I have an application wherein I would like a function to be executed in the same thread when an event is fired. For example:
SomeCode()
{
// Do something...
// Fire event to run SomeOtherCode().
}
SomeOtherCode()
{
// Do something else...
}
I do not want to simply call the function because it will hold things up. SomeOtherFuction() needs to be executed in the same thread because it needs to access the form controls, and I need it to begin execution from an event trigger firing. I am using Microsoft Visual C# 2008 Express Edition. Thanks.
::: EDIT:::
Additional Details: The bottom line is that the contrustor of my form application is taking far too long to complete, and it is causing a significant delay, from when the user launches the application to when the application window appears on the display. This is not a problem on faster computers, but on slower computers it is a big problem. I need to exit the contrustor as soon as possible, thus allowing the framework to draw the application window, and continue initialization outside the constructor. (All essential items would still be initialized inside the constructor.)
An event-triggered function call would be ideal. I would prefer not to use a timer. Interlacing the affected code with Invokes is impractical in my situation and would require much more time to implement than I have to work on this. A simple example of an event-driven function call is all I'm really looking for. Thanks.
From your posts it's seems like you're confusing a few issues. The standard pattern in .Net is for events to run synchronously. The following lines are essentially identical in terms of when they execute.
Option #1
SomeCode();
SomeOtherCode();
Option #2
SomeEvent += delegate { SomeOtherCode(); }
...
SomeCode();
SomeEvent(this,EventArgs.Empty);
If you want to unblock the UI thread and run the code later you'll need to use some mechanism to delay the running of the SomeOtherCode function. The easiest way to do this in a WinForms application is to use a WinForms Timer instance. This will raise an event on the UI thread at a later point in time that you can respond to. It also won't block the UI thread during this time allowing your form to continue processing.
You seem to be asking to run SomeOtherCode() later.
You can call BeginInvoke (either from the UI thread or from any other thread) to queue a function to run during the next message loop:
BeginInvoke(new Action(SomeOtherCode));
It seems that you would want to add an event to the class that exposes the SomeCode method. Then, the class that implements the SomeOtherCode method would attach an event handler that calls the SomeOtherCode method.
It's completely viable to have this done in one class, in case you have some sort of state model where you want to add/remove the call depending on some other logic.
I think you want to put SomeOtherCode into a Task or BackgroundWorker, which would then synchronize with the UI thread to send it updates.
I recently posted on my blog a class that makes updating the UI from a Task as easy as from a BGW. I do recommend using Task rather than BackgroundWorker.
Simialr to what Stephen said, I would recommend that you move as much of that initialization code to a background thread or task. Let the background thread do as much work as possible, then send the necessary window updates to your UI thread via Action<>'s. Here's some quick psuedo-sample code:
protected void LoadMyListInBackground(object state)
{
List<string> myList = Databse.FetchMyList(myParameters); // This take a while, so the UI thread isn't waiting
ShowMyList(myList);
}
protected void ShowMyList(List<string> theList)
{
if(InvokeRequired)
Invoke(new Action<List<string>>(ShowMyList, theList);
else
{
foreach(string item in theList)
myListBox.Items.Add(item);
}
}
In this example the UI thread is free to keep drawing your window while the background thread does the lengthy database work. The problem is, even if you fire an event outside of your constructor, and that event occurs on the UI thread and takes a long time, the user might see the window but that window is going to 'freeze' and possibly appear to be 'crashed' to the user. This technique prevents that and provides a better user experience.
Related
There are a hundred examples in blogs, etc. on how to implement a background worker that logs or gives status to a foreground GUI element. Most of them include an approach to handle the race condition that exists between spawning the worker thread, and creating the foreground dialog with ShowDialog(). However, it occured to me that a simple approach is to force the creation of the handle in the form constructor, so that the thread won't be able to trigger an Invoke/BeginInvoke call on the form prior to its handle creation.
Consider a simple example of a Logger class that uses a background worker thread to log to the foreground.
Assume, also, that we don't want NLog or some other heavy framework to do something so simple and lightweight.
My logger window is opened with ShowDialog() by the foreground thread, but only after the background "worker" thread is started. The worker thread calls logger.Log() which itself uses logForm.BeginInvoke() to update the log control correctly on the foreground thread.
public override void Log(string s)
{
form.BeginInvoke(logDelegate, s);
}
Where logDelegate is just a simple wrapper around "form.Log()" or some other code that may update a progress bar.
