I am using customized user control wrapper. this has one dropdownbox with values populated from database.

here is the code of .ascx.cs file ....

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;

namespace umbracotest
{
    public partial class BookList : System.Web.UI.UserControl, 
        umbraco.editorControls.userControlGrapper.IUsercontrolDataEditor
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!Page.IsPostBack || DropDownList1.SelectedValue.Length.ToString() == "0")
            {
                fillBooks(DropDownList1);

            }
        }

        private void fillBooks(DropDownList DropDownList1)
        {
            book b = new book();
            string st = "select book_id, book_name from book_master";

            DataSet ds = b.fetchdata(st, null); // dynamically fill book type in dropdownlist
            DataTable dt = ds.Tables[0];
            
            if (dt != null)
            {
                DropDownList1.Items.Clear();
                DropDownList1.DataValueField = "BOOK_ID";
                DropDownList1.DataTextField = "BOOK_NAME";
                DropDownList1.DataSource = dt;
                DropDownList1.DataBind();
               

                if (!string.IsNullOrEmpty(_umbval))
                {
                    DropDownList1.SelectedValue = _umbval;
                }

            }
        }

        #region IUsercontrolDataEditor Members

        private string _umbval;

        public object value
        {
            get
            {
                return DropDownList1.SelectedValue;
                //throw new NotImplementedException();
            }
            set
            {
                _umbval = value.ToString();
                //throw new NotImplementedException();
            }
        }

        #endregion
    }
}

now on before publish event I am inserting this dropdown box value to another DB. this work fine....

now I want to replace dropdownbox with checkbox list, so that user can select multiple values and so I can insert multiple records in DB on before publish event.... so how to get and set values for this in above code....

currently my before publish event code is as follows

namespace umbracotest
{
    public class AppBase : umbraco.BusinessLogic.ApplicationBase
    {
        public AppBase()
        {
            Document.BeforePublish += new Document.PublishEventHandler(Document_BeforePublish);
        }

        void Document_BeforePublish(Document sender, PublishEventArgs e)
        {

            Log.Add(LogTypes.Debug, sender.Id, "the document " + sender.Text + " is about to be published");

            if (sender.ContentType.Alias == "BooksPage")
            {
                //BookList bl = new BookList();
                book b = new book();

                b.insertBookNode(sender.getProperty("listOfBooks").Value.ToString());
                
                //sender.ExpireDate = DateTime.Now.AddDays(14);
                sender.Save();
            }


            //cancel the publishing
            //e.Cancel = true;
        }
    }
}

can anybody help.. I am using umbraco 4.7 and new to this.