看MSDN:SetTimer的第一个参数: hWnd [in] Handle to the window to be associated with the timer. This window must be owned by the calling thread. If a NULL value for hWnd is passed in along with an nIDEvent of an existing timer, that timer will be replaced in the same way that an existing non-NULL hWnd timer will be.
hWnd
[in] Handle to the window to be associated with the timer. This window must be owned by the calling thread. If a NULL value for hWnd is passed in along with an nIDEvent of an existing timer, that timer will be replaced in the same way that an existing non-NULL hWnd timer will be.