCompleted events on a specific thread. Getting code to run on a specific thread is a very non-trivial thing to do. You cannot simply inject a call into the thread while it is executing code. That causes horrible re-entrancy problems. The thread has to be 'idle', in a state where inject code doesn't cause trouble.
Having a thread in an idle state is a fairly unnatural condition. You use threads to run code, not for them to be idly spinning its heels. This is however the way a UI thread works. It spends 99% of its time in the message loop, waiting for Windows to tell it to do something. A button click, a paint request, a keyboard press, that sort of thing. While it is inside the message loop, it is in fact idle. A very good time to execute injected code.
Which is what Winforms' Control.Begin/Invoke and WPF's Dispatcher.Begin/Invoke do. They put a delegate in a queue, the queue is emptied and the delegate targets executed by the message loop. The WindowsFormsSynchronizationContext and DispatcherSynchronizationContext classes are the synchronization providers that uses them. Winforms and WPF replace SynchronizationContext.Current with an instance of them. Which in turn gets used by BGW to raise the events. Which makes them run on the UI thread. Which allows you to update the non thread-safe user interface components from a worker thread.
You can probably see where this is heading, a service uses neither. The default synchronization provider doesn't synchronize anything. It simply uses a threadpool thread to call the Send or Post callback. Which is what will happen when you use BGW in a service. Now there is actually no point at all in having these events. You might as well let the DoWork handler call the event handling methods directly. After all, the thread on which DoWork runs is just another threadpool thread as well.
Well, no real harm done, other than making it quite a bit slower.
I've used BackgroundWorker in windows services many times without any ill effect. While its use of SynchronizationContext may be unnecessary, I haven't observed it causing problems or poor performance.
I need an alternative for Dispatcher (.net 3.0) to use for a windows service (done in .net 2.0). Can you give me some idea how to achieve something like that or point me some links?
I know that a dispatcher has a SynchronizationContext behind, but I don't know how I can use a SynchronizationContext into a service.
If you think that I should stick to the Dispatcher (.net 3.0) ... how can I manipulate it (OnServiceStop, OnServiceStart)
edited:
More details (see also...here)
Idea is that I would like to host into my windows service some extensions/plugins which would communicate between each-other through a method ExecuteCommand(type, params).
This method also raises an event to the service in order to receive results if it was executed from inside the plugin. Each plugin could have its own thread from where it calls this method ExecuteCommand so I would like to gather and synchronize all the calls into one thread (main service thread) in order to return the result appropriately.
This is why Dispatcher came into play. But I would like to have, maybe, something in .net 2.0 or do you think Dispatcher is good in my case?
Thanks.
Windows Services don't have anything like the Dispatcher (or message loop in Windows Forms). If you want to marshal from one thread to another, the "target" thread will have to be running its own sort of message loop.
If you could tell us more about what you're trying to achieve, it would make it easier to help you.
EDIT: Okay, it sounds like basically want a producer/consumer queue: one thread waits until something is present in the queue, and processes it. Producers can add to the queue whenever they like.
I have a very simple implementation of a producer/consumer queue in my threading tutorial, but there may be more advanced implementations around. (.NET 4 makes this easy, but it's harder in .NET 2.) If you do take my implementation, you'll want to think about making it generic and adding termination conditions. Joe Albahari has another implementation you should look at, too.
I'm writing a J2ME application. One of the pieces is something that polls the contents of a directory periodically, and, if there are any new things, paints them on the screen. I've done this by having the UI form launch a polling thread with a pointer back to itself, and when the polling thread finds something it calls back to the form and calls a syncrhonized method to update it's display. This seems to work fine.
The question I have is this. In C#/.NET I know it is not nice to have non-UI threads updating the UI, and the correct way to handle this is to delegate it up to the UI thread.
E.g. the following:
public void DoSomeUIThing()
{
if (this.uiComponent.InvokeRequired)
{
this.uiComponent.Invoke(someDelegateThatCallsBackToThis);
}
else
{
this.uiComponent.Text = "This is the update I want to happen";
}
}
Is there a J2ME equivalent for how to manage this process? How about Java? Or does Java/J2ME just play nicer in regard to this? If not, how is this done?
[EDIT] It appears that Swing supports what I'm asking about via the SwingUtilities.invokeLater() and invokeAndWait() methods. Is there an equivalent framework for J2ME?
Regarding Java, what you are describing looks like a SwingWorker (worker thread).
When a Swing program needs to execute a long-running task, it usually uses one of the worker threads, also known as the background threads.
A Swing program includes the following kinds of threads:
Initial threads, the threads that execute initial application code.
The event dispatch thread, where all event-handling code is executed. Most code that interacts with the Swing framework must also execute on this thread.
Worker threads, also known as background threads, where time-consuming background tasks are executed.
Single-thread rule:
Once a Swing component has been realized, all code that might affect or depend on the state of that component should be executed in the event-dispatching thread.
When used in a J2EE context, you need to be careful when you are referencing a SwingWorker from an EJB.
Regarding J2ME, it depends if you are developing your application as a standard MIDlet that will run on any MIDP-enabled device, or for instance as a RIMlet, a CLDC-based application that uses BlackBerry-specific APIs and therefore will run only on BlackBerry devices.
Because unlike MIDP's UI classes, RIM's are similar to Swing in the sense that UI operations occur on the event thread, which is not thread-safe as in MIDP. To run code on the event thread, an application must obtain a lock on the event object, or use invokeLater() or invokeAndWait() – extra work for the developer, but sophistication comes with a price tag.
But for LCDUI, you can access a form from multiple threads.
There are many profiles of Java ME. If you mean MIDP then Display.runSerially is what you want.
For AWT (Swing) you would use EventQueue.invokeLater (SwingUtilities.invokeLater is only necessary due to Java 1.1 not having the EventQueue method - 1.2 is about to celebrate its tenth birthday). For the Common DOM API, use DOMService.invokeLater.
No matter what claims a GUI API may make about thread-safety they are probably wrong (some of the claims of Swing are removed in JDK7 because they are not implementable). In any case, application code unlikely to be thread-safe.
For j2me apps you probably want to keep it simple. The main thing is to touch UI components only in the event thread. The direct way of doing this is to use invokeLater or invokeAndWait. Depending on your libraries you won't have access to anything more than that. In general if these aren't provided in your platform it probably equates to there being no thread support and not being an issue. For example the blackberry does support it.
If you develop under SWT this is accomplished by means of asyncExec() method of Display object. You pass an object implementing Runnable so the UI thread executes the changes done in other thread.
This is an example borrowed from here
public void itemRemoved(final ModelEvent me)
{
final TableViewer tableViewer = this.viewer;
if (tableViewer != null)
{
display.asyncExec(new Runnable()
{
public void run()
{
tableViewer.remove(me.getItem());
}
}
}
}
I can attest that the MIDP UI toolkit is indeed thread-safe, as I have large MIDlets with complex GUI running on millions of phones made by almost every manufacturer, and I have never seen a problem in that regard.