Windows API 关于模态对话框与非模态对话框的两个疑问？

#include <windows.h>
#include "resource.h"

LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM) ;
BOOL CALLBACK BoxDlgProc(HWND, UINT, WPARAM, LPARAM) ;

HWND hDlgModeless;

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
                   PSTR szCmdLine, int iCmdShow)
{
    LPCWSTR szAppName = TEXT("Russian_Box");
    MSG msg;
    HWND hwnd;
    WNDCLASS wndclass;

    wndclass.style= CS_HREDRAW | CS_VREDRAW;
    wndclass.lpfnWndProc= WndProc;
    wndclass.cbClsExtra= 0 ;
    wndclass.cbWndExtra= 0 ;
    wndclass.hInstance= hInstance;
    wndclass.hIcon= LoadIcon(hInstance, szAppName) ;
    wndclass.hCursor= LoadCursor(NULL, IDC_ARROW) ;
    wndclass.hbrBackground= (HBRUSH) GetStockObject(WHITE_BRUSH) ;
    wndclass.lpszMenuName= szAppName;
    wndclass.lpszClassName= szAppName;
    if (!RegisterClass(&wndclass))
    {
        MessageBox( NULL, TEXT("This program requires Windows NT!"),
            szAppName, MB_ICONERROR) ;
        return 0 ;
    }

    hwnd= CreateWindow(szAppName, TEXT("RUSSIAN_BOX"),
        WS_OVERLAPPEDWINDOW &  ~WS_SIZEBOX & ~WS_MAXIMIZEBOX,
        CW_USEDEFAULT, CW_USEDEFAULT,
        CW_USEDEFAULT, CW_USEDEFAULT,
        NULL, NULL, hInstance, szCmdLine) ;

    ShowWindow(hwnd, iCmdShow) ;
    UpdateWindow(hwnd) ;

    hDlgModeless= CreateDialog(hInstance, TEXT("Russian_Box"), hwnd, BoxDlgProc) ;

    while (GetMessage(&msg, NULL, 0, 0))
    {
        if (hDlgModeless == 0 || !IsDialogMessage (hDlgModeless, &msg))
        {
            TranslateMessage(&msg) ;
            DispatchMessage(&msg) ;  
        }
    }
    return msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam,LPARAM lParam)
{
    switch (message)
    {
        case WM_SETFOCUS:
            SetFocus (hDlgModeless);
            return 0;
        case WM_DESTROY:
            DestroyWindow(hDlgModeless);
            PostQuitMessage(0) ;
            return 0 ;
    }
    return DefWindowProc(hwnd, message, wParam, lParam) ;
}

BOOL CALLBACK BoxDlgProc(HWND hDlg, UINT message,WPARAM wParam, LPARAM lParam)
{
    switch (message)
    {
    case WM_SETFOCUS:
        return TRUE;
    case WM_KILLFOCUS:
        SetFocus (hDlg);
        return TRUE;
    case WM_KEYDOWN:
        return TRUE;
    case WM_CHAR:
        return TRUE;
    }
    return FALSE;
}