My program has 2 threads running, thread 1 does something to control a label in a form running on thread 2. So I have to use a delegate and invoke a function in form 1 class to access the label. My code is below and it works perfectly. However, I'm wondering if there is a shorter, better way to do this?
delegate void Change_Status_Call_Back(string status_changed);
public void change_status(string status_changed)
{
if (this.label_status.InvokeRequired)
{
Change_Status_Call_Back obj = new Change_Status_Call_Back(change_status);
this.Invoke(obj, new object[] { status_changed });
}
else
{
this.label_status.Text = status_changed;
}
}
This question is "primarily opinion based". Still, you've touched a pet peeve of mine, so…
You should skip the InvokeRequired check altogether:
public void change_status(string status_changed)
{
this.Invoke((MethodInvoker)(() => this.label_status.Text = status_changed));
}
The framework has to effectively check InvokeRequired anyway, because it's required to support invoking on the UI thread without deadlocking. So the check in your code is redundant. The overhead of always wrapping the method body in a delegate invocation is inconsequential in UI code like this, especially since if you're writing this code, it's probably the case that the method's not going to be called exception when InvokeRequired would be true anyway (i.e. the "fast path" is never taken anyway).
Even better is to use a more modern mechanism for dealing with cross-thread access, such as async/await or the Progress<T> class. Then you never have to write an explicit call to Invoke() at all.
Some time ago, I ranted in more depth here: MSDN’s canonical technique for using Control.Invoke is lame
I would do this:
public void change_status(string status_changed)
{
this.label_status.InvokeSafely(c => c.Text = status_changed);
}
You need this extension method:
public static void InvokeSafely(this Control control, Action<Control> action)
{
if (control.InvokeRequired)
{
control.Invoke((Action)(() => action?.Invoke(control)));
}
else
{
action?.Invoke(control);
}
}
After looking around, I came up with this:
// UPDATE DISPLAY items (using Invoke in case running on BW thread).
IAsyncResult h = BeginInvoke((MethodInvoker)delegate
{
FooButton.Text = temp1;
BarUpdown.Value = temp2;
}
);
EndInvoke(h); // Wait for invoke to complete.
h.AsyncWaitHandle.Close(); // Explicitly close the wait handle.
// (Keeps handle count from growing until GC.)
Details:
I removed if (InvokeRequired) completely. (Discovered from Peter Duniho's answer here.) Invoke() works just fine on the UI thread. In code that runs only on the UI thread, UI actions need no special treatment. In code that runs only on a non-UI thread, wrap all UI actions in an Invoke(). In code that can run on the UI thread -or- a non-UI thread, likewise wrap all UI actions in an Invoke(). Always using Invoke() adds some overhead when running on the UI thread, but: not much overhead (I hope); the actions run less often on the UI thread anyway; and by always using Invoke, you don't have to code the UI actions twice. I'm sold.
I replaced Invoke() with BeginInvoke() .. EndInvoke() .. AsyncWaitHandle.Close(). (Found elsewhere.) Invoke() probably just does BeginInvoke() .. EndInvoke(), so that much is just inline expansion (slightly more object code; slightly faster execution). Adding AsyncWaitHandle.Close() addresses something else: When running on a non-UI thread, Invoke() leaves hundreds of handles that linger until garbage collection. (It's scary to watch Handles count grow in Task Manager.) Using BeginInvoke() .. EndInvoke() leaves lingering handles just the same. (Surprise: Using only BeginInvoke() does not leave the handles; it looks like EndInvoke() is the culprit.) Using AsyncWaitHandle.Close() to explicitly kill the dead handles eliminates the [cosmetic] problem of lingering handles. When running on the UI thread, BeginInvoke() .. EndInvoke() (like Invoke()) leaves no handles, so AsyncWaitHandle.Close() is unnecessary, but I assume it is also not costly.
An IsDisposed test seems safe against race conditions, but I think it is not necessary. I'm worried that BackgroundWorker can Invoke() the operation; while it is pending, a click can trigger a callback on the UI thread that can Close() the form, and then the message loop executes this operation. (Not sure this can happen.)