The problem lies in the race condition that exists; when the background worker thread starts logging before the foreground ShowDialog() is called the form's Handle hasn't yet been created so the BeginInvoke() call fails.
I'm familiar with the various approaches, including using a Form OnLoad event and a timer to create the worker task suspended until the OnLoad event generates a timer message that starts the task once the form is shown, or, as mentioned, using a queue for the messages. However, I think that simply forcing the dialog's handle to create early (in the constructor) ensures there is no race condition, assuming the thread is spawned off by the same thread that creates the dialog.
http://msdn.microsoft.com/en-us/library/system.windows.forms.control.handle(v=vs.71).aspx
MSDN says: "If the handle has not yet been created, referencing this property will force the handle to be created."
So my logger wraps a form, and its constructor does:
public SimpleProgressDialog() {
var h = form.Handle; // dereference the handle
}
The solution seems too simple to be correct. I'm specifically interested in why the seemingly too simple solution is or isn't safe to use.
Any comments? Am I missing something else?
EDIT: I'm NOT asking for alternatives. Not asking how to use NLog or Log4net, etc. if I were, I'd write a page about all of the customer constraints on this app, etc.
By the number of upvotes, there are a lot of other people that would like to know the answer too.
If you are concerned that referencing Control.Handle relies on a side effect in order to create the handle, you can simply call Control.CreateControl() to create it. However, referencing the property has the benefit of not initializing it if it already exists.
As for whether this is safe or not assuming the handle is created, you are correct: as long as you create the handle before spawning the background task on the same thread, you will avoid a race condition.
My two cents: there's no real need to force early handle creation if the logging framework simply maintains a buffer of undisplayed log entries while the handle has not been created. It could be implemented as a Queue, or many other things. Messing with the order of handle creation in .NET makes me squeamish.
I think the only danger is decreased performance. Handle creation is deferred in winforms to speed things up. However, since it sound like this is a one-time operation, it doesn't sound costly, so I think your approach is fine.
You can always check the IsHandleCreated property of your form to see if the handle has been built yet; however, there are some caveats. I've been in a similar spot to yours, where winforms controls are being created/destroyed dynamically with lots of multithreading going on. The pattern we wound up using was quite a bit like this:
private void SomeEventHandler(object sender, EventArgs e) // called from a bg thread
{
MethodInvoker ivk = delegate
{
if(this.IsDisposed)
return; // bail out! Run away!
// maybe look for queued stuff if it exists?
// the code to run on the UI thread
};
if(this.IsDisposed)
return; // run away! killer rabbits with pointy teeth!
if(!this.IsHandleCreated) // handle not built yet, do something in the meantime
DoSomethingToQueueTheCall(ivk);
else
this.BeginInvoke(ivk);
}
The big lesson here is to expect a kaboom if you attempt to interact with your form after it has been disposed. Don't rely on InvokeRequired, since it will return false on any thread if the control's handle hasn't been created yet. Also don't rely solely on IsHandleCreated since that will return false after the control has been disposed.
Basically, you have three flags whose state will tell you what you need to know about the control's initialization state and whether or not you're on a BG thread relative to the control.
The control can be in one of three initialization states:
Uninitialized, no handle created yet
InvokeRequired returns false on every thread
IsHandleCreated returns false
IsDisposed returns false
Initialized, ready, active
InvokeRequired does what the docs say
IsHandleCreated returns true
IsDisposed returns false
Disposed
InvokeRequired returns false on every thread
IsHandleCreated returns false
IsDisposed returns true
Hope this helps.
Since you do create the window on the calling thread you can end up with deadlocks. If the thread that creates the window has no message pump running your BeginInvoke will add your delegate call to the message queue which will never be emptied, if you do not have an Application.Run() on the same thread which will process the window messages.
It is also very slow to send around window messages for each log message. It is much better to have a producer consumer model where your logging thread adds a message to a Queue<string> which is emptied by another thread. The only time you need to lock is when you enqueue or dequeue a message. The consumer thread can wait for an event with a timeout to start processing the next message when either the event was signaled or the timeout (e.g. 100ms) has elapsed.
A thread safe blocking Queue can be found here.
I have a console app that I'm porting to WPF. The application has 3 worker threads, that are all joined to the main thread before some output results are printed to the screen. My understanding is that, if I try and do the same thing in a WPF application, the GUI will be blocked and will not be reponsive to the user. How then can I notify the parent thread that all the threads have completed their work? I think the solution is going to involve delegates and events (or maybe BackgroundWorker?), but it was not clear to me how to get the callback invoked when the thread terminated.
