I am trying to write my first custom datatype using the 'three class method' - inspired very heavily by Tim and Ove - and I have run  into a brick wall...

Requirement : To store an array of values in a single field as parseable XML. I think the actual datatype works fine, shown in the class 'ClassTimeDatatype' below. It's rendering fine in the editor, and when I enter the first row of data and save, the values are persisted to the database. However, when I navigate back to that node in the CMS, the values are not coming back out of the database. I'm sure I'm missing something simple but I cant see what!

If someone who knows what they are doing could glance over this so see what I am missing it would be so useful.

/* Credits:
 *
 * Example Code and Setup:
 * Tim Geyssens (http://www.nibble.be)
 *
 * Template and Documentation:
 * Ove Andersen (http://www.eyecatch.no)
 *
*/
using System;
using umbraco.cms.businesslogic.datatype;

namespace ClassTimeDatatype
{
    public class DataType : AbstractDataEditor
    {
        private PrevalueEditor _prevalueeditor;

        private ClassTimeDatatype _control = new ClassTimeDatatype();

        public DataType()
        {
            base.RenderControl = _control;
            _control.Init += new EventHandler(ControlInit);
            base.DataEditorControl.OnSave += DataEditorControl_OnSave;
           
        }

        public void ControlInit(object sender, EventArgs e)
        {
            _control.LoadDataValue(base.Data.Value);
        }

        public override Guid Id
        {
            get
            {
                return new Guid("68572280-7b8e-49ab-aa74-d2291f29f954");
            }
        }

        public override string DataTypeName
        {
            get
            {
                return "Class Times";
            }
        }

        public override umbraco.interfaces.IDataPrevalue PrevalueEditor
        {
            get
            {
                if (_prevalueeditor == null)
                {
                    _prevalueeditor = new PrevalueEditor(this);
                }
                return _prevalueeditor;
            }
        }

        void DataEditorControl_OnSave(EventArgs e)
        {
            base.Data.Value = _control.GetDataValue();
            _control.LoadDataValue(base.Data.Value);

        }
    }
}

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

using System.Linq;
using System.Xml;
using System.Text;

namespace ClassTimeDatatype
{
    public class ClassTimeDatatype : Panel
    {

        // Datatype Controls

        private List<TextBox> _txtDateBegin;
        private List<TextBox> _txtDateEnd;

        private List<CheckBoxList> _chkRoomList;
        private List<CheckBoxList> _chkWeekdayList;
        private List<TextBox> _txtStartTime;
        private List<TextBox> _txtFinishTime;

        private List<ListItem> roomsTemplate;
        private List<ListItem> weekdaysTemplate;

        private object _dataValue;

        protected override void OnInit(EventArgs e)
        {
           
            roomsTemplate = new List<ListItem>();

            roomsTemplate.Add(new ListItem("Studio 1", "Studio 1"));
            roomsTemplate.Add(new ListItem("Studio 2", "Studio 2"));
            roomsTemplate.Add(new ListItem("Studio 3", "Studio 3"));

            weekdaysTemplate = new List<ListItem>();

            weekdaysTemplate.Add(new ListItem("Monday", "Monday"));
            weekdaysTemplate.Add(new ListItem("Tuesday", "Tuesday"));
            weekdaysTemplate.Add(new ListItem("Wednesday", "Wednesday"));
            weekdaysTemplate.Add(new ListItem("Thursday", "Thursday"));
            weekdaysTemplate.Add(new ListItem("Friday", "Friday"));
            weekdaysTemplate.Add(new ListItem("Saturday", "Saturday"));
            weekdaysTemplate.Add(new ListItem("Sunday", "Sunday"));

            base.OnInit(e);

        }

        internal void LoadDataValue(object dataValue)
        {
            try
            {
                this.Controls.Clear();

                _txtDateBegin = new List<TextBox>();
                _txtDateEnd = new List<TextBox>();
                _chkRoomList = new List<CheckBoxList>();
                _chkWeekdayList = new List<CheckBoxList>();
                _txtStartTime = new List<TextBox>();
                _txtFinishTime = new List<TextBox>();

                _dataValue = dataValue;

                //Takes database value and configes current controls
                if (dataValue != null)
                {
                    XmlDocument retDoc = new XmlDocument();
                    retDoc.LoadXml(dataValue.ToString());

                    foreach (XmlNode node in retDoc.SelectNodes("//ClassTime"))
                    {
                        addControlRow(node);
                    }
                }
            }
            catch
            {
            }
            finally
            {
                addControlRow();
            }
        }

        internal object GetDataValue()
        {
            //Returns value from currently loaded controls for saving to database