Problem: (I will update here when something works.) I changed all my UI updates from running on a UI timer kludge to using Invoke() (as above), and now closing the form fails on a race condition about 20% of the time. If a user click stops my background worker, clicking on close after that works fine. BUT, if the user clicks directly on close, that triggers a callback on the UI thread which Close()s the form; that triggers another that flags the background worker to stop; the background worker continues, and it crashes at EndInvoke() saying "Cannot access a disposed object. Object name: 'MainWin'. at System.Windows.Forms.Control.MarshaledInvoke(Control caller, Delegate method, Object[] args, Boolean synchronous) ...". Adding if (!this.IsDisposed) {} around EndInvoke() .. AsyncWaitHandle.Close() doesn't fix it.
Option: Go back to using a forms timer: Make the BW write its changes into a dozen global "mailbox" variables. Have the timer do FooButton.Text = nextFooButtonText;, etc. Most such assignments will do almost nothing because setting a form field only updates the display if the value actually changes. (For clarity and to reduce copying objects, initialize the mailbox variables to null, and have the timer do if (nextFooButtonText != null) { FooButton.Text = nextFooButtonText; nextFooButtonText = null; }, etc.) The timer puts a new event on the UI message loop every so many milliseconds, which is more silly grinding than the Invoke()s. Updating the display on a timer callback delays each update by [up to] the timer interval. (Yuck.)
WORKING Option: Use only BeginInvoke(). Why make BW wait for each Invoke to finish? 1) temp1 and temp2 seem passed as references - if they get changed after BeginInvoke(), the new value wins. (But that's not so bad.) 2) temp1 and temp2 can go out of scope. (But aren't they safe against being released until the last reference goes away?) 3) Waiting ensures that BW only has one invoked action pending at a time - if the UI thread blocks for a while, BW can't bury it in events. (But my UI thread can't block, at least not at times when my BW is running.)
Option: Put try .. catch around the EndInvoke(). (Yuck.)
I have seen several other tricks suggested:
•Have Close cancel itself, initiate a timer, and then return so that any lingering Invoke()s finish on the UI thread; shortly after that the timer callback does a real Close (found here; from here).
•Kill the background worker thread.
•Alter Program.cs to shut down differently.
Related
I have a windows forms application
on which I need to use a for loop having a large number of Remote Calls around 2000 - 3000 calls,
and while executing the for loop, I loose my control on form and form controls, as it becomes a large process and some time it shows "Not Responding" but if I wait for a long it comes back again, I think I need to use some threading model for that, is there any idea, how can I proceed to solve the issue?
You need to perform the long running operation on a background thread.
There are several ways of doing this.
You can queue the method call for execution on a thread pool thread (See here):
ThreadPool.QueueUserWorkItem(new WaitCallback(YourMethod));
In .NET 4.0 you can use the TaskFactory:
Task.Factory.StartNew(() => YourMethod());
And in .NET 4.5 and later, you can (and should, rather than TaskFactory.StartNew()) use Task.Run():
Task.Run(() => YourMethod());
You could use a BackgroundWorker for more control over the method if you need things like progress updates or notification when it is finished. Drag the a BackgroundWorker control onto your form and attach your method to the dowork event. Then just start the worker when you want to run your method. You can of course create the BackgroundWorker manually from code, just remember that it needs disposing of when you are finished.
Create a totally new thread for your work to happen on. This is the most complex and isn't necessary unless you need really fine grained control over the thread. See the MSDN page on the Thread class if you want to learn about this.
Remember that with anything threaded, you cannot update the GUI, or change any GUI controls from a background thread. If you want to do anything on the GUI you have to use Invoke (and InvokeRequired) to trigger the method back on the GUI thread. See here.
private voidForm_Load(object sender, EventArgs e)
{
MethodInvoker mk = delegate
{
//your job
};
mk.BeginInvoke(callbackfunction, null);
}
private void callbackfunction(IAsyncResult res)
{
// it will be called when your job finishes.
