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).
Related
I need to invoke this: string input_ip_r = listView1.Items[lc].SubItems[1].Text;
so I used
if (InvokeRequired)
{
this.Invoke(new MethodInvoker(function));
return;
}
This worked but now I have put it into a BackgroundWorker and using this
if (InvokeRequired)
{
this.Invoke(new MethodInvoker(bw.RunWorkerAsync));
return;
}
it gives an error that you can only run BackgroundWorker one at a time.
So how do I invoke while in the Backgroundworker?
1) Don't put RunWorkerAsync as the method to invoke. It's not actually running the method that you think. What you should really put there is something like this:
this.Invoke(new MethodInvoker(MethodToUpdateUI));
MethodToUpdateUI should be some new method that you create that specifically does whatever UI updates should be made in this context.
2) There's no need for InvokeRequired. You're in a background thread. Invoking will always be required.
To be honest, the entire patter of if(invoke required) call myself else do stuff is an odd construct which I dislike. InvokeRequired should pretty rarely be used. You should almost always know whether you're in the UI thread or a background thread, if you don't, chances are something wrong (either you're always in one or the other and you just don't know which, or it shouldn't be non-deterministic). Usually this means having methods that must be run in the UI thread. If you're already in the UI thread you just call them, if you're in a background thread and know it then you call Invoke first.
On top of that, Invoke works just fine even if you call it when you're already in the UI thread, so there's really no significant negative consequences to just calling Invoke regardless of whether you're in a background thread or already in the UI thread.
3) Usually it's best to separate code for solving business problems from UI code. It's code smell to be invoking from within DoWork's handler. If this is right near the end, you should probably be adding an event handler to RunWorkerCompleted. If you're calling this periodically to update the UI with progress of the worker, you should be using ReportProgress and handling the ProgressReported event. For getting info from the UI for use in a long running task you should access it before starting the background task. For exceptional cases that aren't any of those, it may be appropriate to use Invoke, but the remaining cases ought to be rare.
I'm not quite sure how you want to use the values, but just to give you an example, you could easily just do this in the BackgroundWorker thread:
private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
string input_ip_r = "";
this.Invoke(new Action(() =>
{
// Don't know what "lc" is (a loop variable?)
input_ip_r = listView1.Items[lc].SubItems[1].Text;
}));
}
See this answer for other ways of doing the same (this is for >= .Net 3.5)
I use a BackgroundWorker and do this:
private void loadNewAsyncToolStripMenuItem_Click(object sender, EventArgs e)
{
this.Text = "RunWorkerAsync()";
backgroundWorkerLoading.RunWorkerAsync();
}
private void backgroundWorkerLoading_DoWork(object sender, DoWorkEventArgs e)
{
UnsafeThreadMethod("hello");
EvenUnsaferThreadMethod();
}
And now the two methods.
private void UnsafeThreadMethod(string text)
{
toolStripLabelRssFeedData.Text = text;
}
private void EvenUnsaferThreadMethod()
{
panelLoading.Visible = true;
}
I don't understand why UnsafeThreadMethod doesn't throw the following exception but EvenUnsaferThreadMethod does.
Cross-thread operation not valid: Control 'panelLoading' accessed from a thread other than the > thread it was created on.
According to the message it's because toolStripLabelRssFeedData was created on the same thread but it wasn't.
I thought that I can't call controls created by the main thread and have to use the ProgressChanged event. What's going on?
And I have a second question. What is the advantage of doing it like this when I can use ProgressChanged? What should I do?
private void EvenUnsaferThreadMethod()
{
if (panelLoading.InvokeRequired)
{
panelLoading.Invoke(new MethodInvoker(() => { EvenUnsaferThreadMethod(); }));
}
else
{
panelLoading.Visible = true;
}
}
To the first question:
the cross-thread exception is deliberately thrown in Debug mode. This means there is (conditional) code checking on InvokeRequired built into most of the GUI controls. Like the Panel.
