The exception is this:
System.InvalidOperationException: Invoke or BeginInvoke cannot be
called on a control until the window handle has been created.
First I'll explain the relations in my app.
There's a form named MainForm and another form named AssetsForm. The MainForm is creating an instance of the AssetsForm in the MainForm's constrcutor but doesn't AssetsForm.Show() it yet.
There is class named AssetsSource which implements the IObservable and sends data for display to the AssetsForm which implements the IObserver. When AssetsForm receives data to display, it creates a BackgroundWorker which handles the data and update a TreeView.
I've implemented the following wrong code to handle the UI updates from the BackgroundWorker:
private void Invoke(Control control, Action action)
{
if (control.InvokeRequired)
{
control.BeginInvoke(action);
}
else
{
control.Invoke(action);
}
}
It's wrong because instead of Invoke(action) I should have written action(); But I will refer to this later. Anyway, an InvalidOperationException was thrown from the Invoke(action) line of code. I can infer that the InvokeRequired evaluated to FALSE, although I update the TreeView from a BackgroundWorker !!
In MSDN it is written about Control.Invoke:
The Invoke method searches up the control's parent chain until it
finds a control or form that has a window handle if the current
control's underlying window handle does not exist yet. If no
appropriate handle can be found, the Invoke method will throw an
exception.
What is the parent chain and what is the window handle ? When the window handle is created ? I guess all this had to do with the fact that the AssetsForm is closed.
When I removed that line and uses only action(); as it should be, the program doesn't crash.
When the AssetsForm is opened before the AssetsSource sends updates to AssetsForm, by debugging I can see that InvokeRequired is evaluated to TRUE and BeginInvoke of the TreeView updates itself.
To sum everything up, I don't understand why when the AssetsForm is closed, then the InvokeRequired is false and the UI update (TreeView) is allowed to be from the thread that didn't create the TreeView.
As long as the window is not shown, Winforms doesn't need to stick on the UI-thread mechanismn. Therefore InvokeRequired returns false.
If you call Show() the window is opened and all UI activities need to be run through the event loop and therefore through the UI-thread.
Background: The restriction to handle UI activities only through the main thread is due to the fact that only one (windows) event loop is handling all UI related activities. To ensure that all activities are running in the correct order, all actions need to be run through one thread (at least in winforms). As long as the form is not shown, no events are triggered and therefore there's no need to enforce that all actions are run through the main thread.
EDIT: Add some background description.
Related
Form1.button_Click(...) {
// Show a dialog form, which runs a method <CheckBalance()> on it's OnLoad Event.
var modemDialog = new ModemDialog("COM25");
modemDialog.ShowDialog();
// the user can't see this dialog form until the method <CheckBalance()> terminates.
}
Is it possible to show first the dialog then run the specified method?
THanks.
That is correct and expected. Winforms UI is inherently single-threaded. Having a function call like "CheckBalance" in the form load event will prevent the form from showing until the form load event completes. Depending on the duration of the task, you have a number of options available to you:
If it's a fast task, compute it ahead of time before showing the form
If it's something the user may want to initiate, move it to a button on the new form, so it's only calculated on the request of the user
If it's a long running task that takes some time, you'll need to move it off in to another thread. Using a BackgroundWorker is recommended.
OnLoad occurs before the form is shown to allow you to initialise the form and variables and what not, which means it is synchronous. The form will not show until you return from that function.
If you want to asynchronously run the CheckBalance() method, then you can use a few techniques, such as utilising the Threading, ThreadPool or Tasks API to shift that work to a background thread, and returning immediately so that the form is shown.
Here is an example of using a Task to perform the same action, but asynchronously so that the form immediately shows:
Action<object> action = () => { CheckBalance(); };
new Task(action).Start();
Please note that if you access the UI thread, you'll need to beware of thread-safety and invocation.
The simple way to make sure your form is visible before CheckBalance is run is to use this code in the form load handler:
this.BeginInvoke((Action)(() => this.CheckBalance()));
This will push the execution of the CheckBalance method onto the UI thread message pump so will execute after all preceding UI code is complete.
Others are correct though that the UI will still be blocked as CheckBalance executes. You probably want to run it on a background thread to prevent this.
I have a WinForm set up and a process that loops until a button is pressed on the form.
When I try to run my code, the form does not even display. I suspect this is because the code gets stuck in the loop and doesn't get far enough to display the WinForm. How can I get the form to display and the loop to run after that point?
If you're looping because you need to do something with the GUI periodically while waiting for input, I suggest using a Timer control and its Tick event.
If you want to do non-GUI things while waiting, a more traditional timer is better suited to the task,
You should probably run the loop in a background thread. The BackgroundWorker class makes this pretty easy to do.
Don't do that.
Windows Forms (like most modern user interface development toolkits) is an event-driven framework. You should never use a loop that "waits" for something to happen; instead you want to use an event that triggers something to happen.
Essentially what's happening is this: WinForms has a loop running a message pump that listens for events from Windows and triggers C# events in response to them. Your code is executing on the same thread as that message pump (it has to, since in WinForms only one thread is allowed to touch any given control). So if you put that thread into a loop, the WinForms code that should be pumping messages isn't, and your user interface appears to hang, since it isn't responding to any messages from Windows. (If you keep clicking it, you will fill up the message queue and get a dialog box that says "This application has stopped responding, do you want to terminate?" or something like that.)