Original Code:
foreach (Thread t in threadList)
{
t.Start();
}
foreach (Thread t in threadList)
{
t.Join();
}
// print some results here
If you are using three BackgroundWorkers, you can use the event RunWorkerCompleted to notice that one of the workers is completed: Before starting the workers set a counter to 3 then decrement and check this counter in the method called by RunWorkerCompleted if it hits 0 you are finished.
You should use three BackgroundWorkers.
You can then handle their RunWorkerCompleted events to find out when the operations finish.
Take a look at this article in MSDN magazine that gives you an example on using BackgroundWorker. Start at Figure 7.
Depends on what you would like to accomplish. What form of communication are you trying to facilitate?
If I were to guess, what you really want is to simply report [or display] your worker results in your application. If this is the case, then in a typical WPF application you have a view model, say
public class AwesomeViewModel : INotifyPropertyChanged
{
// if small fixed number, otherwise, you could use
// an ObservableCollection<T>
public string WorkerResultA { ... }
public string WorkerResultB { ... }
public string WorkerResultC { ... }
}
which is data-bound to your WPF controls. You can simply pass a reference of your view model to each worker thread and they update the class without requiring blocking\waiting on Gui thread. In this manner, each worker reports its results when it completes without intervention from anyone else. This is optimal.
Of course, if you go ahead and do just this, you run into another completely different issue. Which, fyi, is resolvable via Dispatcher. One possible solution here.
As for BackgroundWorker versus explicit Thread control, that is up to you. There are advantages to both, but remember you already have functional code written. That, and in my personal opinion, BackgroundWorker isn't particularly useful.
If you really absolutely positively must implement a more sophisticated synchronization model, then I highly recommend you brush up on ManualResetEvent its cousin AutoResetEvent, Semaphore, keyword lock and concurrent programming in general. Sorry, no shortcuts there :)
Hope this helps!
If you just want to poll the worker threads, you could use something like
bool threadWasDone = thread.Join(0);
Is BackgroundWorker in c# Thread Safe?
The reason I ask this is because I get a
Controls created on one thread cannot
be parented to a control on a
different thread
exception with it. This is my DoWork event code:
private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
var openFile = document.Open(MyFileName);
e.Result = openFile;
}
where document is an UI control that is initialized when the parent form is created. During Open method various properties in document will be filled.
I tried to change the code to invoke, yet the same problem persists. i.e,
document.GetType().GetMethod("Open)".Invoke(document, new object[]{MyFileName})
will yield the same error as the above.
Any idea how to manipulate the document control? In other words, how to make the above code work?
Edit: It was suggested that I use Control.Invoke, but it still didn't work ( both of the threads hanged). This is the code I tried:
private delegate bool OpenFile(string filePath);
private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
OpenFile oF = new OpenFile(document.Open);
var openFile = Invoke(oF, MyFileName); // it doesn't really matter whether I use BeginInvoke or Invoke, or other Control.Invoke, the end result is the same. Both the main thread hosting the document and the thread that launches the UI hanged.
e.Result = openFile;
}
It isn't the thread that's the problem it's the fact that it's trying to call a method on a UI control. In both WPF and WinForms controls can only be called on the UI thread (of which there is typically one). You don't say which you are using but you need to call the Control.Invoke method for WinForms or Dispatcher.Invoke for WPF.
The Invoke() reflection method you show will actually invoke the method on the current thread.
You can either invoke as Mehrdad Afshari suggested, or you can make use of the bgw's progress event which comes back on the UI thread. Or the work completed event which also comes back on the UI thread. The difference between the two is WorkCompleted is fired only once at the end. Progress is fired by you from DoWork.
While it's unclear to me what you exactly mean by thread-safety of a BackgroundWorker, the problem is not that object; Windows Forms controls are designed to be manipulated on a single thread (the UI thread). You should not manipulate Windows Forms objects on different threads. You can invoke actions in the UI thread from other threads by using the Control.Invoke method (the Invoke method you are currently using is provided by reflection and is totally unrelated to this problem):
Invoke(new Action(MethodToRunInUIThread));
void MethodToRunInUIThread() {
// do stuff here.
}
By the way, it doesn't make sense to use a background worker if all you are doing is manipulating UI objects.
If that functionality of the UI Control takes that long to execute, there may not be much you can do. "Freezing" occurs when a long-running operation happens on the UI thread, and if that function of the control was not specifically made thread-safe, it must be run on the main thread.
