VC++的问题，有关映射模式的

Question

这段代码在编译时没错，但是运行时却出错了，求解释！！！
#include<windows.h>
#include<stdlib.h>
#include<string.h>
#include<stdio.h>
int nMode;
LRESULT CALLBACK WndProc(HWND,UINT,WPARAM,LPARAM);
int WINAPI WinMain(HINSTANCE hInstance,HINSTANCE hPrevInst,LPSTR lpszCmdLine,int nCmdShow)
{
HWND hwnd;
MSG Msg;
WNDCLASS wndclass;
char lpszClassName[]="映射模式";
char lpszTitle[]="My_Map_Mode";
wndclass.style=0;
wndclass.lpfnWndProc=WndProc;
wndclass.cbClsExtra=0;
wndclass.cbWndExtra=0;
wndclass.hInstance=hInstance;
wndclass.hIcon=LoadIcon(NULL,IDI_APPLICATION);
wndclass.hCursor=LoadCursor(NULL,IDC_CROSS);
wndclass.hbrBackground=(HBRUSH)GetStockObject(WHITE_BRUSH);
wndclass.lpszClassName=NULL;
wndclass.lpszClassName=lpszClassName;
if(!RegisterClass(&wndclass))
{
MessageBeep(0);
return FALSE;
}
hwnd=CreateWindow(  lpszClassName,
            lpszTitle,
WS_OVERLAPPEDWINDOW,
CW_USEDEFAULT,
CW_USEDEFAULT,
CW_USEDEFAULT,
CW_USEDEFAULT,
NULL,
NULL,
hInstance,
NULL
);
ShowWindow(hwnd,nCmdShow);
UpdateWindow(hwnd);
while(GetMessage(&Msg,NULL,0,0))
{
TranslateMessage(&Msg);
DispatchMessage(&Msg);
} return Msg.wParam;
}
LRESULT CALLBACK WndProc(HWND hwnd,UINT message,WPARAM wParam,LPARAM lParam)
{
HDC hdc;
PAINTSTRUCT ps;
HBRUSH hB1,hB2;
switch(message)
{
case WM_LBUTTONDOWN:
nMode=MM_ISOTROPIC;
InvalidateRect(hwnd,NULL,1);
break;
case WM_RBUTTONDOWN:
nMode=MM_ANISOTROPIC;
InvalidateRect(hwnd,NULL,1);
break;
case WM_PAINT:
hdc=BeginPaint(hwnd,&ps);
SetMapMode(hdc,nMode);
SetWindowExtEx(hdc,150,150,NULL);
SetViewportExtEx(hdc,150,100,NULL);
SetViewportOrgEx(hdc,150,60,NULL);
hB1=(HBRUSH)GetStockObject(WHITE_BRUSH);
hB2=(HBRUSH)GetStockObject(BLACK_BRUSH);
SelectObject(hdc,hB1);
RoundRect(hdc,0,0,150,150,30,30);
SelectObject(hdc,hB2);
Ellipse(hdc,0,10,150,140);
EndPaint(hwnd,&ps);
DeleteObject(hB1);
DeleteObject(hB2);
break;
case WM_DESTROY:
PostQuitMessage(0);
break;
default:
return DefWindowProc(hwnd,message,wParam,lParam);
}
return 0;
}