Listbox multiitem selection on selectedindex change?

I have a list box in which i have multiple value

<asp:ListBox ID="lstbox" runat="server" SelectionMode="Multiple" Width="120px">
<asp:ListItem Value="0">None</asp:ListItem>
<asp开发者_StackOverflow:ListItem Value="8">Rohan</asp:ListItem>
<asp:ListItem Value="16">Jems</asp:ListItem>
<asp:ListItem Value="64">Tena</asp:ListItem>
<asp:ListItem Value="4">Marry</asp:ListItem>
<asp:ListItem Value="2">Pinky</asp:ListItem>
<asp:ListItem Value="32">Coral</asp:ListItem>
<asp:ListItem Value="1">Dev</asp:ListItem>
<asp:ListItem Value="128">Sam</asp:ListItem>
</asp:ListBox>

at once i want to select either one item or more than one and one selected index change i am calling a method

    protected void lstbox_SelectedIndexChanged(object sender, EventArgs e)
    {
        Presenter.getRoles();//filling a grid from database call
    }

    public void getRoles()
    {
        int proid = 0;
        int per=0;
        if (View.UPrivileges.Count > 0)
        {
            proid = Convert.ToInt32(View.UPrivileges.SelectedValue);//listbox selection
            per= Convert.ToInt32(View.Products.SelectedValue);// i have a ddl product checking the value
        }
        if (proid != 0 || per !=0)
        {
            View.Ownerid = per.ToString();
            View.Role = (CRole)proid;
            List<User> list = new List<User>();
            list.AddRange(userDao.GetUserRolesForItems(View.Role, View.Ownerid));// query method call            View.UserListItems.List = list;
            View.UserListItems.TotalRowCount = list.Count;
        }
    }

when i am selecting 1 value then its working fine but for more than one items giving error, I want to know whether its possible with selectedIndexChange or not, if yes how if not then how to do it on one button click

this may help

    foreach (ListItem item in lstbox.Items)
        {
            if (item.Selected)
            {
               //code here

            }
        }