LRESULT CALLBACK SoftwareMainProcedure(HWND hWnd, UINT msg, WPARAM wp, LPARAM lp)
{
    switch (msg)
    {
    case WM_COMMAND:

        switch (wp)
        {

        case draw_plot:
            GraphClass.style          = CS_HREDRAW | CS_VREDRAW;
            GraphClass.lpfnWndProc    = DrawProcedure;
            GraphClass.hInstance      = hInst;
            GraphClass.lpszMenuName  = NULL;
            GraphClass.lpszClassName = L"graphics";

            if (!RegisterClassW(&GraphClass))
            {
                return -1;
            }

            gr_draw = CreateWindow(L"graphics", L"DRAW", WS_VISIBLE | WS_BORDER | WS_MAXIMIZE | WS_HSCROLL | WS_VSCROLL | WS_OVERLAPPEDWINDOW, 0, 0, 800, 700, NULL, NULL, hInst, NULL);

            break;
        case open_plot:
            GraphClass.style          = CS_HREDRAW | CS_VREDRAW;
            GraphClass.lpfnWndProc    = GraphProcedure;
            GraphClass.hInstance      = hInst;
            GraphClass.lpszMenuName  = NULL;
            GraphClass.lpszClassName = L"graphics";


            if (!RegisterClassW(&GraphClass))
            {
                return -1;
            }

            gr_open = CreateWindow(L"graphics", L"OPEN", WS_VISIBLE | WS_BORDER | WS_MAXIMIZE | WS_OVERLAPPEDWINDOW, 0, 0, 800, 700, NULL, NULL, hInst, NULL);

            break;
        default:
            break;
        }
        break;

    case WM_SIZE:
    {
        ...
    }
    break;
    case WM_CREATE:
            ...
    case WM_DESTROY: // close mainwindow
        ExitSoftware();
        break;
    default:
        return DefWindowProc(hWnd, msg, wp, lp);
    }
}