求助, 为什么我在关闭子窗口的时候父窗口自动关闭

// Winnd.cpp : Defines the entry point for the application.
//

#include "stdafx.h"
HINSTANCE g_hInstance;


 //窗口注册
BOOL _register(LPSTR winname, WNDPROC dealproc)
{
        WNDCLASSEX wce = {0};
        wce.cbSize = sizeof(wce);
        wce.style = CS_HREDRAW | CS_VREDRAW;
        wce.lpfnWndProc = dealproc;
        wce.cbClsExtra = 0;
        wce.cbWndExtra = 0;
        wce.hInstance = g_hInstance;
        wce.hIcon = NULL;
        wce.hIconSm = NULL;
        wce.hCursor = NULL;
        wce.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);
        wce.lpszClassName = winname;
        wce.lpszMenuName = NULL;

        ATOM nAtom = RegisterClassEx(&wce);
        if(nAtom == 0)
        {
            return FALSE;
        }
        return TRUE;
}

//主窗口实现
HWND dream(LPSTR winname, LPSTR showname)
{
    HWND hWnd = CreateWindowEx(0, winname, showname, WS_OVERLAPPEDWINDOW, 0, 0, 500, 500, NULL, NULL, g_hInstance, NULL);
    return hWnd;
}

//子窗口实现
HWND child(LPSTR winname, LPSTR showname, HWND hWnd)
{
    HWND hChild = CreateWindowEx(0, winname, showname, WS_OVERLAPPEDWINDOW | WS_CHILD | WS_VISIBLE, 0, 0, 200, 200, hWnd, NULL, g_hInstance, NULL);
    return hChild;
}

//自定义处理函数
LRESULT CALLBACK dealproc(HWND hWnd, UINT nMsg, WPARAM wParam, LPARAM lParam)
{
    switch(nMsg)//关闭程序可以结束进程
    {
    case WM_DESTROY:
        PostQuitMessage(0);//能够使GetMessage返回0
        break;
    }
    return DefWindowProc(hWnd, nMsg, wParam, lParam);
}

//显示窗口
void display(HWND hWnd)
{
    ShowWindow(hWnd, SW_SHOW);
    UpdateWindow(hWnd);
}

//设置消息循环
void message()
{
    MSG nMsg = {0};
    while(GetMessage(&nMsg, NULL, 0, 0))
    {
        TranslateMessage(&nMsg);
        DispatchMessage(&nMsg);
    }
}

int APIENTRY WinMain(HINSTANCE hInstance,
                     HINSTANCE hPrevInstance,
                     LPSTR     lpCmdLine,
                     int       nCmdShow)
{
    g_hInstance = hInstance;
    _register("main", dealproc);
    HWND hWnd = dream("main", "主窗口");
    _register("_child", dealproc);
    HWND hChild = child("_child", "子窗口", hWnd);
    _register("__child", dealproc);
    HWND hChild2 = child("__child", "子窗口", hWnd);
    display(hWnd);
    message();
    return 0;
}