How to edit controls in a system::thread. - c++-cli

Question

I need to be able to add items to a listbox inside of a thread. Code is below:
1. ref class Work
2. {
3. public:
4. static void RecieveThread()
5. {
6. while (true)
7. {
8. ZeroMemory(cID, 64);
9. ZeroMemory(message, 256);
10. if(recv(sConnect, message, 256, NULL) != SOCKET_ERROR && recv(sConnect, cID, 64, NULL) != SOCKET_ERROR)
11. {
12. ID = atoi(cID);
13. String^ meep = gcnew String(message);
14. lbxMessages->Items->Add(meep);
15. check = 1;
16. }
17. }
18. }
19. };
I get the error
Error: a nonstatic member reference must be relative to a specific object on line 14. Is there any way to get it to let me do that? Because if I try to use String^ meep; outside of that Thread it doesn't contain anything. It works PERFECT when I use it within the thread but not outside of it. I need to be able to add that message to the list-box. If anyone can help I would appreciate it.

Answer 1

You don't show how lbxMessages is defined, but I'm going to assume that it's a non-static data member in the same class. If that's the case, then you need to specify which object you want to access lbxMessages on. The simplest way would be to switch the RecieveThread method to be non-static, then you can access this->lbxMessages.
You didn't say which windowing toolkit you're using, but you probably will need to Invoke back onto the UI thread in order to edit the control.

Answer 2

One way is using System::Thread with a ParameterizedThreadStart delegate, which let you pass objects, in this case lbxMessages.
ParameterizedThreadStart^ threadCallback;
threadCallback = gcnew ParameterizedThreadStart(&Work::ReceiveThread);
Thread^ recvThread = gcnew Thread( threadCallback );
recvThread->Start( lbxMessages );
Your static method for running the thread:
static void RecieveThread(Object^ state)
{
ListBox^ lbxMessages = (ListBox^)state;
//...code
}
But.. . there is another problem. Assuming ListBox is a Win32 control, you can make changes in control only from the thread which it was created. So every time you insert a ListBox item, it must be done from the UI's thread. One way is using a SynchronizationContext object.
// Start the thread
array<Object^>^ args = gcnew array<Object^>(2){
lbxMessages,
SynchronizationContext::Current
}
recvThread->Start( args);
Thread method should be something like this:
static void RecieveThread(Object^ state)
{
array<Object^>^ args = (array<Object^>^)state;
ListBox^ lbxMessages = (ListBox^)args[0];
SynchronizationContext^ ctx = (SynchronizationContext^)args[1];
//...code
String^ meep = gcnew String(message);
ctx->Send(gcnew SendOrPostCallback(&Work::SafeListBoxInsert),
gcnew array<Object^>(2){lbxMessages, meep}
);
}
You will need another method to be called from UI's thread and make the changes.
ref class Work{
//...other methods
static void SafeListBoxInsert(Object^ state)
{
array<Object^>^ args = (array<Object^>^)state;
ListBox^ lst = (ListBox^) args[0];
String^ item = (String^) args[1];
lst->Items->Add(item);
}
}