C++ CLI Invoke issues - multithreading

Question

I have a MainForm class (as you'd expect, it is a form) that has a text box on it. I also have another class called 'Application_Server' That does a load of other stuff (not just form-background related, quite a lot of network based stuff etc.).
The Application_Server class runs in it's own thread, but needs to be able to update the controls on the form, for this question, we will stick with just the textbox.
The problem is that even though I am executing the command to set the text of the textBox control via 'Invoke' I am still getting the following exception during runtime:
Additional information: Cross-thread operation not valid: Control
'DebugTextBox' accessed from a thread other than the thread it was
created on.
What could be causing this? I am definitely invoking a delegate within MainForm.
Here are the relevant code segments (cut down for readability):
MainForm.h:
public ref class MainForm : public System::Windows::Forms::Form {
delegate void del_updateDebugText(String^ msg);
del_updateDebugText^ updateDebugText = gcnew del_updateDebugText(this, &MainForm::postDebugMessage);
private: void postDebugMessage(String^ message);
};
MainForm.cpp:
void EagleEye_Server::MainForm::postDebugMessage(String^ message)
{
Monitor::Enter(DebugTextBox);
if (this->DebugTextBox->InvokeRequired)
{
this->Invoke(updateDebugText, gcnew array<Object^> { message });
}
else
{
this->DebugTextBox->AppendText(message);
}
Monitor::Exit(DebugTextBox);
}
And finally, the code calling it:
void ServerAppManager::postDebugMessage(System::String^ message)
{
mainFormHandle->updateDebugText(message);
}
void ServerAppManager::applicationStep()
{
postDebugMessage("Starting\n");
// This is Run in seperate thread in MainForm.cpp
while (s_appState == ApplicationState::RUN)
{
postDebugMessage("Testing\n");
}
}
Thanks!

Answer 1

From background worker called bwSearch we do the call as following from the DoWork event handler:
private: System::Void bwSearch_DoWork(System::Object^ sender, System::ComponentModel::DoWorkEventArgs^ e) {
//... logic
UpdateTxtOutput("Some message");
//... more logic
}
I have a RitchTextBox called txtOutput, also the windows form control containing this code is called frmMain, the UpdateTxtOutput is defined in three parts as follows:
delegate void UpdateTxtOutputDelegate(String^ text);
void UpdateTxtOutput(String^ text)
{
UpdateTxtOutputDelegate^ action = gcnew UpdateTxtOutputDelegate(this, &frmMain::Worker);
this->BeginInvoke(action, text);
}
void Worker(String^ text)
{
txtOutput->AppendText("\t" + text + "\n");
}

Answer 2

I managed to get it working by simplifying the method within the 'MainForm' class to:
void EagleEye_Server::MainForm::postDebugMessage(String^ message)
{
Monitor::Enter(DebugTextBox);
DebugTextBox->AppendText(message);
Monitor::Exit(DebugTextBox);
}
And then moving the 'Invoke' call to the method calling the delegate, not pretty but it works for now. I think the issue may have been caused by the form getting stuck inside an Invoke loop. I say this as I noticed that the form would lock up and stop responding after it hit the recursive Invoke statement.