<form id="form1" runat="server">
    <div>
        <asp:CheckBoxList ID="cblTest" runat="server">
            <asp:ListItem>A</asp:ListItem>
            <asp:ListItem>B</asp:ListItem>
            <asp:ListItem>C</asp:ListItem>
            <asp:ListItem>D</asp:ListItem>
            <asp:ListItem>E</asp:ListItem>
            <asp:ListItem>F</asp:ListItem>
        </asp:CheckBoxList>
        <br />
        <asp:Button ID="btnCV" runat="server" Text="Go" onclick="btnCV_Click"/>
    </div>
    </form>

protected void btnCV_Click(object sender, EventArgs e)
        {
            string value = "";
            for (int i = 0; i < cblTest.Items.Count; i++)
            {
                value += cblTest.Items[i].Selected ? "1" : "0";
            }
            Response.Write(value);
        }