Problem with windows forms and delegates

[nabil035]

I have an OPC server (an application server).

I write this server using C++/CLI.

This server contain callbacks (delegates)


When the client connects to the server (enter the core of a callback function onconnectclient) I want to change the text of a text box!!!

onclientcallback is defined in a seperate class called Callbacks

In this class I have an attribute called mainform which is the main form of my application (called by application::run)

so I call in the function mainform->TextBox->Text = "text" (1)

and nothing happen (compilation succeds)

I debuged the application and when attending (1) it retrun to the top of the function onclientcallback

I don't understand the problem, should I for example declare the delegate in the initialise componenet and if yes how and why??

Please help or give me any idea!

[nabil035]

Well I understand that the problem is a cross-thread exception


So, please tell me how can I fix this problem without changing the structure of my application.

[weaknessforcats]

Are you able to step through the code with the debugger?

I am curious about your comment of the "debugger returns to the top". If the debugger can't step through the code, the code was not compiled. Visual Studio will do that if there is no logical path to the code.

[nabil035]

In my case the debugger can't step into the code when it attends the statement where the callback(that I defined) try to change the text of the label.

Now I understand more the problem, in winforms we cannot change the controls from an extern process(extern to the one that created the control)
==>an exeption is raised but don't don't block the execution, but the debugger return to the top of the callback

I find a solution using control::invoke()

[pootle]

In my case the debugger can't step into the code when it attends the statement where the callback(that I defined) try to change the text of the label.

Now I understand more the problem, in winforms we cannot change the controls from an extern process(extern to the one that created the control)
==>an exeption is raised but don't don't block the execution, but the debugger return to the top of the callback

I find a solution using control::invoke()

Hi Nabil,

You are right. In C++/CLI or in any other .NET language for that matter, you cannot change WinForms properties or members(e.g. via accessors) outside of the main gui thread. This is to prevent problems where two threads are modifying data concurrently.

However, I urge not to use Invoke. Please consider using BeginInvoke instead. You can find a lot of articles on the internet about Invoke vs. BeginInvoke.

Normally you would do something like:

delegate void InvokeDelegate();
void MyForm::onAnEvent() {
      if (InvokeRequired) {
         BeginInvoke(gcnew InvokeDelegate(this, &MyForm::onAnEvent));      
      }
      else {
         // Do your code here in the GUI thread
      }
}
 

The whole "InvokeRequired" thing is a bit of a pain, but eventually you avoid problems such as the one you describe.

As an extra point, do you have CLR exceptions enabled in your debugger? If not, you will probably miss the problem altogether and end up at some totally unrealted part of the code.

HTH

[nabil035]

Hi pootle,

thx for your clear ideas.


but I don't understand the last part of your reply, anyway I have CLR exceptions enabled in my debugger.