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Window Handles
After creating a window, the creation function returns a window handle that uniquely identifies the window. An application uses this handle in other
functions to direct their actions to the window. A window handle has the HWND data type; an application must use this type when declaring a variable that
holds a window handle.
The Win32 API includes several special constants that can replace a window
handle in certain functions. For example, an application can use HWND_TOPMOST in
the SendMessageTimeout function, HWND_BROADCAST in the SendMessage function, or HWND_DESKTOP in the MapWindowPoints function.
Although the NULL constant is not a window handle, you can use it in some
functions to specify that no window is affected. For example, specifying NULL for
the CreateWindowEx function's hwndParent parameter creates a window that has no parent or owner. Some functions may
return NULL instead of a handle, indicating that the given action applies to no
window.
An application can use the FindWindow function to discover whether a window with the specified class name or window
name exists in the system. If such a window exists, FindWindow returns the handle of the window. To limit the search to the child windows of
a particular application, use the FindWindowEx function. The IsWindow function determines whether a window handle identifies a valid, existing
window.
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Win32 Multimedia Programmer's Reference (mmedia.hlp)
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