添加一行:

Add a line:

protected void CreateUserWizard1_CreatingUser(object sender, LoginCancelEventArgs e)
    {
        if (RadioButtonList1.SelectedIndex == -1)
        { 
            e.Cancel=true;  // <----- Add this line.
            return;//not to proceed
        }
    }

请参见 MSDN [ ^ ].