            StringBuilder sb = new StringBuilder();

            sb.Append("<ClassTimes>");
            for (int i = 0; i < _txtDateBegin.Count; i++)
            {
                if (!string.IsNullOrEmpty(_txtDateBegin[i].Text) || !string.IsNullOrEmpty(_txtDateEnd[i].Text) || !string.IsNullOrEmpty(getCheckBoxes(_chkRoomList[i])) || !string.IsNullOrEmpty(getCheckBoxes(_chkWeekdayList[i])) || !string.IsNullOrEmpty(_txtStartTime[i].Text) || !string.IsNullOrEmpty(_txtFinishTime[i].Text))
                sb.AppendFormat("<ClassTime dateBegin=\"{0}\" dateEnd=\"{1}\" room=\"{2}\" weekDay=\"{3}\" startTime=\"{4}\" finishTime=\"{5}\"/>", _txtDateBegin[i].Text, _txtDateEnd[i].Text, getCheckBoxes(_chkRoomList[i]), getCheckBoxes(_chkWeekdayList[i]), _txtStartTime[i].Text, _txtFinishTime[i].Text);
            }
            sb.Append("</ClassTimes>");

            return sb.ToString();
        }


        protected override void Render(System.Web.UI.HtmlTextWriter writer)
        {
            // (Prints out the prevalue editor control)

            writer.WriteLine("<table>");
          
            writer.Write("<tr>");
            writer.Write("<th>Date Begin (yyyy-mm-dd)</th>");
            writer.Write("<th>Date End (yyyy-mm-dd)</th>");
            writer.Write("<th>Rooms</th>");
            writer.Write("<th>Weekdays</th>");
            writer.Write("<th>Start Time (hh:mm)</th>");
            writer.Write("<th>End Time (hh:mm)</th>");
            writer.Write("</tr>");

            for (int i = 0; i < _txtDateBegin.Count; i++)
            {
                writer.Write("<tr>");
                writer.Write("<th>"); _txtDateBegin[i].RenderControl(writer); writer.Write("</th>");
                writer.Write("<th>"); _txtDateEnd[i].RenderControl(writer); writer.Write("</th>");
                writer.Write("<th>"); _chkRoomList[i].RenderControl(writer); writer.Write("</th>");
                writer.Write("<th>"); _chkWeekdayList[i].RenderControl(writer); writer.Write("</th>");
                writer.Write("<th>"); _txtStartTime[i].RenderControl(writer); writer.Write("</th>");
                writer.Write("<th>"); _txtFinishTime[i].RenderControl(writer); writer.Write("</th>");
                writer.Write("</tr>");
            }

            writer.Write("</table>");
           
        }

        private void addControlRow()
        {

            int count = _txtDateBegin.Count;

            TextBox txtDateBegin = new TextBox();
            txtDateBegin.ID = string.Format("txtDateBegin_{0}", count);
            txtDateBegin.CssClass = "umbDateTimePicker";
            _txtDateBegin.Add(txtDateBegin);
            Controls.Add(txtDateBegin);

            TextBox txtDateEnd = new TextBox();
            txtDateEnd.ID = string.Format("txtDateEnd_{0}", count);
            txtDateEnd.CssClass = "umbDateTimePicker";
           
            _txtDateEnd.Add(txtDateEnd);
            Controls.Add(txtDateEnd);

            CheckBoxList chkRoomList = new CheckBoxList();
            chkRoomList.ID = string.Format("chkRoomList_{0}", count);
            chkRoomList.Items.AddRange(roomsTemplate.ToArray());

            _chkRoomList.Add(chkRoomList);
            Controls.Add(chkRoomList);

            CheckBoxList chkWeekdayList = new CheckBoxList();
            chkWeekdayList.ID = string.Format("chkWeekdayList_{0}", count);
            chkWeekdayList.Items.AddRange(weekdaysTemplate.ToArray());

            _chkWeekdayList.Add(chkWeekdayList);
            Controls.Add(chkWeekdayList);

            TextBox txtStartTime = new TextBox();
            txtStartTime.ID = string.Format("txtStartTime_{0}", count);

            _txtStartTime.Add(txtStartTime);
            Controls.Add(txtStartTime);

