两个参数都是可NULL的。指定1个或2个查找条件都可以。
lpClassName:
If lpClassName is NULL, it finds any window whose title matches the lpWindowName parameter.
lpWindowName:
The window name (the window's title). If this parameter is NULL, all window names match.
参考msdn:
https://docs.microsoft.com/zh-cn/windows/win32/api/winuser/nf-winuser-findwindoww