When the application's main window is a dialog, how to SetIcon for child dialog

MFC >> When the application's main window is a dialog, how to SetIcon for child dialog

The main window of the application is a dialog, the sub dialog is pop up by
the main window.

I have set the sub dialog to show on taskbar, but I cannot set the taskbar
icon, use SetIcon doesn't work, how to solve this problem?

Thanks for your great help!
Jack

Visual Studio355