}
use MethodInvoker is the easiest way.
Obviously, you need to use background threads. I suggest you read this free e-book.
I have a UserControl with a TreeView control called mTreeView on it. I can get data updates from multiple different threads, and these cause the TreeView to be updated. To do this, I've devised the following pattern:
all data update event handlers must acquire a lock and then check for InvokeRequired; if so, do the work by calling Invoke. Here's the relevant code:
public partial class TreeViewControl : UserControl
{
object mLock = new object();
void LockAndInvoke(Control c, Action a)
{
lock (mLock)
{
if (c.InvokeRequired)
{
c.Invoke(a);
}
else
{
a();
}
}
}
public void DataChanged(object sender, NewDataEventArgs e)
{
LockAndInvoke(mTreeView, () =>
{
// get the data
mTreeView.BeginUpdate();
// perform update
mTreeView.EndUpdate();
});
}
}
My problem is, sometimes, upon startup, I will get an InvalidOperationException on mTreeView.BeginUpdate(), saying mTreeView is being updated from a thread different than the one it was created. I go back in the call stack to my LockAndInvoke, and lo and behold, c.InvokeRequired is true but the else branch was taken! It's as if InvokeRequired had been set to true on a different thread after the else branch was taken.
Is there anything wrong with my approach, and what can I do to prevent this?
EDIT: my colleague tells me that the problem is that InvokeRequired is false until the control is created, so this is why it happens on startup. He's not sure what to do about it though. Any ideas?
It is a standard threading race. You are starting the thread too soon, before the TreeView is created. So your code sees InvokeRequired as false and fails when a split second later the native control gets created. Fix this by only starting the thread when the form's Load event fires, the first event that guarantees that all the control handles are valid.
Some mis-conceptions in the code btw. Using lock is unnecessary, both InvokeRequired and Begin/Invoke are thread-safe. And InvokeRequired is an anti-pattern. You almost always know that the method is going to be called by a worker thread. So use InvokeRequired only to throw an exception when it is false. Which would have allowed diagnosing this problem early.
When you marshal back to the UI thread, it's one thread--it can do only one thing at at time. You don't need any locks when you call Invoke.
The problem with Invoke is that it blocks the calling thread. That calling thread usually doesn't care about what get's completed on the UI thread. In that case I recommend using BeginInvoke to marshal the action back to the UI thread asynchronously. There are circumstances where the background thread can be blocked on Invoke while the UI thread can be waiting for the background thread to complete something and you end up with a deadlock: For example:
private bool b;
public void EventHandler(object sender, EventArgs e)
{
while(b) Thread.Sleep(1); // give up time to any other waiting threads
if(InvokeRequired)
{
b = true;
Invoke((MethodInvoker)(()=>EventHandler(sender, e)), null);
b = false;
}
}
... the above will deadlock on the while loop while because Invoke won't return until the call to EventHandler returns and EventHandler won't return until b is false...
Note my use of a bool to stop certain sections of code from running. This is very similar to lock. So, yes, you can end up having a deadlock by using lock.
Simply do this:
public void DataChanged(object sender, NewDataEventArgs e)
{
if(InvokeRequired)
{
BeginInvoke((MethodInvoker)(()=>DataChanged(sender, e)), null);
return;
}
// get the data
mTreeView.BeginUpdate();
// perform update
mTreeView.EndUpdate();
}
This simply re-invokes the DataChanged method asynchronously on the UI thread.
The pattern as you have shown it above looks 100% fine to me (albeit with some extra unnecessary locking, however I can't see how this would cause the problem you have described).
As David W points out, the only difference between what you are doing and this extension method is that you directly access mTreeView on the UI thread instead of passing it in as an argument to your action, however this will only make a difference if the value of mTreeView changes, and in any case you would have to try fairly hard to get this to cause the problem you have described.
Which means that the problem must be something else.
