Creating window in another thread(not main thread)

Question

I've got a function:

HWND createMainWindow(P2p_Socket_Machine * toSend){

    HWND hMainWnd = CreateWindow( 
        L"Class",/*(LPCWSTR) nameOfConference.c_str()*/L"Chat",  WS_OVERLAPPED | WS_MINIMIZEBOX | WS_SYSMENU, 
    CW_USEDEFAULT, 0, 600,400, 
    (HWND)NULL, (HMENU)NULL, 
    /*(HINSTANCE)hlnstance*/NULL, NULL 
    ); 

    if (!hMainWnd) { 
        MessageBox(NULL, L"Cannot create main window", L"Error", MB_OK); 
        return 0; 
    }

    CreateWindowA("LISTBOX",NULL, WS_CHILD|WS_VISIBLE|WS_BORDER|WS_VSCROLL|LBS_NOTIFY|LBS_MULTIPLESEL,310,30,255,275,hMainWnd,(HMENU)List_Box,NULL,NULL);

    CreateWindowExA(NULL,"BUTTON", "Refresh", WS_TABSTOP|WS_VISIBLE|WS_CHILD|BS_DEFPUSHBUTTON,385,310,100,24,hMainWnd,(HMENU)Button_Refresh, NULL ,NULL);

    CreateWindowExA(NULL,"BUTTON", "Send", WS_TABSTOP|WS_VISIBLE|WS_CHILD|BS_DEFPUSHBUTTON,385,334,100,24,hMainWnd,(HMENU)Button_Send, NULL ,NULL);

    CreateWindowExA(NULL,"BUTTON", "New", WS_TABSTOP|WS_VISIBLE|WS_CHILD|BS_DEFPUSHBUTTON,385,354,100,24,hMainWnd,(HMENU)Button_New, NULL ,NULL);

    CreateWindowA("EDIT",0,WS_BORDER|WS_VISIBLE|WS_CHILD|ES_LEFT|ES_MULTILINE|WS_VSCROLL|WS_DISABLED,
    10,30,265,275,hMainWnd,(HMENU)Text_Box_Get,NULL,NULL);

    CreateWindowA("EDIT",0,WS_BORDER|WS_VISIBLE|WS_CHILD|ES_LEFT|ES_MULTILINE|WS_VSCROLL,
    10,320,265,45,hMainWnd,(HMENU)Text_Box_Send,NULL,NULL);

    SetWindowLongPtr(hMainWnd,GWLP_USERDATA,(LONG_PTR)toSend);

    ShowWindow(hMainWnd, SW_SHOW); 
    //UpdateWindow(hMainWnd);

    return hMainWnd;

}

And this is main part of my program:

int WINAPI WinMain(HINSTANCE hlnstance, HINSTANCE hPrevInstance, LPSTR IpCmdLine, int 
nCmdShow) 
{
WNDCLASSEX wc; 
    wc.cbSize = sizeof(wc); 
    wc.style = CS_HREDRAW | CS_VREDRAW; 
    wc.lpfnWndProc = MyFunc; 
    wc.cbClsExtra = 0; 
    wc.cbWndExtra = 0; 
    wc.hInstance = hlnstance; 
    wc.hIcon = LoadIcon(NULL, IDI_APPLICATION); 
    wc.hCursor = LoadCursor(NULL, IDC_ARROW); 
    wc.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
    wc.lpszMenuName = NULL; 
    wc.lpszClassName = L"Class"; 
    wc.hIconSm = LoadIcon(NULL, IDI_APPLICATION);
HWND toSend = createMainWindow(P2pSocket);

//some code

hThread = CreateThread(NULL, 0, ClientThread, 
            Message2, 0, &dwThreadId);

        if (hThread == NULL)
        {
            cout<<"Create thread filed";
            exit(10);
        }


    while (GetMessage(&msg, NULL, 0, 0)) { 

        TranslateMessage(&msg); 
        DispatchMessage(&msg);

    }

    return msg.wParam;   

When i call function createMainWindow() in main part of my program it works as it should, but when i run it in my thread(ClientThread) it doesn't work. i've read that i should create windows only in main thread. Is it true? And if it's true, what is the simplest way to call this function from another thead to be done in main thread?

