Populating ListBox with the selected Items from other LisBox.

 <asp:ListBox ID="ListBox1" SelectionMode="Multiple" runat="server">
 <asp:ListItem>1</asp:ListItem>       
 <asp:ListItem>2</asp:ListItem>        
 <asp:ListItem>3</asp:ListItem>
 </asp:ListBox>
 <asp:ListBox ID="ListBox2" 
SelectionMode="Multiple"  runat="server">
</asp:ListBox>

<asp:Button ID="Button1" runat="server" Text="Button" 
onclick="Button1_Click" />

 protected void Button1_Click(object sender, EventArgs e)
    {
        foreach (ListItem item in ListBox1.Items)
        {
            if (item.Selected == true)
            {
                ListBox2.Items.Add(item);
            }
        }
    }