The only thing that I can think of is that you may have created mTreeView on a thread other than the UI thread - if this is the case then accessing the tree view will be 100% safe, however if you try and add that tree view to a form which was created on a different thread then it will go bang with an exception similar to the one that you describe.
Im trying to upload some images using the Flickr.net API.The Images are uploaded but the User Interface freezes.I have inserted the code for uploading in a Background worker
private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
foreach (var item in imagelist)
{
flickr.UploadPicture(item, Path.GetFileName(item), null, null, true, false, true);
}
MessageBox.Show("Success");
}
The flickr object is created earlier from another form and passed to this form. I call the worker with if(worker.IsBusy==false){backgroundWorker1.RunWorkerAsync();} when a button is clicked.
Two common causes for this, your snippet is way too brief to narrow down which it might be. First is the ReportProgress method, the event handler runs on the UI thread. If you call it too often then the UI thread can get flooded with invoke requests and spend too much time to handle them. It doesn't get around to doing its regular duties anymore, like responding to paint requests and processing user input. Because as soon as it is done handling a invoke request, there's another one waiting to get dispatched. The UI thread isn't actually frozen, it just looks like it is. The net effect is the same. You'll need to fix it by slowing down the worker or call ReportProgress less often.
The second cause is your flicker object not being thread-safe and itself ensuring that it is used in a thread-safe way. By marshaling the call from the worker thread to the UI thread automatically. This is very common for COM components, this kind of marshaling is a core feature of COM. Again the UI thread isn't actually frozen, but it still won't handle paint and input since it is busy uploading a photo. You'll need to fix it by creating the flicker object on the worker thread. With good odds that you can't do this with a BackgroundWorker, such a component often needs an STA thread that pumps a message loop. Which requires Thread.SetApartmentState() and Application.Run().
If you are doing something like:
while(worker.IsBusy)
{
}
to wait for it to finish, this will hang because it ties up the UI thread in the loop and since the background worker needs to invoke onto the UI thread to set the busy property safely there is a dead lock.
Anytime the beginner asks something like: How to update the GUI from another thread in C#?, the answer is pretty straight:
if (foo.InvokeRequired)
{
foo.BeginInvoke(...)
} else {
...
}
But is it really good to use it? Right after non-GUI thread executes foo.InvokeRequired the state of foo can change. For example, if we close form right after foo.InvokeRequired, but before foo.BeginInvoke, calling foo.BeginInvoke will lead to InvalidOperationException: Invoke or BeginInvoke cannot be called on a control until the window handle has been created. This wouldn't happen if we close the form before calling InvokeRequired, because it would be false even when called from non-GUI thread.
Another example: Let's say foo is a TextBox. If you close form, and after that non-GUI thread executes foo.InvokeRequired (which is false, because form is closed) and foo.AppendText it will lead to ObjectDisposedException.
In contrast, in my opinion using WindowsFormsSynchronizationContext is much easier - posting callback by using Post will occur only if thread still exists, and synchronous calls using Send throws InvalidAsynchronousStateException if thread not exists anymore.
Isn't using WindowsFormsSynchronizationContext just easier? Am I missing something? Why should I use InvokeRequired-BeginInvoke pattern if it's not really thread safe? What do you think is better?
WindowsFormsSynchronizationContext works by attaching itself to a special control that is bound to the thread where the context is created.
So
if (foo.InvokeRequired)
{
foo.BeginInvoke(...)
} else {
...
}
Can be replaced with a safer version :
context.Post(delegate
{
if (foo.IsDisposed) return;
...
});
Assuming that context is a WindowsFormsSynchronizationContext created on the same UI thread that foo was.
This version avoid the problem you evoke :
Right after non-GUI thread executes foo.InvokeRequired the state of foo can change. For example, if we close form right after foo.InvokeRequired, but before foo.BeginInvoke, calling foo.BeginInvoke will lead to InvalidOperationException: Invoke or BeginInvoke cannot be called on a control until the window handle has been created. This wouldn't happen if we close the form before calling InvokeRequired, because it would be false even when called from non-GUI thread.