---

Thanks everyone. Now i know the problem, but i'm stuck with solution. My client thread code is:

while(1){

    vector<HWND> AllHandlers;

    pair<string,string> Answer = Pointer->receiveMsgByUdp();

    if(!Pointer->isMyLocalAddress(Answer.first)){

        int type = messageUdpContentType(Answer.second);

        switch(type){

        case 0 :

            Pointer->sendMsgToIpUdp(Answer.first,"<?xml version='1.0'?><accepted/>");
            AllHandlers = getAllHandlersOfElementsOnWindowsByIdentityCode(Pointer->getAllHandlers(),List_Box);
            for(vector<HWND>::iterator j = AllHandlers.begin();j!=AllHandlers.end();j++)
                if(SendMessageA(*j, LB_FINDSTRINGEXACT, 0, (LPARAM)Answer.first.c_str())==LB_ERR)
                    SendMessageA(*j, LB_ADDSTRING, 0, (LPARAM)Answer.first.c_str());

            break;

        case 1 :
            AllHandlers = getAllHandlersOfElementsOnWindowsByIdentityCode(Pointer->getAllHandlers(),List_Box);
            for(vector<HWND>::iterator j = AllHandlers.begin();j!=AllHandlers.end();j++)
                if(SendMessageA(*j, LB_FINDSTRINGEXACT, 0, (LPARAM)Answer.first.c_str())==LB_ERR)
                    SendMessageA(*j, LB_ADDSTRING, 0, (LPARAM)Answer.first.c_str());

            break;

        case 2 :
            AllHandlers = getAllHandlersOfElementsOnWindowsByIdentityCode(Pointer->getAllHandlers(),List_Box);
            for(vector<HWND>::iterator j = AllHandlers.begin();j!=AllHandlers.end();j++)
                if((i = SendMessageA(*j, LB_FINDSTRINGEXACT, 0, (LPARAM)Answer.first.c_str()))!=LB_ERR)
                    SendMessageA(*j,LB_DELETESTRING, 0, (LPARAM)Answer.first.c_str());

            break;

        case 3 :

            userReply = MessageBoxW(NULL, L"Принять приглашение на конференцию?",
                    L"", MB_YESNO | MB_ICONQUESTION); 
            if (userReply==IDYES){

                //todo: Проверка на создание встречи, в которой уже состоишь
                string nameOfConf = fetchNameOfInviteConf(Answer.second);
                Pointer->createConference(nameOfConf);
                HWND toSendTo = createMainWindow(Pointer);
                Pointer->setHandlerInfo(nameOfConf,toSendTo);               
                Pointer->addNewMemberToConference_ServerType(nameOfConf,Answer.first);
                string toSend = string("<?xml version='1.0'?><inviteAccepted>") + nameOfConf + string("</inviteAccepted>");
                Pointer->sendMsgToIpUdp(Answer.first,toSend);

            }
            break;

        case 4 :

            string nameOfConf = fetchNameOfInviteAcceptConf(Answer.second);

            toSend.clear();
            Participants.clear();
            Participants = Pointer->getCurrentParticipants(nameOfConf);
            toSend+="<?xml version='1.0'?>";
            toSend+="<conference>";
            toSend+="<nameOfConference>";
            toSend+=nameOfConf;
            toSend+="</nameOfConference>";
            for(vector<string>::iterator i = Participants.begin();i!=Participants.end();i++){
                toSend+="<participant>" + *i + "</participant>";
            }
            toSend+="</conference>";



            Pointer->addNewMemberToConference_ClientType(nameOfConf,Answer.first);

            Pointer->sendToIpTcp(Answer.first,toSend);

            break;

    }

the function receiveMsgByUdp() stops this thread until receives message. I apologize for lack of knowledge, but what functions can i use or another stuff to solve this. Should i rewrite my method receiveMsgByUdp() to be asynchronous or how can i call function createMainWindow() to be run on main thread?About the last variant : how can i do this in pure winapi, i couldn't found any simple examples. Can someone give code snippet. Thanks one more time)

Answer 1

You can indeed create windows in threads other than the main UI thread. However, those windows will have affinity to the thread that created them and you will need to run a message pump in each and every thread that creates windows.

So whilst you can do what you ask, Win32 is really designed to work with all windows in a process having affinity to the same thread. There's really nothing to be gained from creating multiple UI threads. All you will succeed in doing is making your life extraordinarily and needlessly complex.