Re: How to safely Form.Invoke from another thread? - C# Discussion Boards

25-Jul-07 1:19

Hello,

how do I safely invoke a delegate on my windows form if I can't eliminate the possibility that the form gets disposed BETWEEN me checking whether it is disposed and the invoke code itself?

Randomly I'm getting an exception that says I can't access the disposed object. It's absolutely clear that I can't access a disposed object, but how do I make sure it's indeed NOT disposed if it can still get disposed right after I check whether it is disposed? Smile | :)

Consider this example that looks correct, but produces the above mentioned exception from time to time:

if (!MyForm.IsDisposed)
{
    // what if the form gets disposed BETWEEN the above line
    // of code and the following line of code?
    MyForm.Invoke(MyDelegate);
}

Form.Disposing doesn't help neither.

Thanks very much for any input,
Michal

-- modified at 7:24 Wednesday 25th July, 2007

michal.kreslik25-Jul-07 1:44

25-Jul-07 1:44

It helps to set a flag that the form is closing and check for that flag before the invoke, but it's not an elegant solution and it might still throw an exception if the flag is set after checking for the flag and the form gets disposed before executing the invoke.

Any ideas?

Thanks,
Michal

Tormod Fjeldskaar25-Jul-07 1:50

Tormod Fjeldskaar

25-Jul-07 1:50

How about wrapping both your protected override void Dispose(bool disposing)-method and your invoke-procedure in a lock(...)-block?

Ref.: http://msdn2.microsoft.com/en-us/library/c5kehkcz(vs.80).aspx

michal.kreslik25-Jul-07 2:47

25-Jul-07 2:47

Cool, thanks a lot! This solutions seems to work.

Next time, I'll be wiser and will use lock for this kind of situation.

Thanks much again!!

Michal

Luc Pattyn25-Jul-07 7:29

Luc Pattyn

25-Jul-07 7:29

are you kidding ?

the Dispose() method is the one that typically will be executed by the finalizer thread.
Do you think it will appreciate your lock ?
if the finalizer cant do its job, the GC has less memory to manage and recycle;
do you want the GC to run out of memory for no good reason ?

I am not sure what the right answer is, but what you propose seems very wrong to me. Confused | :confused:

Luc Pattyn

try { [Search CP Articles] [Search CP Forums] [Forum Guidelines] [My Articles] }
catch { [Google] }

michal.kreslik25-Jul-07 11:41

25-Jul-07 11:41

Actually, it does not appreciate my lock Smile | :)

If I'm forcing the Dispose() to wait on the lock until all Invokes are done and until I make sure there are no Invokes going to be done in the future, I'm effectively blocking that form's Window Message Queue.

If I'm blocking the Window Message Queue, the Invoke never returns.

Obviously, the same happens (WMQ block => Invoke never returns) if I try, for instance, to block after Form.Closing event etc.

So I'm back at the beginning Smile | :)

Any suggestions? Smile | :)

Thanks,
Michal

Luc Pattyn25-Jul-07 12:15

Luc Pattyn

25-Jul-07 12:15

I'm sure I would have some suggestions (or at least comments) if I felt I understood
your problem. You seem to somehow Dispose() of a form, while you may still need it.

So I need more info (contextual I guess).

For what it's worth: I typically dont dispose of forms, I just close them.
I realize you may want to avoid Close() so you can play with Hide() and Show() again,
but even then the last time you could Close() rather than Dispose(), couldnt you ?

Smile | :)

Luc Pattyn

try { [Search CP Articles] [Search CP Forums] [Forum Guidelines] [My Articles] }
catch { [Google] }

michal.kreslik25-Jul-07 21:43

25-Jul-07 21:43

Hi, Luc,

thanks for your comment. I'm not disposing of the form myself. I'm just closing it. But the whole point is in not allowing the form to get disposed before all possible Invokes on it are done.

I can briefly outline what is going on in my app: a server application is calling my client app every 100 ms via COM and refreshes its state. Sometimes it happens that the server stops calling my client app and I need to detect that.

For that purpose I've written a simple timer class that's started together with the app. The core class of my app stores the timestamp of the last COM call and my timer class checks every 200 ms whether the difference between DateTime.Now() and last COM call timestamp is higher than maximum allowed. If it is, it Invokes a method on a form that's telling me the connection has been lost. If it's not, it Invokes a method on a form that's telling me the connection is OK.

Obviously, the timer runs on a separate thread from the form's thread. When I'm closing the form (which does not close the application itself), I need to make sure that the timer executes the last Invoke on the form and shuts itself off.

And there's the problem. In order to make sure that the timer executes the last Invoke on the form safely, I need to prevent the form from disposing of itself before that last Invoke is done. But if I'm preventing the form from disposing of itself by putting a lock anywhere between the moment when the form starts closing and the moment when it starts getting disposed of, I'm effectively blocking the window message queue, so the last Invoke (which is executed while the form waits on a lock) never returns.

