GetTopWindow function (winuser.h)

Examines the Z order of the child windows associated with the specified parent window and retrieves a handle to the child window at the top of the Z order.

HWND GetTopWindow(
  [in, optional] HWND hWnd
);

[in, optional] hWnd

Type: HWND

A handle to the parent window whose child windows are to be examined. If this parameter is NULL, the function returns a handle to the window at the top of the Z order.

Type: HWND

If the function succeeds, the return value is a handle to the child window at the top of the Z order. If the specified window has no child windows, the return value is NULL. To get extended error information, use the GetLastError function.

Conceptual

GetNextWindow

GetWindow

Reference

Windows