学习MFC必须弄懂的一个程序

#include <windows.h>
#include <stdio.h>
LRESULT CALLBACK WinSunProc(
HWND hwnd,      // handle to window
UINT uMsg,      // message identifier
WPARAM wParam, // first message parameter
LPARAM lParam   // second message parameter
);
int WINAPI WinMain(
HINSTANCE hInstance,      // handle to current instance
HINSTANCE hPrevInstance, // handle to previous instance
LPSTR lpCmdLine,          // command line
int nCmdShow              // show state
)
{
WNDCLASS wndcls;
wndcls.cbClsExtra=0;
wndcls.cbWndExtra=0;
wndcls.hbrBackground=(HBRUSH)GetStockObject(BLACK_BRUSH);
wndcls.hCursor=LoadCursor(NULL,IDC_CROSS);
wndcls.hIcon=LoadIcon(NULL,IDI_ERROR);
wndcls.hInstance=hInstance;
wndcls.lpfnWndProc=WinSunProc;
wndcls.lpszClassName="Weixin2003";
wndcls.lpszMenuName=NULL;
wndcls.style=CS_HREDRAW | CS_VREDRAW;
RegisterClass(&wndcls);
HWND hwnd;
hwnd=CreateWindow("Weixin2003","北京维新科学技术培训中心",WS_OVERLAPPEDWINDOW,
   0,0,600,400,NULL,NULL,hInstance,NULL);
ShowWindow(hwnd,SW_SHOWNORMAL);
UpdateWindow(hwnd);
MSG msg;
while(GetMessage(&msg,NULL,0,0))
{
   TranslateMessage(&msg);
   DispatchMessage(&msg);
}
return 0;
}
LRESULT CALLBACK WinSunProc(
HWND hwnd,      // handle to window
UINT uMsg,      // message identifier
WPARAM wParam, // first message parameter
LPARAM lParam   // second message parameter
)
{
switch(uMsg)
{
case WM_CHAR:
   char szChar[20];
   sprintf(szChar,"char is %d",wParam);
   MessageBox(hwnd,szChar,"weixin",0);
   break;
case WM_LBUTTONDOWN:
   MessageBox(hwnd,"mouse clicked","weixin",0);
   HDC hdc;
   hdc=GetDC(hwnd);
   TextOut(hdc,0,50,"计算机编程语言培训",strlen("计算机编程语言培训"));
   ReleaseDC(hwnd,hdc);
   break;
case WM_PAINT:
   HDC hDC;
   PAINTSTRUCT ps;
   hDC=BeginPaint(hwnd,&ps);
   TextOut(hDC,0,0,"维新培训",strlen("维新培训"));
   EndPaint(hwnd,&ps);
   break;
case WM_CLOSE:
   if(IDYES==MessageBox(hwnd,"是否真的结束？","weixin",MB_YESNO))
   {
    DestroyWindow(hwnd);
   }
   break;
case WM_DESTROY:
   PostQuitMessage(0);
   break;
default:
   return DefWindowProc(hwnd,uMsg,wParam,lParam);
}
return 0;
}