What does HWND do in ShellExecute?

Question

I use ShellExecute to do something, and the first parameters is HWND , the documentation on MSDN says:

A handle to the parent window used for displaying a UI or error messages. This value can be NULL if the operation is not associated with a window.

but I find whichever HWND value gets the same result.

for example :

ShellExecute(0, 'open', 'c:\', nil, nil, SW_SHOWNORMAL);

ShellExecute(Self.Handle, 'open', 'c:\', nil, nil, SW_SHOWNORMAL);

ShellExecute(123456, 'open', 'c:\', nil, nil, SW_SHOWNORMAL);

just gets the same thing (Opens disk C), so I wonder what's the use using different HWND?

by the way, when 'HWND = 0' is the DeskTop's Handle used?

Answer 1

That HWND is used as the owner window for any UI that is shown as a result of the call to ShellExecute. For example, any error message dialogs will be owned by that window.

The implications of a window being owned are described in the MSDN documentation. Key excerpts:

Being owned places several constraints on a window.

• An owned window is always above its owner in the z-order.
• The system automatically destroys an owned window when its owner is destroyed.
• An owned window is hidden when its owner is minimized.

The important one is the first one. If you are calling ShellExecute from a GUI app then you want any windows to be owned by the window that is currently active in your app. So pass MyForm.Handle.

If you have no GUI in your app then pass 0.

In the code samples, the call to ShellExecute is not showing any UI at all. So it makes no difference what you pass. But if your calls resulted in an error dialog being shown, then the window handle that you pass would become relevant.