Normally, you'd want to separate the "document" functionality away from the control that displays it. This way, your document could be loaded on a separate, independent thread and be displayed later when ready. Otherwise, the control itself would have to implement a multi-threaded load routine to slow loading freezes.
Since you've specified this is a third party control in your comments, you may be out of luck here.
BackgroundWorker is a thread based structure. The thread-safety matter is about functions when doing simultaneous tasks. Maybe what you ask for is about winforms controls which are accessed through a unique thread, that of the user interface thread.
You need to use Control.BeginInvoke() in DoWork. This executes the delegate asynchronously and so will ensure the calling thread will not "hang".
Control.Invoke() will execute the delegate on the other thread also, but will cause the calling thread to wait for it to complete.
Generally in Windows Forms you are better off using Control.BeginInvoke() wherever possible to help avoid deadlocking between threads that can occur when one thread waits for another, as with Control.Invoke().
If the "document" object inherits from System.Windows.Forms.Control, you can simply call document.BeginInvoke(myDelegate).
However if it is actually some other component that encapsulates GUI controls, it may expose some way to call BeginInvoke. Check the documentation (if any). If there is no such ability, then unfortunately it is probably just not designed to support multi-threaded applications.
It looks like you are confused about the various Invoke/BeginInvoke types (understandable). This earlier question: What is the difference between Invoke and BeginInvoke? and Jon Skeets answer should help clarify things.
#Graviton, a related task with an answer is found here. The person was using BackgroundWorker to update a textbox, same concept applies (yours is only a single worker thread).
If I have Thread A which is the main Application Thread and a secondary Thread. How can I check if a function is being called within Thread B?
Basically I am trying to implement the following code snippit:
public void ensureRunningOnCorrectThread()
{
if( function is being called within ThreadB )
{
performIO()
}
else
{
// call performIO so that it is called (invoked?) on ThreadB
}
}
Is there a way to perform this functionality within C# or is there a better way of looking at the problem?
EDIT 1
I have noticed the following within the MSDN documentation, although Im a dit dubious as to whether or not its a good thing to be doing! :
// if function is being called within ThreadB
if( System.Threading.Thread.CurrentThread.Equals(ThreadB) )
{
}
EDIT 2
I realise that Im looking at this problem in the wrong way (thanks to the answers below who helped me see this) all I care about is that the IO does not happen on ThreadA. This means that it could happen on ThreadB or indeed anyother Thread e.g. a BackgroundWorker. I have decided that creating a new BackgroundWorker within the else portion of the above f statement ensures that the IO is performed in a non-blocking fashion. Im not entirely sure that this is the best solution to my problem, however it appears to work!
Here's one way to do it:
if (System.Threading.Thread.CurrentThread.ManagedThreadId == ThreadB.ManagedThreadId)
...
I don't know enough about .NET's Thread class implementation to know if the comparison above is equivalent to Equals() or not, but in absence of this knowledge, comparing the IDs is a safe bet.
There may be a better (where better = easier, faster, etc.) way to accomplish what you're trying to do, depending on a few things like:
what kind of app (ASP.NET, WinForms, console, etc.) are you building?
why do you want to enforce I/O on only one thread?
what kind of I/O is this? (e.g. writes to one file? network I/O constrained to one socket? etc.)
what are your performance constraints relative to cost of locking, number of concurrent worker threads, etc?
whether the "else" clause in your code needs to be blocking, fire-and-forget, or something more sophisticated
how you want to deal with timeouts, deadlocks, etc.
Adding this info to your question would be helpful, although if yours is a WinForms app and you're talking about user-facing GUI I/O, you can skip the other questions since the scenario is obvious.
Keep in mind that // call performIO so that it is called (invoked?) on ThreadB implementation will vary depending on whether this is WinForms, ASP.NET, console, etc.
If WinForms, check out this CodeProject post for a cool way to handle it. Also see MSDN for how this is usually handled using InvokeRequired.
If Console or generalized server app (no GUI), you'll need to figure out how to let the main thread know that it has work waiting-- and you may want to consider an alternate implementation which has a I/O worker thread or thread pool which just sits around executing queued I/O requests that you queue to it. Or you might want to consider synchronizing your I/O requests (easier) instead of marshalling calls over to one thread (harder).
If ASP.NET, you're probably implementing this in the wrong way. It's usually more effective to use ASP.NET async pages and/or to (per above) synchronize snchronizing to your I/O using lock{} or another synchronization method.