Apparently the ToolstripLabel does not make this check. Since it does not derive from Control that could be because it is outside the scope of this safety net.
Since the standard disclaimer "Any instance members are not guaranteed to be thread safe" applies to the ToolstripLabel I would just go with the normal InvokeRequired logic when setting the Text.
For your first question, I am not entirely sure, but a review from online seems to show that sometimes this will not throw an exception, but it will not update the label. Is that the case here? Is your label being updated along with having no exception?
However, I can answer you second question right now. The ProgressChanged event is meant for exactly what it sounds like. It is supposed to be called to let the UI thread know the status of the backgroundworker so that it can update itself appropriately. The original calling thread (UI in this case) is the one that is used for the ProgressChanged, so when it updates it does not need to call Invoke. But, this should really only be done for showing the progress of a background worker.
Now, if it is not an update that you are trying to pass to the calling method, then I would suggest just passing your return data back through the RunWorkerCompleted event. This passes all of your final data back up to the original (UI) thread, so that it can update the UI without any need for an Invoke.
So, yes your call to Invoke will work, though. However, understanding what each of the other events are for can help you understand why to use one way over another. Maybe a ProgressChanged event fits better? It can also declutter your code from having unnecessary invokes.
Update to first q
I still cannot find anything about the toolstrip not needing the invoke. In fact I am finding the opposite using google searches like "toolstriplabel no cross thread exception" or "toolstriplabel invoke", etc. However, as henk mentioned, the toolstriplabel doesn't inherit from control so that might explain why no invoke is required. However, my suggestion is to assume that it will act like any other UI control and make sure it is updated on the UI thread to be safe. do not rely on quirks. Better safe than sorry, you never know if things like this might change, especially since it is logically a UI item to most..,
The advantage of your second choice is that it works :)
All UI elements are created on main UI thread and, what is more important from this question perspective, is that can be acessed only within that thread.
This is the reason why your first case fails and that is the reason your second case will work. Invoke()... will redirect required merhod call to the main UI thread.
Hope this helps.
I'm a bit confused in terms of the dispatcher. Say I'm on a background thread doing some kind of long operation. I'd like to update the ui thread I understand I do this through the dispatcher. My question is, do i call the dispatcher statically like: Dispatcher.BeginInvoke(mywork) ... Or on the control i want to update: mytextbox.Dispatcher.BeginInvoke(mywork)
It's worth noting that calling Dispatcher.BeginInvoke isn't a static call: it's an implicit this.Dispatcher.BeginInvoke. If you can use this call, you're likely writing your code from within a control or window already. In that case, you're probably safe to call either since most of the time there will be one UI thread per application.
The actual static call would be Dispatcher.CurrentDispatcher.BeginInvoke, which is not something you want to call (see my comment on Hasan Khan's answer for why).
EDIT: Calling Application.Current.Dispatcher is not a bad thing. (And, for clarity, it's an instance property, not a static - being called on a static/singleton instance of Application.) This property will return the Dispatcher for the thread that the app was created with, and typically that's the thread that the UI is created on aswell - so Application.Current.Dispatcher returns the same Dispatcher as myWindow.Dispatcher would.
The static call Dispatcher.CurrentDispatcher (that I warned against) returns a Dispatcher for the thread you call it from. If you call it from a background thread, you'll get a new Dispatcher created especially for that thread - which often isn't what's wanted.
First I think it is important to understand, that the Dispatcher is not designed to handle big background operations. It is designed to queue work on the UI thread of an object. Here is a worthwhile MSDN article about the .NET threading model and Dispatcher:
Threading Model, Overview and the Dispatcher
Saying that the standard way of implementing the Dispatcher.BeginInvoke method would be to call it on the control:
startStopButton.Dispatcher.BeginInvoke(
DispatcherPriority.Normal, new NextPrimeDelegate(CheckNextNumber)
);
Hope that helps!