The correct solution is to do one of the following:
Use a Timer
Use a BackgroundWorker
Use a ThreadPool
Another solution that would work, but is not a good idea is:
Use Application.DoEvents() -- but please don't actually do this
Your form loading is freezing because the UI of a windows form runs in a single thread. And the logic that you put on the Load event of this form is running on that thread.
You can run your loop on a separate thread easily by using a BackgroundWorker component on your windows form. On the event DoWork of your background worker, you place the code that has the loop that should run without block your UI. On the Form.Load event, you can start the background worker component by calling the method RunWorkerAsync. On the event handler of your button, you place a code to stop the background worker by calling CancelAsync method.
The article How to: Implement a Form That Uses a Background Operation shows exactly how to accomplish it.
About your comment on not being able to update the Text of a TextBox from a your background worker component. It happens because it is not allowed to modify the state of a windows forms control from a different thread (your background worker code is running on a separated thread) MSDN documentation says:
Access to Windows Forms controls is not inherently thread safe. If you have two or more threads manipulating the state of a control, it is possible to force the control into an inconsistent state. Other thread-related bugs are possible, such as race conditions and deadlocks. It is important to make sure that access to your controls is performed in a thread-safe way.
A sample of how you can update the state of your windows forms controls from your background thread will be similar to the one below (assuming that the new value is already stored on a String variable named text):
// InvokeRequired required compares the thread ID of the
// calling thread to the thread ID of the creating thread.
// If these threads are different, it returns true.
if (this.textBox1.InvokeRequired)
{
SetTextCallback d = new SetTextCallback(SetText);
this.Invoke(d, new object[] { text });
}
else
{
this.textBox1.Text = text;
}
I borrowed this code snipped from How to: Make Thread-Safe Calls to Windows Forms Controls article. It can provide you more information about how to deal with multi-threaded windows forms.
You can use the form load event to trigger the start of the loop.
So it would handle the event Me.Load
However is it necessary for your loop to be happening inside of the UI?
This happens because your loop is keeping the window function from processing messages, such as those that tell it to repaint itself. Place a call to Application.DoEvents() inside of your loop to allow the UI to continue to function.
However, you need to ask yourself why you're looping like this in the first place. If you're, say, copying a bunch of files, this might make sense. For most tasks, though, responding to a timer tick should do the trick and won't block the UI.
You should run your loop in a background thread using the BackgroundWorker component.
Remember that the background thread cannot directly interact with the UI controls.
To report the progress on the UI, you should call the BackgroundWorker's ReportProgress method in the background thread, and handle the ProgressChanged event to update the UI.
You can call the CancelAsync method when the Button is clicked, and loop until the CancellationPending property is true.
I've got a Windows Forms (C#) project with multiple comboboxes/listboxes etc that are populated when the form loads.
The problem is that the loading of the comboboxes/listboxes is slow, and since the loading is done when the form is trying to display the entire form isn't shown until all the controls have been populated. This can in some circumstances be 20+ seconds.
Had there been a Form_finished_loaded type of event I could have put my code in there, but I can't find an event that is fired after the form is done drawing the basic controls.
I have one requirement though - the loading has to be done in the main thread (since I get the items from a non-threading friendly COM-application).
I have found one potential solution, but perhaps there is a better way?
I can create a System.Timer.Timer when creating the form, and have the first Tick be called about 1 second later, and then populate the lists from that tick. That gives the form enough time to be displayed before it starts filling the lists.
Does anyone have any other tips on how to delay the loading of the controls?
There is the Shown event that "occurs whenever the form is first displayed.". Also you may want to use the BeginUpdate and EndUpdate functions to make the populating of your combobox faster.
It has that certain smell of workaround, but this approach should fulfil your needs:
private bool _hasInitialized = false;
private void Form1_Shown(object sender, EventArgs e)
{
if (!_hasInitialized)
{
ThreadPool.QueueUserWorkItem(state =>
{
Thread.Sleep(200); // brief sleep to allow the main thread
// to paint the form nicely
this.Invoke((Action)delegate { LoadData(); });
});
}
}
private void LoadData()
{
// do the data loading
_hasInitialized = true;
}
What it does is that it reacts when the form is shown, checks if it has already been initialized before, and if not it spawns a thread that will wait for a brief moment before calling the LoadData method on the main thread. This will allow for the form to get painted properly. The samething could perhaps be achieve by simply calling this.Refresh() but I like the idea of letting the system decide how to do the work.
I would still try to push the data loading onto a worker thread, invoking back on the main thread for populating the UI (if it is at all possible with the COM component).
Can you get your data from a web service that calls the COM component?
That way, you can display empty controls on a Locked form at the start, make Asynchronous calls to get the data, and on return populate the respective combos, and once all of them are loaded, you can unlock the form for the user to use.
You could listen for the VisibleChanged event and the first time it's value is true you put your initialization code.
Isn't FormShown the event you're looking for?
