crossthreading issue

[Andrew Bullock]

Hi, I've got a wierd problem...

Here is my code:

public partial class frmMain : Form
{
private MYClass myClass;

private void btnRun_Click(ob ject sender, EventArgs e)
{
myClass = new MYClass(new CallBack(doStuf f));
Thread t1 = new Thread(new ThreadStart(myC lass.Run));
t1.Start();
}

private void doStuff()
{
pictureBox1.Ima ge = myClass.getPict ure();

}
}

///////////////////////////////////////

public delegate void CallBack();
class MYClass
{
private CallBack CallBackMethod;

public MYClass(CallBac k cb)
{
CallBackMethod = cb;
}
public void Run()
{
// do something useful
CallBackMethod( );
}
public Bitmap getPicture()
{
// return an img
}
}
///////////////////////////////////////////////////////////////////


in doStuff() i grab data back from myClass like this:

myClass.getData ();

and then display it in pictureboxes, listboxes etc in the normal way
(not using invoke - because i thought the point in callbacks was to
avoid invoke). SOMETIMES (ive not worked out when/why) I get a
cross-threading error:

"Cross-thread operation not valid: Control '' accessed from a thread
other than the thread it was created on."

When the debugger takes me to break mode to display the error, if i just
click the run button again, it continues fine :s Sometimes this error
does not occur at all.

Can anyone tell me why, and then what I can do about it?


Thanks
Andrew

[Michael Nemtsev]

RE: crossthreading issue

You shouldn't call doStuff directly if u are not in UI thread. Use *Invoke
for this reason

Detailed explanation is here

http://www.yoda.arachsys.com/csharp/threads/winforms.shtml

[color=blue]
> Hi, I've got a wierd problem...
>
> Here is my code:
>
> public partial class frmMain : Form
> {
> private MYClass myClass;
>
> private void btnRun_Click(ob ject sender, EventArgs e)
> {
> myClass = new MYClass(new CallBack(doStuf f));
> Thread t1 = new Thread(new ThreadStart(myC lass.Run));
> t1.Start();
> }
>
> private void doStuff()
> {
> pictureBox1.Ima ge = myClass.getPict ure();
>
> }
> }
>
> ///////////////////////////////////////
>
> public delegate void CallBack();
> class MYClass
> {
> private CallBack CallBackMethod;
>
> public MYClass(CallBac k cb)
> {
> CallBackMethod = cb;
> }
> public void Run()
> {
> // do something useful
> CallBackMethod( );
> }
> public Bitmap getPicture()
> {
> // return an img
> }
> }
> ///////////////////////////////////////////////////////////////////
>
>
> in doStuff() i grab data back from myClass like this:
>
> myClass.getData ();
>
> and then display it in pictureboxes, listboxes etc in the normal way
> (not using invoke - because i thought the point in callbacks was to
> avoid invoke). SOMETIMES (ive not worked out when/why) I get a
> cross-threading error:
>
> "Cross-thread operation not valid: Control '' accessed from a thread
> other than the thread it was created on."
>
> When the debugger takes me to break mode to display the error, if i just
> click the run button again, it continues fine :s Sometimes this error
> does not occur at all.
>
> Can anyone tell me why, and then what I can do about it?[/color]

--
WBR,
Michael Nemtsev :: blog: http://spaces.msn.com/laflour

"At times one remains faithful to a cause only because its opponents do not
cease to be insipid." (c) Friedrich Nietzsche

[Stoitcho Goutsev \(100\)]

Re: crossthreading issue

Andrew,

Callbacks doesn't avoid using Control.Invoke. Calling a callback is the same
as calling a method directly - it executes in the context of the calling
thread. If you use .NET 2.0 and ther BackgroundWorke r than its Events
ProgressChanged and RunWorkerComple te are executed in the UI thread so you
don't need to call Control.Invoke from within these event hadnlers.


