This issue affects both my VB.NET and C# Winform applications. When closing Form containing Adobe PDF Viewer conreol, the Form takex some 10 seconds to close. I tracked the issue down to Dispose method. Normally, in C# all it takes it so steal focus from Adobe PDF Control by adding lines to FormClosing event handler but in VB.NET, even if I steal focus, it still takes way too long for the Form to close. I tried to explicitly dispose the control and Implicitly - by disposing the containing Form. I tried empty string in LoadFile() method. I tried new reference to new interop library. Nothing works.
Also, if you would suggest a different API/library to replace AxAcroPDFlib I'd love to read what you use.
I now have a solution. Early testing is promising.
In short, add Controls.Remove(yourPdfViewerControl) to FormClosing event handler. Of course this will offer instant alleviation of the problem but may cause memory leak. After you have removed the control from Container you could assugn null to its memory address and hope that memory manager will do its job. So in order to prevent the leak, I created a class holding one memory space for PdfViewer control. If a Form needs a viewer, it requests it from an object that is accessible from any form to prevent memory leaks. To avoid Dispose method invocation when Form closes, make sure you remove this Control from Controls container.
I can provide sample code if needed.
Related
A older application loads some forms using implicit instances:
form2.showdialog()
Sometime between VS2008 32-bit and VS2013 64-bit, the forms stopped being initialized when they are reloaded. For example, if you load a form, close the form (using the Close method), and load the form again, the classes and controls (and, I assume, the form) are not initialized as new instances.
Re-initialization can be accomplished by putting me.dispose() in the FormClosed event, or by using an explicit instance of the form:
Using frm As New Form2
frm.ShowDialog()
End Using
Is there a good reason to use one of these methods over the other, or is there another method that should be used to cause a form to be initialized when it is reloaded?
Dispose will be called automatically if the form is shown using the Show method. If another method such as ShowDialog is used (your case it is), or the form is never shown at all, you must call Dispose yourself within your application. You can also handle the dispose by moving it from the designer file into the code file and handle things there as well.
On the other hand, Using statement typically makes your application safer to maintain and less prone to deadlocks and other misbehavior related to the lifecycle of the resource. I would stick by using this approach.
Also you cant put Me.Dispose in the Form Closed event (possible issues). If your using ShowDialog it will fail as it will dispose your objects first, if you need them they are gone.
Here's more on dispose: https://msdn.microsoft.com/en-us/library/aw58wzka(v=vs.110).aspx
The Form object and its child controls are not automatically disposed when you display the form with ShowDialog(). That sounds pretty quirky but this was done for a very good reason. After ShowDialog returns DialogResult.OK, you are normally going to obtain the dialog results. What nobody likes is that failing because of a ObjectDisposedException. Which would be likely to occur since the dialog results are often stored in controls.
You should always use the Using statement to ensure the form object and all of its controls are disposed.
A possible corner case is intentionally not disposing it because you like the redisplay the dialog with the original entered values. Which is not completely wrong, it is however a very expensive way to preserve those values. Those undisposed window objects cost an arm and a leg in system resources.
Pretty clear explanation from MSDN
Unlike non-modal forms, the Close method is not called by the .NET
Framework when the user clicks the close form button of a dialog box
or sets the value of the DialogResult property. Instead the form is
hidden and can be shown again without creating a new instance of the
dialog box. Because a form displayed as a dialog box is hidden instead
of closed, you must call the Dispose method of the form when the form
is no longer needed by your application.
When ShowDialog() called and closed, instance of the form will remain in the memory, and can be used again, for example get a result from some public property.
If you not using anymore this form, you need to call Dispose method to dispose form and form's controls
Dim myform As New MyDialogForm()
myform.ShowDialog()
Dim result As Object = myForm.SelectedResult()
myform.Dispose() 'need to call manually, if instance not used anymore
When you use Using keyword then Dispose method will be executed automatically at the end of the Using block
Dim result As Object
Using myform As New MyDialogForm()
myform.ShowDialog()
result = myForm.SelectedResult()
End Using 'myform.Dispose will be called
Bottom line: Both methods doing a same things.
But Using block will call Dispose method automatically
P.S. Putting Me.Dispose in the FormClosed eventhandler then
- instance of the form will stay in the memory even form was closed
- and will work only until you tried using disposed controls again. If you will try to show disposed object then ObjectDisposedException will be thrown.
