Хочу сделать кнопке собственную процедуру обработки сообщений (WM_LBUTTONDOWN). Подскажите пожалуйста, почему не работает код:

#include <windows.h>

char szTitle[] = "Buttonz";
char szWindowClass[] = "Buttonz window class";

ATOM MyRegisterClass(HINSTANCE hInstance);
BOOL InitInstance(HINSTANCE, int);
LRESULT CALLBACK    WndProc(HWND, UINT, WPARAM, LPARAM);

HINSTANCE hInst;
HWND hMainWnd;
HWND hBtn;

int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
    MSG msg;

    MyRegisterClass(hInstance);

    if (!InitInstance (hInstance, nCmdShow)) return FALSE;

    while (GetMessage(&msg, NULL, 0, 0)) 
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    return msg.wParam;
}

ATOM MyRegisterClass(HINSTANCE hInstance)
{
    WNDCLASSEX wcex;

    wcex.cbSize = sizeof(WNDCLASSEX); 

    wcex.style        = CS_HREDRAW | CS_VREDRAW;
    wcex.lpfnWndProc    = (WNDPROC)WndProc;
    wcex.cbClsExtra    = 0;
    wcex.cbWndExtra    = 0;
    wcex.hInstance    = hInstance;
    wcex.hIcon        = LoadIcon(hInstance, (LPCTSTR)IDI_BUTTON);
    wcex.hCursor    = LoadCursor(NULL, IDC_ARROW);
    wcex.hbrBackground= (HBRUSH)(COLOR_BTNFACE+1);
    wcex.lpszMenuName    = NULL;
    wcex.lpszClassName= szWindowClass;
    wcex.hIconSm    = LoadIcon(wcex.hInstance, (LPCTSTR)IDI_SMALL);

    return RegisterClassEx(&wcex);
}

BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
   hInst = hInstance;

   hMainWnd = CreateWindowEx(WS_EX_CLIENTEDGE | WS_EX_TOPMOST,
       szWindowClass, szTitle,
       WS_OVERLAPPEDWINDOW | WS_BORDER | WS_CLIPCHILDREN,
       CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL);

   if (!hMainWnd) return FALSE;

   ShowWindow(hMainWnd, nCmdShow);
   UpdateWindow(hMainWnd);

   return TRUE;
}

static LRESULT CALLBACK ButtProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    HDC dc;
    HBRUSH brush;
    RECT rect;
    PAINTSTRUCT ps;

    switch (message)
    {
        
    case WM_ERASEBKGND:
        dc = GetDC(hWnd);
        brush = (HBRUSH)GetStockObject(BLACK_BRUSH);
        brush = (HBRUSH)SelectObject(dc, brush);
        GetClientRect(hWnd, &rect);
        FillRect(dc, &rect, 0);
        SelectObject(dc, brush);
        ReleaseDC(hWnd, dc);
        return 1;
        break;

    case WM_PAINT:
        dc = BeginPaint(hWnd, &ps);
        brush = (HBRUSH)COLOR_BTNFACE+1;
        brush = (HBRUSH)SelectObject(dc, brush);
        GetClientRect(hWnd, &rect);
        FillRect(dc, &rect, 0);

        DrawEdge(dc, &rect, EDGE_RAISED, BF_TOPLEFT);
        DrawEdge(dc, &rect, EDGE_RAISED, BF_BOTTOMRIGHT);

        DrawText(dc, "the", -1, &rect, DT_CENTER | DT_VCENTER | DT_SINGLELINE);

        EndPaint(hWnd, &ps);
        break;

    case WM_LBUTTONDOWN:
        MessageBox(NULL, "Pressed", "Button", MB_OK); // !!!!!!!!!! - не работает
        break;
    }
    return 0;
}


LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    switch (message) 
    {
    case WM_CREATE:
        hBtn = CreateWindowEx(0, "BUTTON", "Ok", WS_CHILD | WS_VISIBLE | WS_CLIPCHILDREN,
                      100, 10, 80, 80,
                      hWnd, NULL, hInst, NULL);

        SetWindowLong(hBtn, GWL_WNDPROC, (LONG)&ButtProc);
        break;
    case WM_DESTROY:
        PostQuitMessage(0);
        break;
    default:
        return DefWindowProc(hWnd, message, wParam, lParam);
    }
   return 0;
}