Beware of some special cases with WindowsFormsSynchronizationContext.Post if you play with multiple message loops or multiple UI threads :
WindowsFormsSynchronizationContext.Post will execute the delegate only if there still is a message pump on the thread where it was created. If there isn't nothing happens and no exception is raised.Also if another message pump is later attached to the thread (Via a second call to Application.Run for example) the delegate will execute (It's due to the fact that the system maintain a message queue per thread without any knowledge about the fact that someone is pumping message from it or not)
WindowsFormsSynchronizationContext.Send will throw InvalidAsynchronousStateException if the thread it's bound to isn't alive anymore. But if the thread it's bound to is alive and doesn't run a message loop it won't be executed immediately but will still be placed on the message queue and executed if Application.Run is executed again.
None of these cases should execute code unexpectedly if IsDisposed is called on a control that is automatically disposed (Like the main form) as the delegate will immediately exit even if it's executed at an unexpected time.
The dangerous case is calling WindowsFormsSynchronizationContext.Send and considering that the code will be executed: It might not, and there is now way to know if it did anything.
My conclusion would be that WindowsFormsSynchronizationContext is a better solution as long as it's correctly used.
It can create sublte problems in complex cases but common GUI applications with one message loop that live as long as the application itself will always be fine.
Who said InvokeRequired / Control.BeginInvoke is preferred? If you ask me, in most cases it's an anti pattern for the exact reasons you mentioned. The question you linked to has many answers, and some actually do suggest using the synchronization context (including mine).
While it's true that any given control could be disposed by the time you're trying to access it from the posted delegate, that's easily solved using Control.IsDisposed (as your delegate is executing on the UI thread so nothing can dispose controls while it's running):
public partial class MyForm : Form
{
private readonly SynchronizationContext _context;
public MyForm()
{
_context = SynchronizationContext.Current
//...
}
private MethodOnOtherThread()
{
//...
_context.Post(status =>
{
// I think it's enough to check the form's IsDisposed
// But if you want to be extra paranoid you can test someLabel.IsDisposed
if (!IsDisposed) {someLabel.Text = newText;}
},null);
}
}
I have a windows forms application
on which I need to use a for loop having a large number of Remote Calls around 2000 - 3000 calls,
and while executing the for loop, I loose my control on form and form controls, as it becomes a large process and some time it shows "Not Responding" but if I wait for a long it comes back again, I think I need to use some threading model for that, is there any idea, how can I proceed to solve the issue?
You need to perform the long running operation on a background thread.
There are several ways of doing this.
You can queue the method call for execution on a thread pool thread (See here):
ThreadPool.QueueUserWorkItem(new WaitCallback(YourMethod));
In .NET 4.0 you can use the TaskFactory:
Task.Factory.StartNew(() => YourMethod());
And in .NET 4.5 and later, you can (and should, rather than TaskFactory.StartNew()) use Task.Run():
Task.Run(() => YourMethod());
You could use a BackgroundWorker for more control over the method if you need things like progress updates or notification when it is finished. Drag the a BackgroundWorker control onto your form and attach your method to the dowork event. Then just start the worker when you want to run your method. You can of course create the BackgroundWorker manually from code, just remember that it needs disposing of when you are finished.
Create a totally new thread for your work to happen on. This is the most complex and isn't necessary unless you need really fine grained control over the thread. See the MSDN page on the Thread class if you want to learn about this.
Remember that with anything threaded, you cannot update the GUI, or change any GUI controls from a background thread. If you want to do anything on the GUI you have to use Invoke (and InvokeRequired) to trigger the method back on the GUI thread. See here.
private voidForm_Load(object sender, EventArgs e)
{
MethodInvoker mk = delegate
{
//your job
};
mk.BeginInvoke(callbackfunction, null);
}
private void callbackfunction(IAsyncResult res)
{
// it will be called when your job finishes.
}
use MethodInvoker is the easiest way.
Obviously, you need to use background threads. I suggest you read this free e-book.