If you not using form anymore then Using block will be best method
I'm trying to use a bunch of webbrowsers in some background threads. This works no problem when I use webbrowser controls that i have placed on the form in design view but now when they are created at runtime.
I declare the webbrowsers array globally:
Dim webbroswers(-1) As WebBrowser
The following code is on the main thread:
ReDim Preserve webbroswers(somenum)
For i = 0 To sumnum
webbroswers(currentbrowsermax + i) = New WebBrowser
Next
Then this code is run on the background thread:
If webbroswers(num).InvokeRequired Then
webbroswers(num).Invoke(Sub() webbroswers(num).Navigate(someurl))
Else
webbroswers(num).Invoke(Sub() webbroswers(num).Navigate(someurl))
The program crashes at this point with the following error:
Unable to get the window handle for the 'WebBrowser' control. Windowless ActiveX controls are not supported.
Any help on this would be great. Also if anyone knows how to suppress script errors then I think this might help. I've tried: WebBrowser(num).ScriptErrorsSuppressed = True but this doesn't work (it doesn't work elsewhere in my code when running on the main thread either) Thanks!
The Control.InvokeRequired and Invoke members use the Handle property to figure out what thread owns the control. Problem is, the Handle is null for the web browsers that you created. A control only has a valid handle when you made it visible on a form. Which you didn't do. It will then try to create the handle but that's a fail whale, an ActiveX control like WebBrowser needs a valid Parent. Without Me.Control.Add(), as was done in your original version, it won't have one.
The workaround is simple, you just need another control with a valid Handle property. Any will do, it only cares about the thread that owns the handle, not the value of the handle.
You have one: your form. Use Me.InvokeRequired and Me.Invoke() instead. Or Application.OpenForms(0) if you can't easily get a reference to the form object, best avoided.
I have a UI element in my application where a Panel is used to host one of several potential custom UserControls. The Panel itself is hosted in a standardised UserControl that I am using something like a non-modal dialog that I'm calling a 'pane'.
The method I use is to instantiate a new instance of the standard pane, then with logic instantiate one of the several optional hosted controls inside it using Panel.Controls.Add(control). I then add the new pane to the interface control in a set location, again with a Control.Controls.Add(control), followed by a control.BringToFront() to maximise its z position.
This all works well, however when the time comes to hide the pane and destroy it, I cannot seem to fully get rid of it. Originally I was simply using Control.Controls.Remove(control) and for good measure setting the pane's Parent property to Nothing. This would have the desired effect of making the pane disappear, and my assumption was that now the control was unreferenced, that GC would dispose of it.
What I am seeing however is that the control still blits instantaneously onto the screen when the next outer hosting TabControl changes tab page, implying it still exists somewhere. I can confirm that this is not a graphical issue and the pane object persists using the VS Watch window's 'Make Object ID'. (At least I think this is proof, that without a code-accessible reference I can still directly see the object and its properties continue to exist.)
I have tried replacing
Control.Controls.Remove(pane)
pane.Parent = Nothing
with
pane.Dispose()
GC.Collect()
where the Dispose call I can confirm both removes the control from its parent's Controls collection and sets its Parent property to Nothing, but appears to do no more. It persists after forced GC and still blits onscreen occasionally.
This all leads to my original question, what is the proper way to remove and fully destroy controls after they have served their purpose?
According to this article from MSDN it seems like you might be experiencing side affects from the object being on the finalization queue.
A Dispose method should call the GC.SuppressFinalize method for the object it is disposing. If the object is currently on the finalization queue, GC.SuppressFinalize prevents its Finalize method from being called.
Translation: The finalize method isn't being called, and so the resources associated with your control are not being released. After a bit more digging, I found that you should
Always call Dispose before you release your last reference to the Component. Otherwise, the resources it is using will not be freed until the garbage collector calls the Component object's Finalize method.
From this article.
So either you need to release your last reference OR you need to call the components finalize method directly so your GC.Collect() will work.
I'm trying to figure out the lifetime of the tmpTabPages in the following bit of code. Lets assume the form has an empty TabControl named MyTabControl, that there's a collection of strings called NameCollection.