Thanks much for any help.

Michal

michal.kreslik25-Jul-07 23:13

GROUP BY RECORD IN GRIDVIEW & EXPORT TO EXCEL

25-Jul-07 23:13

OK, so I resolved the issue. It's certainly not the most elegant solution, but it works. I will still appreciate if someone tells me how this should be done "professionally" Smile | :)

Instead of a lock, I'm using an ordinary while loop. The while loop has an advantage over lock of being able to execute something while it's waiting for a signal. Lock simply sits there and does nothing until the lock is released.

So I placed the while loop inside the Form.Closing() event handler. This while loop is blocking the form from closing itself (and thus disposing of itself). Inside this loop (apart from waiting for a signal) I'm executing Application.DoEvents() every 50 ms. This ensures that the Invoke, waiting on the second thread, gets processed.

If you've got some better suggestion, please tell me.

Thanks,
Michal

krishanpanda25-Jul-07 1:07

krishanpanda

25-Jul-07 1:07

how to display group by data in gridview
and export it to excel.
example
GROUP1
CONTENT1-A CONTENT1-B
CONTENT1-A CONTENT1-B
GROUP2
CONTENT1-A CONTENT1-B
CONTENT1-A CONTENT1-B
AND EXPORT THE EXACT INTO EXCEL

kp_chat@yahoo.co.in

Need VSD for asp.net case study

Banjo Ayorinde25-Jul-07 1:05

Re: Need VSD for asp.net case study

25-Jul-07 1:05

Am relatively new to asp.net and need a challenging case study. Pls send me a VSD to prepare myself for one of the best job around.

Thanks.

UglyxXXx25-Jul-07 23:25

UglyxXXx

25-Jul-07 23:25

Hei U buy by your self a**hole Roll eyes | :rolleyes:

DataGridView

sangramkp25-Jul-07 0:48

sangramkp

25-Jul-07 0:48

How to iterate through every cell of a datagridview?

Re: DataGridView

Tormod Fjeldskaar25-Jul-07 1:00

Tormod Fjeldskaar

25-Jul-07 1:00

foreach(DataGridViewRow row in yourDataGridView.Rows)<br />
{<br />
  foreach(DataGridViewCell cell in row.Cells)<br />
  {<br />
    /* do your stuff */<br />
  }<br />
}

datagrid focus

praveenkumar palla25-Jul-07 0:45

praveenkumar palla

25-Jul-07 0:45

present functionality:
on load the datagridview's selection mode is "FullRowSelect".
on clicking "Rename" button i am changing the selection mode to "CellSelect"(only the column is editable) and selecting that cell of the selected row index and then double clicking on that cell we able type(cursor is focusing to that cell) the code is

dgvLasers.EditingPanel.Visible = true;
bEdit = true;
dgvLasers.SelectionMode = DataGridViewSelectionMode.CellSelect;
dgvLasers.Rows[intIndex].Cells[1].Selected = true;

but a small change required
on clicking "Rename" button only the cell shold be selected and cursor should be focused to that cell (i.e. without clicking on the cell we should able to type or replace the text)

please help me on this

Thanks in advance

praveen Kumar palla

Re: datagrid focus

Nouman Bhatti25-Jul-07 2:46

Nouman Bhatti

25-Jul-07 2:46

set
datagridview.readonly = false
datagridview.editmode = editonenter

then write your code

Hiding Form after loading another

Banjo Ayorinde25-Jul-07 0:12

Re: Hiding Form after loading another

25-Jul-07 0:12

How do I hide a from from which 've just loaded another.

I load formB after meeting certain criteria and wish to close the formA from wish 've loaded formB

Eduard Keilholz25-Jul-07 0:21

Eduard Keilholz

25-Jul-07 0:21

form.Hide();

BTW, don't double-post...

.: I love it when a plan comes together :.
http://www.zonderpunt.nl

Re: Hiding Form after loading another

PhilDanger25-Jul-07 3:51

PhilDanger

25-Jul-07 3:51

Another idea would be to use the Main application program as a staging ground for each form.

//---code inside FormA ---
if(conditionMet){
    DialogResult = DialogResult.OK;
    this.Close();
}


//code inside Main

FormA formA = new FormA();
Application.Run(formA);
if(formA.DialogResult == DialogResult.OK){
     FormB formB = new FormB();
     Application.Run(formB);
}

Hiding Form after loading another

Banjo Ayorinde25-Jul-07 0:09

Creating a web based report in win app

25-Jul-07 0:09

How do I hide a from from which 've just loaded another.

I load formB after meeting certain criteria and wish to close the formAioud form from wish I load formB

Banjo Ayorinde25-Jul-07 0:02