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.
Related
I am facing an issue with communication between threads in a C#.NET application.
Hope someone will guide me in the right direction about the possible solutions.
I have an application in C#.NET.It is a windows form application.
My application has two threads - One thread is the main thread (UI thread) and the other one is the child thread. Lets call the child thread the "workerThread"
There is only one form used in the application.Lets call this form the "MainForm"
The child thread is started when the MainForm loads (used the form's "Load" event handler to start the thread)
In the MainForm class, I have a variable named "stopWork" which is a public boolean variable and it serves as a flag to indicate whether the child thread should continue working or should it stop
I have another class (besides the MainForm class) which contains the method that I execute in the the child thread. Lets call this second class the "WorkerClass".
I pass a reference to the current form (the MainForm) into the constructor of the "WorkerClass"
I have a button "stop" in the main form which sets "stopWork" to "true" if its clicked and then calls "workerThread.Join()" to wait for the child thread to finish excecution.
In the child thread, the method "doWork" keeps checking the status of "parentForm.stopWork" inside a for loop. If "stopWork" is set to "true" then the loop breaks and subsequently the method ends.
Now, the issue is, once I am clicking the "stop" button ,the application hangs.
I am pasting parts of the code below so that it is easier to understand :
public partial class MainForm : Form
{
Thread workerThread = null;
ThreadStart workerThreadStart = null;
WorkerClass workerClass = null;
public bool stopWork = true;
/*.......... some code ............*/
private void MainForm_Load(object sender, EventArgs e)
{
workerThreadStart = new ThreadStart(startWork);
workerThread = new Thread(workerThreadStart);
stopWork = false;
workerThread.Start();
}
private void startWork()
{
workerClass = new WorkerClass(this);
}
private void buttonStop_Click(object sender, EventArgs e) //"stop" button
{
if (workerThread != null)
{
if (workerThread.IsAlive == true)
{
stopWork = true;
workerThread.Join();
}
}
}
/*.......... some more code ............*/
}
public class WorkerClass
{
MainForm parentForm=null;
/*......... some variables and code ........*/
public WorkerClass(MainForm parentForm)
{
this.parentForm=parentForm;
}
/* .............. some more code ...........*/
public void doWork()
{
/*.......... some variables and code ...........*/
for(int i=0;i<100000;i++)
{
// ** Here is the check to see if parentForm has set stopWork to true **
if(parentForm.stopWork==true)
break;
/*......... do some work in the loop ..........*/
}
}
/********* and more code .........*/
}
I think I may know where the problem lies.
The problem is in the "doWork" method in the child thread trying to access "stopWork" variable in the parent form when already the parent form is blocked by calling the "workerThread.Join()" method. So ,I think this is a "deadlock" problem.
Am I right in identifying the problem ? Or am I wrong and the problem lies somewhere else ?
In case this is indeed a deadlock, what are the possible solutions to solve this ?
I did a bit of googling and found lots of resources on thread synchronisation and how to avoid deadlocks. But I could not understand how to apply them specifically to my problem.
I would really appreciate any help or guidance on resolving this issue.
Yes, the code you wrote is highly vulnerable to deadlock. The BackgroundWorker class is especially prone to cause this kind of deadlock.
The problem is located in code we can't see in your snippet, the WorkerClass. You are surely doing something there that affects the UI in one way or another, always the primary reason to consider creating a thread in the first place. You probably use Control.Invoke() to have some code run on the UI thread and update a control. Perhaps also to signal that the worker thread is completed and, say, set the Enable property of a button back to true.
That's deadlock city, such code cannot run until the UI thread goes idle, back to pumping its message loop. It will never be idle in your case, it is stuck in Thread.Join(). The worker thread can't complete because the UI thread won't go idle, the UI thread can't go idle because the worker thread isn't finishing. Deadlock.
BackgroundWorker has this problem too, the RunWorkerCompleted event cannot run unless the UI thread is idle. What you need to do is not block the UI thread. Easier said than done, BGW can help you get this right because it runs an event when it completes. You can have this event do whatever you now do in the code past the Thread.Join() call. You'll need a boolean flag in your class to indicate that you are in the 'waiting for completion' state. This answer has relevant code.
Use a BackgroundWorker for this task instead. When you want to stop the task's execution, call the background worker's CancelAsync method.
Generally speaking, rolling your own threading code (on any platform) is a recipe for disaster if you don't have an expert-level understanding of multithreading (and even then it's still dangerous).