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
For Each itm In NameCollection
Dim tmpTabPage as New TabPage(itm.toString)
'Add Controls to tmpTabPage
MyTabControl.TabPages.Add(tmpTabPage)
Next
End Sub
Since the scope of the tmpTabPage is the For/Next block, typically it's lifetime would be until the end of the block right? But since it is added to a collection that has a scope outside of the block does it get the same lifetime as the collection, or in this case the MyTabControl? Finally, if I call MyTabControl.TabPages.Clear will the tmpTabPages in the collection be destroyed or will they just sit around taking up memory?
The big deal about classes derived from Control (including TabPage) is the Dispose() method. They are immune from automatic garbage collection, Winforms keeps an internal table that maps the Handle of a control to the control reference. That's why, say, your main form doesn't suddenly get garbage collected, even though your program doesn't keep a reference to it.
Adding the TabPage to the TabControl's collection takes care of automatic disposal. The same applies for the TabControl, it would be added to the form's Controls collection. The normal chain of events is that either your program or the user closes the form. The Form class iterates its child controls and calls their Dispose() method. TabControl does the same thing in its Dispose() method, disposing the tab pages. The Windows window gets destroyed in the process, removing the Handle from that mapping table and now allowing the garbage collector to, eventually, collect the managed wrapper for the controls.
There is a nasty trap that gets many Winforms programmers in trouble. If you remove a control from its parent's collection then you get the responsibility of disposing it yourself. Removing it does not automatically dispose it. Winforms keeps the native window alive by temporarily re-parenting the control to a hidden window named the "parking window". Nice feature, it allows you to move a control from one parent to another without having to destroy and re-create the control.
But the keyword there is "temporarily". It is only temporarily if you next reparent the control. So it gets moved from the parking window to the new parent. If you don't actually reparent it then it will stay alive for ever on the parking window. Gobbling up resources until the program terminates. This is otherwise known as a leak. It can crash your program when Windows refuses to create another window when you've already created 10,000 of them.
The ControlCollection.Clear() method is especially harmful here. It does not dispose the controls, they all get moved to that parking window. If that's not intended, it rarely is, you'll have to call Dispose() on them yourself.
Objects in .NET become eligible for garbage collection when there's no way of getting at them. In this case, there will be a way of getting at the TabPage via the TabPages collection, until either it's removed from the collection or the tab control itself becomes eligible for collection.
Now when an object becomes eligible for garbage collection, that doesn't mean it's garbage collected straight away - the garbage collection runs at various times according to some fairly complex heuristics, and there are also "generations" of memory which make things harder to predict.
But basically:
You don't need to worry that an object that's been added to a collection will be mysteriously collected and cause problems
You generally don't need to worry that objects will leak memory forever. There are certainly situations where you do need to take some active steps to make sure that an object is eligible for collection when you're no longer using it, but they're relatively rare. (Typically they are related to static variables and/or events, in my experience.)
because soemthing has a reference to the controls they will not be disposed of.
yes the lifetime would be till the end of the procedure if you were not adding a reference to them to the colelction.
the clear will remove the objects from the collection and they will get garbage collected provided there are no other references to them (which there shuoldnt be in the situation you describe)
You add only a reference of the TabPage object to the collection not the object TmpTabPage. The tmpTabPage object in this case you use it only for allocating memory.
We have a question regarding VB.Net 2008.
We are used control array in vb.net and third party timer controls.
When handle received from external application to timer control event procedure,
after this form becomes blank and controls disappear.
What we have to do to persist the controls.
You asked what you have to do to persist the controls. It's not clear whether you mean winforms or webforms, but I can answer for both possibilities:
If it's the former, you have it backwards. The default behavior of all controls is that they are "persisted" until you tell them otherwise. If anything disappears, it's because you have code somewhere that tells it to. That's where you need to start looking.
If it's webforms/ASP.Net, the problem is that you don't understand the page lifecycle. Everything that raises server events, including your third party timer controls, causes a post back. That's how events work - the browser posts the form back to the same url in such a way that the server knows to call a your event code at the right time. The thing here is that as far your server is concerned, it's still just a new http request, and that means you're working with a brand new instance of your page class every time this happens. If you've previously added some controls to your page, it doesn't matter. That was an old instance that was discarded and probably disposed by the time the page was visible in the user's browser. If you want to keep those controls, you need to make sure you add them to the page on every postback.