Adding Items to ListBox Control

<table align="center">
<tr>
<td>
Enter Customer Id
</td>
<td>
<asp:TextBox ID="txtCustomerID" runat="server"></asp:TextBox>
    <asp:RequiredFieldValidator ID="RequiredFieldValidator12" runat="server" 
                             ControlToValidate="txtCustomerID" ValidationGroup="v1" 
                             ErrorMessage="*" Width="53px"></asp:RequiredFieldValidator>
           <cc1:AutoCompleteExtender ID="txtCustomerID_AutoCompleteExtender" 
                runat="server" DelimiterCharacters="" Enabled="True" ServicePath="~/Admin/AutoComplete.asmx" ServiceMethod="GetSerialNumber" 
                 MinimumPrefixLength="1" CompletionInterval="3" EnableCaching="true" CompletionSetCount="12"                                                   
                TargetControlID="txtCustomerID">
            </cc1:AutoCompleteExtender> 
</td>
</tr>
<tr>
<td></td>
<td>
            <asp:Button ID="btnAddtoList" runat="server" Text="Add to list" class="mybutton"
        onclick="btnAddtoList_Click" ValidationGroup="v1" />
    <asp:Button ID="btnRemoveFromList" runat="server" Text="Remove from List" class="mybutton"
                onclick="btnRemoveFromList_Click" />
        </td>
        </tr>
  <tr>
        <td></td>
        <td>
        <asp:ListBox ID="lstUsers" runat="server"></asp:ListBox>            
        </td>
        </tr>
</table>

protected void btnAddtoList_Click(object sender, EventArgs e)
    {
        System.Web.UI.WebControls.ListItem li = new System.Web.UI.WebControls.ListItem(txtCustomerID.Text);
        string Serial = txtCustomerID.Text;
        dt = ee.GetUserID(Serial);
        if (dt.Rows.Count == 0)
        {
            txtCustomerID.Focus();
            txtCustomerID.Text = "";
            ClientScript.RegisterStartupScript(this.GetType(), "Alert", "alert('Invalid Serial No');", true);
        }
        else
        {
            if (!lstUsers.Items.Contains(li))
            {
                lstUsers.Items.Add(li);
            }
            else
            {
                ClientScript.RegisterStartupScript(this.GetType(), "Alert", "alert('Customer Id already exists');", true);
            }
        }        
        txtCustomerID.Text = "";
    }

 public DataTable GetUserID(string Serial)
    {
        SqlConnection con = new SqlConnection(str);
        SqlCommand cmd = new SqlCommand("sp_getuserid", con);
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.Parameters.AddWithValue("@serial", Serial);
        SqlDataAdapter ada = new SqlDataAdapter(cmd);
        DataTable dt = new DataTable();
        ada.Fill(dt);
        return dt;
    }

[System.Web.Script.Services.ScriptService]
public class AutoComplete : System.Web.Services.WebService {

    public AutoComplete () {

        //Uncomment the following line if using designed components 
        //InitializeComponent(); 
    }

    [WebMethod]
    public string[] GetSerialNumber(string prefixText, int count)
    {
        if (count == 0)
        {
            count = 10;
        }
        DataTable dt = GetRecords(prefixText);
        List<string> items = new List<string>(count);
        for (int i = 0; i < dt.Rows.Count; i++)
        {
            string strName = dt.Rows[i][0].ToString();
            items.Add(strName);
        }
        return items.ToArray();
    }
    public DataTable GetRecords(string strName)
    {
        string strConn = ConfigurationManager.ConnectionStrings["cons"].ConnectionString;
        SqlConnection con = new SqlConnection(strConn);
        SqlCommand cmd = new SqlCommand();
        cmd.Connection = con;
        cmd.CommandType = System.Data.CommandType.Text;
        cmd.Parameters.AddWithValue("@Name", strName);
        cmd.CommandText = "Select propertyvaluesstring from dbo.aspnet_Profile where propertyvaluesstring like '%'+@Name+'%'";
        DataSet objDs = new DataSet();
        SqlDataAdapter dAdapter = new SqlDataAdapter();
        dAdapter.SelectCommand = cmd;
        con.Open();
        dAdapter.Fill(objDs);
        con.Close();

        DataTable dt = new DataTable();
        dt.Columns.Add("serialno");
        for (int i = 0; i < objDs.Tables[0].Rows.Count; i++)
        {
            string temp = objDs.Tables[0].Rows[i]["propertyvaluesstring"].ToString();
            string[] temp1 = temp.Split('M');
            dt.Rows.Add("M"+temp1[1].ToString());

        }
        return dt;       
    }    
}

 protected void btnRemoveFromList_Click(object sender, EventArgs e)
    {
        if (lstUsers.Items.Count <= 0)
        {
            ClientScript.RegisterStartupScript(this.GetType(), "Alert", "alert('List is empty');", true);
        }
        if (lstUsers.SelectedIndex == -1)
        {
            ClientScript.RegisterStartupScript(this.GetType(), "Alert", "alert('Select the users to remove');", true);
        }
        else
        {
            lstUsers.Items.Remove(lstUsers.SelectedItem.Text);
            ClientScript.RegisterStartupScript(this.GetType(), "Alert", "alert('Customer Id removed successfully');", true);
        }
    }