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.
please consider the following scenario for .net 2.0:
I have an event that is fired on system.Timers.Timer object. The subscriber then adds an item to a Windows.Forms.Listbox upon receiving the event. This results in a cross-thread exception.
My question is what would be the best way to handle this sort of situation. The solutions that I have come up with is as follows:
private delegate void messageDel(string text);
private void ThreadSafeMsg(string text)
{
if (this.InvokeRequired)
{
messageDel d = new messageDel(ThreadSafeMsg);
this.Invoke(d, new object[] { text });
}
else
{
listBox1.Items.Add(text);
listBox1.Update();
}
}
// event
void Instance_Message(string text)
{
ThreadSafeMsg(text);
}
Is this the optimum way to handle this in .net 2? What about .net 3.5?
There's no point in using Control.InvokeRequired, you know that it always is. The Elapsed event is raised on a threadpool thread, never the UI thread.
Which makes it kinda pointless to use a System.Timers.Timer, just use the System.Windows.Forms.Timer. No need to monkey with Control.Begin/Invoke, you can't crash your program with an ObjectDisposedException when the event is raised just as the user closes the form.
You have a cross thread exception because you are trying to access items from outside the UI thread. Delegates are necessary in order to hook into the message pump and make the UI change.
If you use the Form Timer, then you'll be in the UI thread. You'll have the same problem, however, if you use a BackgroundWorkerThread and you'll need a delegate there as well.
See Threading in Windows Forms
It is pretty much the same in .net 3.5, since it is related to Windows Forms and cross-threading when you are accessing the UI thread from some another working thread.
However, you can make the code smaller, by using the generic Action<> and Func<>, avoiding creating manually the delegates.
Something like this:
private void ThreadSafeMsg(string text)
{
if (this.InvokeRequired)
this.Invoke(new Action<string>(ThreadSafeMsg), new object[] { text });
else
{
// Stuff...
}
}
The easiest solution in your case - is using System.Windows.Forms.Timer class, but in general case you may use following solution to access you GUI-stuff from non-GUI thread (this solution applicable for .net 2.0 but it more elegant for .net 3.5):
public static class ControlExtentions
{
public static void InvokeIfNeeded(this Control control, Action doit)
{
if (control.InvokeRequired)
control.Invoke(doit);
else
doit();
}
}
And you may use it like this no mater from what thread, from UI or from another one:
this.InvokeIfNeeded(()=>
{
listBox1.Items.Add(text);
listBox1.Update();
});
Depending upon what your action is doing, Control.BeginInvoke may be better than Control.Invoke. Control.Invoke will wait for the UI thread to process your message before it returns. If the UI thread is blocked, it will wait forever. Control.BeginInvoke will enqueue a message for the UI thread and return immediately. Because there's no way to avoid an exception if a control gets disposed immediately before you try to BeginInvoke it, you need to catch (possibly swallow) the exception (I think it may be either ObjectDisposedException or IllegalOperationException depending upon timing). You also need to set a flag or counter when you're about to post a message and clear or decrement it in the message handler (probably use Threading.Interlocked.Increment/Decrement), to ensure that you don't enqueue an excessive number of messages while the UI thread is blocked.
i am working with a winforms control that is both a GUI element and also does some internal processing that has not been exposed to the developer. When this component is instantiated it may take between 5 and 15 seconds to become ready so what i want to do is put it on another thread and when its done bring it back to the gui thread and place it on my form. The problem is that this will (and has) cause a cross thread exception.
Normally when i work with worker threads its just with simple data objects i can push back when processing is complete and then use with controls already on the main thread but ive never needed to move an entire control in this fashion.
Does anyone know if this is possible and if so how? If not how does one deal with a problem like this where there is the potential to lock the main gui?
You don't need to lock the GUI, you just need to call invoke:
Controls in Windows Forms are bound to
a specific thread and are not thread
safe. Therefore, if you are calling a
control's method from a different
thread, you must use one of the
control's invoke methods to marshal
the call to the proper thread. This
property can be used to determine if
you must call an invoke method, which
can be useful if you do not know what
thread owns a control. ref
Here is how it looks in code:
public delegate void ComponentReadyDelegate(YourComponent component);
public void LoadComponent(YourComponent component)
{
if (this.InvokeRequired)
{
ComponentReadyDelegate e = new ComponentReadyDelegate(LoadComponent);
this.BeginInvoke(e, new object[]{component});
}
else
{
// The component is used by a UI control
component.DoSomething();
component.GetSomething();
}
}
// From the other thread just initialize the component
// and call the LoadComponent method on the GUI.
component.Initialize(); // 5-15 seconds
yourForm.LoadComponent(component);
Normally calling the LoadComponent from another thread will cause a cross-thread exception, but with the above implementation the method will be invoked on the GUI thread.
