请教高手大哥们（API）：关于按钮的自绘问题？？

简要代码如下：OnCreate(...)
{
   CreateWindowEx(0,.....,style | BS_OWNERDRAW..);
}
OnDrawItem(lpdis)
{
//Create compatible DC
HDC hdcMem=CreateCompatibleDC(lpdis->hDC);
HBITMAP hbmp=LoadBitmap(app.m_hInstance,MAKEINTRESOURCE(IDB_STYLE1));HBITMAP hbmpOrg=(HBITMAP)SelectObject(hdcMem,hbmp);
//设置button样式为圆角矩形
RECT rc;
memcpy(&rc,&lpdis->rcItem);
HRGN hrgn=CreateRoundRectRgn(rc.left,rc.top,rc.right,rc.bottom,15,15);
SetWindowRgn(GetDlgItem(hwnd,lpdis->CtlID),hrgn,TRUE);
StretchBlt(lpdis->hDC,lpdis->rcItem.left,lpdis->rcItem.top,lpdis->rcItem.right-lpdis->rcItem.left,lpdis->rcItem.bottom-lpdis->>rcItem.top,hdcMem,0,0,rc.right,rc.bottom,SRCCOPY);
SelectObject(hdcMem,hbmpOrg);
DeleteDC(hdcMem);
DeleteObject(hbmp);
DeleteObject(hrgn);}
为什么按钮开始显示为圆角矩形，下一次再显示时就变成了矩形了。