--
HTH
Stoitcho Goutsev (100)

"Andrew Bullock" <andrewREMOVEbu llockTHIS@ANDnt lworldTHIS.com> wrote in
message news:7xZKf.4353 0$Dn4.18786@new sfe3-gui.ntli.net...[color=blue]
> Hi, I've got a wierd problem...
>
> Here is my code:
>
> public partial class frmMain : Form
> {
> private MYClass myClass;
>
> private void btnRun_Click(ob ject sender, EventArgs e)
> {
> myClass = new MYClass(new CallBack(doStuf f));
> Thread t1 = new Thread(new ThreadStart(myC lass.Run));
> t1.Start();
> }
>
> private void doStuff()
> {
> pictureBox1.Ima ge = myClass.getPict ure();
>
> }
> }
>
> ///////////////////////////////////////
>
> public delegate void CallBack();
> class MYClass
> {
> private CallBack CallBackMethod;
>
> public MYClass(CallBac k cb)
> {
> CallBackMethod = cb;
> }
> public void Run()
> {
> // do something useful
> CallBackMethod( );
> }
> public Bitmap getPicture()
> {
> // return an img
> }
> }
> ///////////////////////////////////////////////////////////////////
>
>
> in doStuff() i grab data back from myClass like this:
>
> myClass.getData ();
>
> and then display it in pictureboxes, listboxes etc in the normal way
> (not using invoke - because i thought the point in callbacks was to
> avoid invoke). SOMETIMES (ive not worked out when/why) I get a
> cross-threading error:
>
> "Cross-thread operation not valid: Control '' accessed from a thread other
> than the thread it was created on."
>
> When the debugger takes me to break mode to display the error, if i just
> click the run button again, it continues fine :s Sometimes this error does
> not occur at all.
>
> Can anyone tell me why, and then what I can do about it?
>
>
> Thanks
> Andrew
>[/color]

[Willy Denoyette [MVP]]

Re: crossthreading issue


"Andrew Bullock" <andrewREMOVEbu llockTHIS@ANDnt lworldTHIS.com> wrote in
message news:7xZKf.4353 0$Dn4.18786@new sfe3-gui.ntli.net...
| Hi, I've got a wierd problem...
|
| Here is my code:
|
| public partial class frmMain : Form
| {
| private MYClass myClass;
|
| private void btnRun_Click(ob ject sender, EventArgs e)
| {
| myClass = new MYClass(new CallBack(doStuf f));
| Thread t1 = new Thread(new ThreadStart(myC lass.Run));
| t1.Start();
| }
|
| private void doStuff()
| {
| pictureBox1.Ima ge = myClass.getPict ure();
|
| }
| }
|
| ///////////////////////////////////////
|
| public delegate void CallBack();
| class MYClass
| {
| private CallBack CallBackMethod;
|
| public MYClass(CallBac k cb)
| {
| CallBackMethod = cb;
| }
| public void Run()
| {
| // do something useful
| CallBackMethod( );
| }
| public Bitmap getPicture()
| {
| // return an img
| }
| }
| ///////////////////////////////////////////////////////////////////
|
|
| in doStuff() i grab data back from myClass like this:
|
| myClass.getData ();
|
| and then display it in pictureboxes, listboxes etc in the normal way
| (not using invoke - because i thought the point in callbacks was to
| avoid invoke). SOMETIMES (ive not worked out when/why) I get a
| cross-threading error:
|
| "Cross-thread operation not valid: Control '' accessed from a thread
| other than the thread it was created on."
|
| When the debugger takes me to break mode to display the error, if i just
| click the run button again, it continues fine :s Sometimes this error
| does not occur at all.
|
| Can anyone tell me why, and then what I can do about it?
|
|
| Thanks
| Andrew
|

Just to add what Michael and Stoitcho said, there is no reason to run this
on a separate thread, the action "getPicture " only takes a few milliseconds
to complete, so you won't freeze the UI thread at all.

Willy.

[Andrew Bullock]

Re: crossthreading issue

Stoitcho Goutsev (100) wrote:[color=blue]
> Andrew,
>
> Callbacks doesn't avoid using Control.Invoke. Calling a callback is the same
> as calling a method directly - it executes in the context of the calling
> thread. If you use .NET 2.0 and ther BackgroundWorke r than its Events
> ProgressChanged and RunWorkerComple te are executed in the UI thread so you
> don't need to call Control.Invoke from within these event hadnlers.
>
>[/color]


Ace, thanks for that :)

Andrew