            TextBox txtFinishTime = new TextBox();
            txtFinishTime.ID = string.Format("txtFinishTime_{0}", count);

            _txtFinishTime.Add(txtFinishTime);
            Controls.Add(txtFinishTime);

        }

        private void addControlRow(XmlNode node)
        {

            int count = _txtDateBegin.Count;

            TextBox txtDateBegin = new TextBox();
            txtDateBegin.ID = string.Format("txtDateBegin_{0}", count);
            txtDateBegin.Text = node.Attributes["dateStart"].Value;
            _txtDateBegin.Add(txtDateBegin);
            Controls.Add(txtDateBegin);

            TextBox txtDateEnd = new TextBox();
            txtDateEnd.ID = string.Format("txtDateEnd_{0}", count);
            txtDateEnd.Text = node.Attributes["dateEnd"].Value;
            _txtDateEnd.Add(txtDateEnd);
            Controls.Add(txtDateEnd);

            CheckBoxList chkRoomList = new CheckBoxList();
            chkRoomList.ID = string.Format("chkRoomList_{0}", count);
            chkRoomList.Items.AddRange(roomsTemplate.ToArray());
            selectCheckBoxes(chkRoomList, node.Attributes["room"].Value);
            _chkRoomList.Add(chkRoomList);
            Controls.Add(chkRoomList);

            CheckBoxList chkWeekdayList = new CheckBoxList();
            chkWeekdayList.ID = string.Format("chkWeekdayList_{0}", count);
            chkWeekdayList.Items.AddRange(weekdaysTemplate.ToArray());
            selectCheckBoxes(chkWeekdayList, node.Attributes["weekday"].Value);
            _chkWeekdayList.Add(chkWeekdayList);
            Controls.Add(chkWeekdayList);

            TextBox txtStartTime = new TextBox();
            txtStartTime.ID = string.Format("txtStartTime_{0}", count);
            txtStartTime.Text = node.Attributes["startTime"].Value;
            _txtStartTime.Add(txtStartTime);
            Controls.Add(txtStartTime);

            TextBox txtFinishTime = new TextBox();
            txtFinishTime.ID = string.Format("txtFinishTime_{0}", count);
            txtFinishTime.Text = node.Attributes["finishTime"].Value;
            _txtFinishTime.Add(txtFinishTime);
            Controls.Add(txtFinishTime);

        }

        private string getCheckBoxes(CheckBoxList checkBoxList)
        {
            StringBuilder sb = new StringBuilder();

            foreach (ListItem item in checkBoxList.Items)
            {
                if (item.Selected) sb.AppendFormat("{0},", item.Value);
            }

            return sb.ToString();
        }

        private void selectCheckBoxes(CheckBoxList checkBoxList, string fieldValue)
        {
            string[] valueList = fieldValue.Split(new string[] { "," }, StringSplitOptions.RemoveEmptyEntries);

            foreach (ListItem item in checkBoxList.Items)
            {
                item.Selected = valueList.Contains(item.Value);
            }

        }

    }
}

using System;
using System.Web.UI;
using System.Web.UI.WebControls;
using umbraco.BusinessLogic;
using umbraco.cms.businesslogic.datatype;
using umbraco.DataLayer;
using umbraco.interfaces;
using System.Collections.Generic;
using System.Xml;
using System.Text;

using System.Linq;

namespace ClassTimeDatatype
{
    public class PrevalueEditor : PlaceHolder, IDataPrevalue
    {
        #region IDataPrevalue Members

        // Referenced datatype
        private readonly BaseDataType _datatype;

        public PrevalueEditor(DataType dataType)
        {
            _datatype = dataType;
            SetupChildControls();
        }

        private void SetupChildControls()
        {
        }

        public Control Editor
        {
            get
            {
                return this;
            }
        }

        protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);
        }

        public void Save()
        {
            _datatype.DBType = (DBTypes)Enum.Parse(typeof(DBTypes), DBTypes.Ntext.ToString(), true);
            SqlHelper.ExecuteNonQuery("delete from cmsDataTypePreValues where datatypenodeid = @dtdefid", SqlHelper.CreateParameter("@dtdefid", _datatype.DataTypeDefinitionId));
        }

        protected override void Render(HtmlTextWriter writer)
        {
            base.Render(writer);
        }

        #endregion

        public static ISqlHelper SqlHelper
        {
            get
            {
                return Application.SqlHelper;
            }
        }

    }
}