Multiple selected items in ListBox.

<asp:ListBox ID="ListBox1" SelectionMode="Multiple"  runat="server">
        <asp:ListItem>one</asp:ListItem>
        <asp:ListItem>two</asp:ListItem>
        <asp:ListItem>three</asp:ListItem>
        </asp:ListBox> 

private void GetSelectedItems()
    {
        string[] str = { "one", "three" };
        foreach (string item in str)
        {
            for (int i = 0; i <= ListBox1.Items.Count - 1; i++)
            {
                if (ListBox1.Items[i].Text == item)
                { ListBox1.Items[i].Selected = true; }
            }
        }
    }