What you are trying to do is the opposite of what the InvokeRequired property of a windows form control does, so if it's a window form application, you could just use the property of your main form:
if (InvokeRequired) {
// running in a separate thread
} else {
// running in the main thread, so needs to send the task to the worker thread
}
The else part of your snippet, Invoking PerformIO on ThreadB is only going to work when ThreadB is the Main thread running a Messageloop.
So maybe you should rethink what you are doing here, it is not a normal construction.
Does your secondary thread do anything else besides the performIO() function? If not, then an easy way to do this is to use a System.Threading.ManualResetEvent. Have the secondary thread sit in a while loop waiting for the event to be set. When the event is signaled, the secondary thread can perform the I/O processing. To signal the event, have the main thread call the Set() method of the event object.
using System.Threading;
static void Main(string[] args)
{
ManualResetEvent processEvent = new ManualResetEvent(false);
Thread thread = new Thread(delegate() {
while (processEvent.WaitOne()) {
performIO();
processEvent.Reset(); // reset for next pass...
}
});
thread.Name = "I/O Processing Thread"; // name the thread
thread.Start();
// Do GUI stuff...
// When time to perform the IO processing, signal the event.
processEvent.Set();
}
Also, as an aside, get into the habit of naming any System.Threading.Thread objects as they are created. When you create the secondary thread, set the thread name via the Name property. This will help you when looking at the Threads window in Debug sessions, and it also allows you to print the thread name to the console or the Output window if the thread identity is ever in doubt.
From time to time my applications GUI stops redrawing.
There a lot of threads that are firing all kinds of events (like timers or network data ready etc.). Also there are a lot of controls that are subscribing these events. Because of that, all the event handlers play the InvokeRequired/Invoke game.
Now I figured out that when the GUI freezes a lot of threads are waiting for Invoke() to return. Looks like the message pump stopped pumping.
The handlers look like this:
private void MyEventHandler( object sender, EventArgs e ) {
if ( InvokeRequired ) {
Invoke( new EventHandler( MyEventHandler ), sender, e );
return;
}
SetSomeStateVariable();
Invalidate();
}
Any ideas?
Solution: BeginInvoke(). Looks like you should always use BeginInvoke() if you have lots of CrossThread-Events...
Thanks.
Thanks everybody.
EDIT: Looks like BeginInvoke() really solved it. No freezing until now.
Invoke waits until the event is handled in the GUI thread. If you want it to be asynchronous use BeginInvoke()
Deadlock perhaps? Do you make sure that the events are never fired while holding a lock?
Are you able to see this with a debugger attached? If so, make it freeze and then hit the "pause" button - and see what the UI thread is doing.
Note that if you are able to get away with BeginInvoke instead of Invoke, life is a bit easier as it won't block.
Also note that you don't need the "new EventHandler" bit - just
Invoke((EventHandler) MyEventHandler, sender, e);
should be fine.
From watching this question, I can see that you're not going to get any answers that will fix the problem immediately, as most of them require you to debug the event, and it happens so infrequently that this is nearly impossible. So, let me suggest you make some code changes that might help you identify the culprit in the field.
I suggest that you create a static class whose sole purpose is to handle all your Invoke calls. I would suggest that this class has a method that takes a Control, (to call Invoke on) an Action (the method to be invoked), and a description (containing the information you would need to know to identify the method and what it is going to do).
Within the body of this method, I suggest you enqueue this information (method, description) and return immediately.
The queue should be serviced by a single thread, which pops the action/message pair off the queue, records the current time and the Action's description in a pair of properties, and then Invokes() the Action. When the Action returns, the description and time are cleared (your DateTime can be nullable, or set it to DateTime.Max). Note, since all Invokes are marshalled one at a time onto the UI thread, you're not losing anything by servicing the queue by a single thread here.
Now, here's where we get to the point of this. Our Invoking class should have a heartbeat System.Threading.Timer thread. This should NOT be a windows.forms.timer object, as that runs on the UI thread (and would be blocked when the ui is blocked!!!).
The job of this timer is to periodically peek at the time the current Action was Invoked. If DateTime.Now - BeginTime > X, the heartbeat timer will decide that this Action has blocked. The heartbeat timer will LOG (however you log) the DESCRIPTION recorded for that Action. You now have a recording of what was happening at the time your UI locked up and can debug it better.
I know it's not an answer to your problem, but at least by doing this you can get a good idea about what's going on at the time you're blocked.
The most likely answer (deadlock) has already been suggested.
Another way to simulate this behaviour is to reduce the number of pool threads and IO completion ports; you haven't called ThreadPool.SetMaxThreads(...) by any chance?