This is not a real answer, but since so many people seem to believe that Win32 forbids creating children in other threads than the parent, I feel obliged to post a demonstration to the contrary.

The code below demonstrates creation of a child window on a parent belonging to a different process. It accepts a window handle value as a command-line parameter and creates a child window on that parent.

// t.cpp

#include <windows.h>
#include <stdio.h>

#define CLASS_NAME L"fykshfksdafhafgsakr452"


static LRESULT CALLBACK WindowProc( HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam )
{
    switch ( msg )
    {
        case WM_DESTROY:
            PostQuitMessage(0);
            break;

        case WM_PAINT:
        {
            PAINTSTRUCT ps;
            BeginPaint(hwnd, &ps);
            EndPaint(hwnd, &ps);
            break;
        }
    }
    return DefWindowProc(hwnd, msg, wParam, lParam);
}



int main( int argc, char* argv[] )
{
    HWND parent = (argc >= 2) ? (HWND)strtoul(argv[1], 0, 0) : (HWND)0;
    printf("parent: 0x%x
", parent);

    WNDCLASS wc = {};
    wc.lpfnWndProc = WindowProc;
    wc.hInstance = (HINSTANCE)GetModuleHandle(NULL);
    wc.lpszClassName = CLASS_NAME;
    wc.hbrBackground = (HBRUSH)(COLOR_ACTIVECAPTION + 1);
    if ( !RegisterClass(&wc) )
    {
        printf("%d: error %d
", __LINE__, GetLastError());
        return 0;
    }

    const DWORD style = WS_CHILD | WS_VISIBLE;

    HWND hwnd = CreateWindow(CLASS_NAME, L"Test", style, 50, 50, 100, 100,
                             parent, 0, wc.hInstance, 0);

    if ( !hwnd )
    {
        printf("%d: error %d
", __LINE__, GetLastError());
        return 0;
    }

    MSG msg;
    while ( GetMessage(&msg, 0, 0, 0) )
        DispatchMessage(&msg);

    return 0;
}

Compile this with the following command (using MSVC command line environment):

cl /EHsc /DUNICODE /D_UNICODE t.cpp user32.lib

Then use Spy++ or some other tool to obtain handle value of any window -- e.g. of Notepad or the browser you re viewing this site in. Let s assume it s 0x00001234. Then run the compiled sample with t.exe 0x1234. Use Ctrl-C to terminate t.exe (or just close the console window).

When a child window is created, it can interact with the parent window via SendMessage. But, note that SendMessage across thread boundary blocks threads unlike PostMessage. If the parent window s thread is waiting for the child thread, and the child thread is trying to create a window whose parent is in that thread, then it s a deadlock.

In general, I don t think it s a good idea to make child-parent relationship across the threads. It can very easily make deadlock.

There are a lot of replies here saying that you MUST NOT attempt to have child and parent windows on different threads, and rather emphatically stating that it will not work.

If that was so, Windows would put some safeguards in and simply fail out when you attempted to call CreateWindow. Now, there definitely are thread coupling issues that can cause major issues, but, that said with those constraints, this is a supported scenario.

This is an interesting question: A lot of old school win32 guys have told me you CANNOT do this. In researching this, I found this forum: SendMessage(). My current theory is that CreateWindowEx() must send a message (via SendMessage(), so it blocks) to the parent window to ask for permission to exist (or at least to notify of its existence)... Anyhow, as long as that parent thread is free to process these messages, it all works...

A window is tied to the thread that creates it (more specifically, to that thread s message queue). A parent window cannot reside in a different thread than its child windows.