::GetClientRect(HWND,&LPRECT); return 0 ?

最近在写一个控件CMenuButton，初衷是提供设置偏移量的接口，可是ASSERT总是中断，调试发现::GetClientRect()返回的rect都是0值。
Google了一下，原来：
Q:“为何在OnCreate调用取出的就是0 呢？
在OnInitialUpdate中是可以调用的，返回数据也是正确的 ”

A:“OnCreate时还没创建出客户区 ”

void CMenuButton::SetOffset(int OffsetL, int OffsetR, int OffsetTop, int OffsetBottom)
{
   //if out of client bound, then hold it in the bound
   RECT client_rect;
   ::GetClientRect(m_hWnd, &client_rect);//返回值总是0?

   OffsetL = OffsetL<0 ? 0 : OffsetL;
   OffsetR = OffsetR<0 ? 0 : OffsetR;
   OffsetTop = OffsetTop<0 ? 0 :OffsetTop;
   OffsetBottom = OffsetBottom<0 ? 0 :OffsetBottom;

   ATLASSERT(( (OffsetL+OffsetR < client_rect.right-client_rect.left) && (OffsetTop+OffsetBottom < client_rect.bottom-client_rect.top)));

   m_nOffsetLeft = OffsetL;
   m_nOffsetRight = OffsetR;
   m_nOffsetTop = OffsetTop;
   m_nOffsetBottom = OffsetBottom;
   m_nItemWidth = client_rect.right-client_rect.left-m_nOffsetLeft-m_nOffsetRight;
   ATLASSERT(m_nItemWidth > 0);
   m_nItemHeight2 = m_nItemHeight1 = client_rect.bottom-client_rect.top-m_nOffsetBottom-m_nOffsetTop;
   ATLASSERT(m_nItemHeight1>0);
}

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