C# threading question: Invoke() fails with ObjectDisposedException

[HuntrCkr]

I have had a similar problem many times. In a lot of my apps, I have a textbox that sits on a form, and any thread can write a line to it. I have poured over documentation, and never found a solution to whether the form has been closed or not. So, I simply added the following :

private volatile bool formClosed = false;

private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
formClosed = true;
}

delegate void ProgressValueDelegate(int value);
public void SetProgressValue(int value)
{
if (this.InvokeRequired)
{
ProgressValueDelegate pvd = new ProgressValueDelegate(SetProgressValue);
this.Invoke(pvd, new object[] { value });
}
else
{
if( formClosed == false )
{
m_progressBar.Value = value;
}
}
}

BTW, you can also move the if to before the if (this.InvokeRequired) to save yourself an Invoke call if it won't be doing anything in anycase.

[Luc Pattyn]

HuntrCkr wrote:

you can also move the if

Not a good idea. Invoke puts the job in the input queue of the main thread; a "close this form" command could already be awaiting execution. You need to perform the test as late as possible. :)

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[HuntrCkr]

In fact, it doesn't make a difference. Common misunderstanding. Let me first put in the code snippet

public void SetProgressValue(int value)
{
if( formClosed == true )
{
return;
}

if (this.InvokeRequired)
{
   ProgressValueDelegate pvd = new ProgressValueDelegate(SetProgressValue);                
   this.Invoke(pvd, new object\[\] { value });
}
else
{
    m\_progressBar.Value = value;
}

}

When SetProgressValue is called from another thread, the first thing that happens is the check if the form is closed already. Let's take you hypothetical situation where a FormClosing event is queued, but has not been executed yet. formClosed is false, so now this.InvokeRequired is checked. It's true so a call to SetProgressValue is queued as well. The FormClosing event fires, and formClosed is set to true. Next, the queued call to SetProgressValue occurs. First thing, we check formClosed which is now true, so the function is terminated immediately. The common misconception is that Invoke simply transfers the call to the correct thread, but in fact the function is called all over again. Another thing many people are not aware of is that the call to Invoke actually halts the thread until the Invoked delegate has been executed in the GUI thread, which can cause major slowdowns. Rather use BeginInvoke. It does the same thing in this scenario, but does not block the calling thread.

[Fabio Franco]

It seems that, for some reason, your dialog is beeing disposed (and with it, all it's controls). It's really difficult to know why without more code. But have you tried handling FormClosing event? There you could do this: e.Cancel = true; this.Hide(); Maybe that will work out for you, hiding instead of closing. Let me know if it solves your problem Regards, Fábio

[Luc Pattyn]

My mistake. You're right about the test, if it sits on the top of the method, it gets executed twice so it can't go wrong. However executing it twice all the time may be a high price to pay for potentially saving one thread switch when the form closes. I do not really agree on the BeginInvoke issue. While technically correct, I don't like the SetProgressValue method to behave synchronously when called on the main thread and asynchronously when called anywhere else. If there is a potential asynchronous operation, it should be reflected in the method name (BeginSetProgressValue?) and it'd better be consistent (as in: don't test InvokeRequired, just call BeginInvoke). Furthermore, having a mix of such Invoke and BeginInvoke patterns (several Controls each using their own little SetProgressValue method) could result in incomprehensible app behavior. :)

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[HuntrCkr]

Luc Pattyn wrote:

My mistake. You're right about the test, if it sits on the top of the method, it gets executed twice so it can't go wrong. However executing it twice all the time may be a high price to pay for potentially saving one thread switch when the form closes.

Hey, no problem. I make mistakes all the time. Hence my motto There are two ways to write bug free software. Only the third method works though ;P As for the performance cost argument, I think it's highly dependent on the scenario.

Luc Pattyn wrote:

I do not really agree on the BeginInvoke issue.

I agree with you. In a large project or any other production level code, things should be done consistently, and all the things you mention are true. That is why I stated that in this simplified scenario, BeginInvoke doesn't really matter. If the function was even remotely more complex, or had more than 3 lines to execute with the synchronous vs. asynchronous path, it would be far more important. Glad we could agree :)

[Peter Trevor]

I'm not sure how to satisfy Luc's issue over mixing sync and async behaviours, but how about this variation:

public void SetProgressValue(int value)
{
MethodInvoker method = delegate
{
if (!formClosed)
{
m_progressBar.Value = value;
}
};

if (InvokeRequired)
    BeginInvoke(method);
else
    method.Invoke();

}

First, you don't need to worry about keeping the delegate's parameters aligned with the method's parameters (makes maintenance easier). Second, you shouldn't test boolean values against true/false (just take their value). Third, it uses the async BeginInvoke when not on the GUI thread.

[CodeHawkz]

Hi there, I would love to get my hands on your code and experiment. I am way too lazy to write a new one. If you think you can write a small app that simulates your prob or share your code itself, please do so. I would love to solve the riddle. =]

[Ware Work]

at ProgressForm.SetProgressDescription(String description)

It appears to be calling SetProgressDescription and not SetProgressValue that you have included previously. Is this the issue?

WarePhreak Programmers are tools to convert caffiene to code.

[Luc Pattyn]

I like it, especially if you scrap the "Begin" of BeginInvoke. :)

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[JOAT MON]

Just a thought - Have you checked to see if the thread is still alive? It may be that the closing form is triggering the thread itself to close or change states, preventing the continued use without restarting it.

Jack of all trades ~ Master of none.

[Patrick Fox]

How are you hiding the dialog box? After some googling, it appears you aren't the only one facing this problem. Most helpful responses seem to suggest using try / catch and ignoring ObjectDisposedExceptions. This might work, because if the problem happens when the dialog is hidden, the user won't expect the progress bar to increment, because it's not visible... http://www.devnewsgroups.net/windowsforms/t38313-parkingwindow-objectdisposedexception-system-windows-forms.aspx[^] http://www.tech-archive.net/Archive/DotNet/microsoft.public.dotnet.languages.csharp/2006-08/msg01372.html[^]

[Jason J Chase]

When developing apps that use worker threads which like to report back a status, a good technique I have used on projects is to set up a data structure (class or struct) that holds member variables for all the "stats" that the threads must update. Each thread updates the fields in the class/struct, without caring who may be "listening". For dialogs that like to report the status of these threads (i.e. using progress bars, updating text boxes etc), they query the class / struct member variables. This way the relationship is decoupled and state-free. This is a technique we used a lot in the Win32 programming model, and it works just as well in .Net.