While in most cases using either DispatcherObject.Dispatcher (all dependency objects and controls inherit from DispatcherObject, among others) or Application.Current.Dispatcher is the right thing to do, as there's usually only one UI thread, there can be multiple UI threads and different windows can use different dispatchers. In that case, it's important to update the control using its dispatcher. It's stored in in its Dispatcher property (inherited from DispatcherObject), any other control in this window and the window itself.
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.
I'm trying to make some sense out of an application Ive been handed in order to track down the source of an error. Theres a bit of code (simplified here) which creates four threads which in turn populate list views on the main form. Each method gets data from the database and retrieves graphics from a resource dll in order to directly populate an imagelist and listview.
From what Ive read on here (link) updating UI elements from any thread other than the UI thread should not be done, and yet this appears to work?
Thread t0 = new Thread(new ThreadStart(PopulateListView1));
t0.IsBackground = true;
t0.Start();
Thread t1 = new Thread(new ThreadStart(PopulateListView2));
t1.Start();
Thread t2 = new Thread(new ThreadStart(PopulateListView3));
t2.Start();
Thread t3 = new Thread(new ThreadStart(PopulateListView4));
t3.Start();
The error itself is a System.InvalidOperationException "Image cannot be added to the ImageList." which has me wondering if the above code is linked in some way.
Iis this method of populating the UI recommended and if not what are the possible complications resulting from it?
Update:
I may have given some misinformation by referring to a 'form'. The application is a windows forms application but the code is from a plugin application based on a usercontrol. The threads are created inside an initialise method publically exposed by this control. The listviews etc are also part of this plugin usercontrol.
DO NOT USE threads for that - if you have to do that async, use WOrkItems on a THreadPool. Thread usage in general should be reseved for long running items - a THreadPool or the new .NET 4.0 tasks API are way better suited for that.
UI elements should only be namipulated from the element creation thread. It "works" or not depending on what version of the .net framework you use or what the control really is if you break this.
Within your threads methods, such as DoWork() for the BackgroundWorker class, for example, you will need to instiate a delegate method to populate your UI control. Then, verifying whether your UI control requires to be invoked (InvokeRequired property), then invoking it when it'srequired to.
private delegate IList<MyObject> PopulateUiControl();
private void myThread_DoWork(object sender, DoWorkEventArgs e) {
PopulateUiControl myDelegate = FillUiControl;
while(uiControl.InvokeRequired)
uiControl.Invoke(myDelegate);
}
private IList<MyObject> FillUiControl() {
uiControl.Items = myThreadResultsITems;
}
It is not a precise working code, as I can't take the time to do the research, etc. but this shall put you in the path to succeed.
In the end, I agree with the others, try to avoid such things in the future, as it can get tricky to debug or reveal some strange behaviour. Perhaps .NET 4 has some improvements on the topic as Microsoft has worked hard to make parallelism easy for the use of multicore processors for developers.
As others have stated, you cannot update your UI from any thread other than the one it was created by.
If a thread wants to update the UI, it needs to invoke a method on the UI control on the same thread that created it using BeginInvoke.
PS: I am assuming when you say UI, you mean a WindowsForms and not WPF. I have used the above solution successfully in WinForms.
Update: Here are a couple of articles that explain the concept in-depth:
* Threading in Windows Forms
* WinForms UI Thread Invokes: An In-Depth Review of Invoke/BeginInvoke/InvokeRequred
Also, related question from SO: In WinForms, why can't you update UI controls from other threads?
Never update UI from a worker thread. Program may work sometimes, but this is undefined behavior. Add this line to the program initialization code:
Control.CheckForIllegalCrossThreadCalls = true;
After this every incorrect UI update attempt fails, allowing to fix all errors in the code.
If this is done because getting the values fr the listviews takes time, then get the values in a background worker and then use the main thread to bind the data to the listview.