InvokeRequired tells you if:
the caller must call an invoke method
when making method calls to the
control because the caller is on a
different thread than the one the
control was created on.
ref
Update:
So if I understand you correctly the control object is created on a thread other than the GUI thread, therefore even if you were able to pass it to the GUI thread you still won't be able to use it without causing a cross-thread exception. The solution would be to create the object on the GUI thread, but initialize it on a separate thread:
public partial class MyForm : Form
{
public delegate void ComponentReadyDelegate(YourComponent component);
private YourComponent _component;
public MyForm()
{
InitializeComponent();
// The componet is created on the same thread as the GUI
_component = new YourComponent();
ThreadPool.QueueUserWorkItem(o =>
{
// The initialization takes 5-10 seconds
// so just initialize the component in separate thread
_component.Initialize();
LoadComponent(_component);
});
}
public void LoadComponent(YourComponent component)
{
if (this.InvokeRequired)
{
ComponentReadyDelegate e = new ComponentReadyDelegate(LoadComponent);
this.BeginInvoke(e, new object[]{component});
}
else
{
// The component is used by a UI control
component.DoSomething();
component.GetSomething();
}
}
}
Without knowing too much about the object. To avoid cross thread exceptions, you can make the initial thread invoke a call (Even if you are calling from a thread).
Copied and pasted from one of my own applications :
private delegate void UpdateStatusBoxDel(string status);
private void UpdateStatusBox(string status)
{
listBoxStats.Items.Add(status);
listBoxStats.SelectedIndex = listBoxStats.Items.Count - 1;
labelSuccessful.Text = SuccessfulSubmits.ToString();
labelFailed.Text = FailedSubmits.ToString();
}
private void UpdateStatusBoxAsync(string status)
{
if(!areWeStopping)
this.BeginInvoke(new UpdateStatusBoxDel(UpdateStatusBox), status);
}
So essentially the threaded task will call the "Async" method. Which will then tell the main form to begininvoke (Actually async itself).
I believe there is probably a shorter way to do all of this, without the need for creating delegates and two different methods. But this way is just ingrained into me. And it's what the Microsoft books teach to you do :p
The BackgroundWorker class is designed for exactly this situation. It will manage the thread for you, and let you start the thread, as well as cancel the thread. The thread can send events back to the GUI thread for status updates, or completion. The event handlers for these status and completion events are in the main GUI thread, and can update your WinForm controls. And the WinForm doesn't get locked. It's everything you need. (And works equally well in WPF and Silverlight, too.)
The control must be created and modified from the UI thread, there's no way around that.
In order to keep the UI responsive while doing long-running initialization, keep the process on a background thread and invoke any control access. The UI should remain responsive, but if it doesn't, you can add some wait time to the background thread. This is an example, using .Net 4 parallel tools: http://www.lovethedot.net/2009/01/parallel-programming-in-net-40-and_30.html
If interaction with the specific control being initialized can't be allowed until initialization finishes, then hide or disable it until complete.
Lets say I have a component called Tasking (that I cannot modify) which exposes a method “DoTask” that does some possibly lengthy calculations and returns the result in via an event TaskCompleted. Normally this is called in a windows form that the user closes after she gets the results.
In my particular scenario I need to associate some data (a database record) with the data returned in TaskCompleted and use that to update the database record.
I’ve investigated the use of AutoResetEvent to notify when the event is handled. The problem with that is AutoResetEvent.WaitOne() will block and the event handler will never get called. Normally AutoResetEvents is called be a separate thread, so I guess that means that the event handler is on the same thread as the method that calls.