When you say that you cannot use a background thread because of COM what do you mean? I am using many COM components within my apps and running them on background threads.
If you create a new thread as an STAThread you can probably load the ComboBox/ListBox on a Non-UI thread. IIRC the ThreadPool allocates worker threads as MTAThread so you'll need to actually create a thread manually instead of using ThreadPool.QueueUserWorkItem.
I am having some threading issues with a large app I am working on (getting cross-thread exceptions). Is there a way to find the thread name/id that a particular control was created on?
The error occurs when I try to add a new control to my control's control collection. I can't really create a small, reproducible sample so I will describe it as best as I can.
I have a main control that sits on a form, call it _mainControl. In its constructor I instantiate an instance of another control, something like
ChildControl _childControl = new ChildControl();
Now _childControl exists but I do not add it to _mainControls collection yet.
Eventually, _mainControl receives an event notification that I should add the control. In the event handler I check if this.InvokeRequired and if it is, I Invoke the handler, something like the following:
AddControlEventHander(...)
{
if(InvokeRequired)
{
BeginInvoke(new MethodInvoker(AddControlEventHander);
return;
}
Controls.Add(_childControl);
}
The exception is always thrown at Controls.Add ("Cross-thread operation not valid: Control '_item' accessed from a thread other than the thread it was created on").
Now, what I don't understand is how this is possible. I created _childControl on the same thread that _mainControl was created on. When I look at the threads window while I am debugging, the current thread name/id is the same when I call Control.Add as it was when the _childControl was added. However, the thing that confuses me the most are the following calls from _mainControl:
InvokeReuqired == false;
_childControl.InvokeRequired == false;
_childControl._item.InvokeRequired == true; //I made _item public just to try this and it returns true!
How is that possible? Is it possible for _childControl to be created on one thread while its children are created somehow on another? All of _childControl's children are created during initialization, as is normally done.
If anyone has any tips/suggestions as to what might be going on please let me know.
Thanks.
Edit:
In case anyone is interested, I found out what was happening. I was curious as to how a control can be created on one thread and it's children created on a another thread even though the InitializeComponent was all done on the same thread. So, I found out which thread the child was being created on using code similar to what Charles suggested below. Once I knew that, I at least knew which thread to focus on. Then I broke on the OnHandleCreated event of the child control and found the issue.
One thing I didn't know was that the handle of a control is created when the control is first made visible, not when it is created. So a thread that didn't own the control was trying to set it's visibility to true. So I added a check to see if InvokeRequired and thought that would do the trick. However, something I really didn't expect is that calling InvokeRequired will create the handle of the control if it is not created yet! This in effect causes the control to be created on the wrong thread and always return false for InvokeRequired. I got around this by touching the Handle property of the control so that it is created before InvokeRequired is called.
Thanks for the help guys :)
To get the owner thread for a control, try this:
private Thread GetControlOwnerThread(Control ctrl)
{
if (ctrl.InvokeRequired)
ctrl.BeginInvoke(
new Action<Control>(GetControlOwnerThread),
new object[] {ctrl});
else
return System.Threading.Thread.CurrentThread;
}
Can child controls be on a different thread from the parent (container control)? Yes, it all depends on what thread was running when the control was constructed (new'ed up)
You always have to check InvokeRequired... Because you never know what thread might be calling into the method you are coding... Whether you need to check InvokeRequired separately for each child control, depends on how sure you are that all the controls were created on the same thread or not. If all controls are created when the form is created, in the same initialization routine, then you are probably safe to assume they were all created on the same thread
In my C# Windows Forms application, I have a user control that contains other controls and does its own logic. One of them is a delayed call (Timer-invoked) that does certain things after the user has finished a keyboard input (live filter text). It accesses the other controls for this, one of them is that text input control. This method is invoked 500 ms after the last input event.
Now I have a problem when the delayed call is running while the application is terminating. When I enter some text, then wait about 500 ms (it seems to work every time) and then press Alt+F4 to close the window, the application throws a NullReferenceException while trying to access the text input control. This doesn't happen when I close the window immediately after the last input or a second or more after.
It seems that the control is being disposed or something and its methods cannot access the child controls anymore. So, when the control is being put in that state (by whomever and whatever that state exaclty is), those timer need to be stopped first so that the controls can be safely disposed.
I have already tried to stop the timer in the OnHandleDestroyed method (overridden) and at the beginning of the Designer-generated Dispose method. Nothing helped.
This procedure works fine in regular Forms when stopping the timers in the overridden OnFormClosed method, before calling base.OnFormClosed(). I just cannot find a suitable event in a user control.
Try this in your UserControl:
bool isDisposed;
protected override void Dispose(bool disposeManaged)
{
if(!isDisposed)
{
if(disposeManaged)
{
//Dispose your timer here
}
isDisposed = true;
}
}
Another possibility is that one of your UI classes doesn't do its cleanup. Eg. it registers itself for an event but doesn't deregister when it's manually disposed. It is never collected by the GC and when the event is fired for the next time, it tries to access some members that were set to null during the Dispose(...) call before.
Another possibility is that you have a more complex race condition within your code but it's hard to say from here.