Essentially I want to turn an asynchronous call, where the results are returned via an event, into a synchronous call (ie call DoSyncTask from another class) by blocking until the event is handled and the results placed in a location accessible to both the event handler and the method that called the method that started the async call.
public class SyncTask
{
TaskCompletedEventArgs data;
AutoResetEvent taskDone;
public SyncTask()
{
taskDone = new AutoResetEvent(false);
}
public string DoSyncTask(int latitude, int longitude)
{
Task t = new Task();
t.Completed = new TaskCompletedEventHandler(TaskCompleted);
t.DoTask(latitude, longitude);
taskDone.WaitOne(); // but something more like Application.DoEvents(); in WinForms.
taskDone.Reset();
return data.Street;
}
private void TaskCompleted(object sender, TaskCompletedEventArgs e)
{
data = e;
taskDone.Set(); //or some other mechanism to signal to DoSyncTask that the work is complete.
}
}
In a Windows App the following works correctly.
public class SyncTask
{
TaskCompletedEventArgs data;
public SyncTask()
{
taskDone = new AutoResetEvent(false);
}
public string DoSyncTask(int latitude, int longitude)
{
Task t = new Task();
t.Completed = new TaskCompletedEventHandler(TaskCompleted);
t.DoTask(latitude, longitude);
while (data == null) Application.DoEvents();
return data.Street;
}
private void TaskCompleted(object sender, TaskCompletedEventArgs e)
{
data = e;
}
}
I just need to replicate that behaviour in a window service, where Application.Run isn’t called and the ApplicationContext object isn’t available.
I've had some trouble lately with making asynchronous calls and events at threads and returning them to the main thread.
I used SynchronizationContext to keep track of things. The (pseudo)code below shows what is working for me at the moment.
SynchronizationContext context;
void start()
{
//First store the current context
//to call back to it later
context = SynchronizationContext.Current;
//Start a thread and make it call
//the async method, for example:
Proxy.BeginCodeLookup(aVariable,
new AsyncCallback(LookupResult),
AsyncState);
//Now continue with what you were doing
//and let the lookup finish
}
void LookupResult(IAsyncResult result)
{
//when the async function is finished
//this method is called. It's on
//the same thread as the the caller,
//BeginCodeLookup in this case.
result.AsyncWaitHandle.WaitOne();
var LookupResult= Proxy.EndCodeLookup(result);
//The SynchronizationContext.Send method
//performs a callback to the thread of the
//context, in this case the main thread
context.Send(new SendOrPostCallback(OnLookupCompleted),
result.AsyncState);
}
void OnLookupCompleted(object state)
{
//now this code will be executed on the
//main thread.
}
I hope this helps, as it fixed the problem for me.
Maybe you could get DoSyncTask to start a timer object that checks for the value of your data variable at some appropriate interval. Once data has a value, you could then have another event fire to tell you that data now has a value (and shut the timer off of course).
Pretty ugly hack, but it could work... in theory.
Sorry, that's the best I can come up with half asleep. Time for bed...
I worked out a solution to the async to sync problem, at least using all .NET classes.
Link
It still doesn't work with COM. I suspect because of STA threading. The Event raised by the .NET component that hosts the COM OCX is never handled by my worker thread, so I get a deadlock on WaitOne().
someone else may appreciate the solution though :)
If Task is a WinForms component, it might be very aware of threading issues and Invoke the event handler on the main thread -- which seems to be what you're seeing.
So, it might be that it relies on a message pump happening or something. Application.Run has overloads that are for non-GUI apps. You might consider getting a thread to startup and pump to see if that fixes the issue.
I'd also recommend using Reflector to get a look at the source code of the component to figure out what it's doing.
You've almost got it. You need the DoTask method to run on a different thread so the WaitOne call won't prevent work from being done. Something like this:
Action<int, int> doTaskAction = t.DoTask;
doTaskAction.BeginInvoke(latitude, longitude, cb => doTaskAction.EndInvoke(cb), null);
taskDone.WaitOne();
My comment on Scott W's answer seems a little cryptic after I re-read it. So let me be more explicit:
while( !done )
{
taskDone.WaitOne( 200 );
Application.DoEvents();
}
The WaitOne( 200 ) will cause it to return control to your UI thread 5 times per second (you can adjust this as you wish). The DoEvents() call will flush the windows event queue (the one that handles all windows event handling like painting, etc.). Add two members to your class (one bool flag "done" in this example, and one return data "street" in your example).
That is the simplest way to get what you want done. (I have very similar code in an app of my own, so I know it works)
Your code is almost right... I just changed
t.DoTask(latitude, longitude);
for
new Thread(() => t.DoTask(latitude, longitude)).Start();
TaskCompleted will be executed in the same thread as DoTask does. This should work.