ASP.NET DropDownList with OptionGroup support

<browsers>
  <browser refID="Default">
    <controlAdapters>
      <adapter controlType="System.Web.UI.WebControls.AdRotator"
				   adapterType="My.Namespace.Project.Code.Adapters.AdRotatorAdaptor" />
    </controlAdapters>
  </browser>
</browsers>

ListItem item6 = new ListItem("Allosaurus", "6");
item6.Attributes["OptionGroup"] = "Dinosaurs";

ListItem item7 = new ListItem("Trex", "7");

    public class DropDownListAdapter : System.Web.UI.WebControls.Adapters.WebControlAdapter {

        public static readonly string OPTGROUPATTR = "OptionGroup";

        Dictionary<string, string> optionGroups = new Dictionary<string, string>();

        protected override void RenderContents(HtmlTextWriter writer) {
            DropDownList list = this.Control as DropDownList;

            string currentOptionGroup;
            List<string> renderedOptionGroups = new List<string>();

            foreach(ListItem item in list.Items) {
                string keyString = item.Value + item.Text;
                if(item.Attributes[OPTGROUPATTR] == null && !optionGroups.ContainsKey(keyString)) {
                    RenderListItem(item, writer);
                } else {
                    if (item.Attributes[OPTGROUPATTR] != null)
                        currentOptionGroup = item.Attributes[OPTGROUPATTR];
                    else
                    {
                        currentOptionGroup = optionGroups[keyString];
                    }

                    if(renderedOptionGroups.Contains(currentOptionGroup)) {
                        RenderListItem(item, writer);
                    } else {
                        if(renderedOptionGroups.Count > 0) {
                            RenderOptionGroupEndTag(writer);
                        }

                        RenderOptionGroupBeginTag(currentOptionGroup, writer);
                        renderedOptionGroups.Add(currentOptionGroup);

                        RenderListItem(item, writer);
                    }
                }
            }

            if(renderedOptionGroups.Count > 0) {
                RenderOptionGroupEndTag(writer);
            }
        }

        private void RenderOptionGroupBeginTag(string name, HtmlTextWriter writer) {
            writer.WriteBeginTag("optgroup");
            writer.WriteAttribute("label", name);
            writer.Write(HtmlTextWriter.TagRightChar);
            writer.WriteLine();
        }

        private void RenderOptionGroupEndTag(HtmlTextWriter writer) {
            writer.WriteEndTag("optgroup");
            writer.WriteLine();
        }

        private void RenderListItem(ListItem item, HtmlTextWriter writer) {

            this.Page.ClientScript.RegisterForEventValidation(this.Control.UniqueID, item.Value);

            writer.WriteBeginTag("option");
            writer.WriteAttribute("value", item.Value, true);

            if(item.Selected) {
                writer.WriteAttribute("selected", "selected", false);
            }

            foreach(string key in item.Attributes.Keys) {
                writer.WriteAttribute(key, item.Attributes[key]);
            }

            writer.Write(HtmlTextWriter.TagRightChar);
            HttpUtility.HtmlEncode(item.Text, writer);
            writer.WriteEndTag("option");
            writer.WriteLine();
        }

        //
        // Summary:
        //     Saves view state information for the control adapter.
        //
        // Returns:
        //     An System.Object that contains the adapter view state information as a System.Web.UI.StateBag.
        protected override object SaveAdapterViewState()
        {
            DropDownList list = this.Control as DropDownList;
            string currentOptionGroup;
            string keyString;

            foreach (ListItem item in list.Items)
            {
                if (item.Attributes[OPTGROUPATTR] != null)
                {
                    currentOptionGroup = item.Attributes[OPTGROUPATTR];
                    keyString = item.Value + item.Text;
                    optionGroups[keyString] = currentOptionGroup;
                }
            }


            return optionGroups;
        }


        //
        // Summary:
        //     Loads adapter view state information that was saved by System.Web.UI.Adapters.ControlAdapter.SaveAdapterViewState()
        //     during a previous request to the page where the control associated with this
        //     control adapter resides.
        //
        // Parameters:
        //   state:
        //     An System.Object that contains the adapter view state information as a System.Web.UI.StateBag.
        protected override void LoadAdapterViewState(object state)
        {

            optionGroups = state as Dictionary<string, string>;    
        }

    }

Imports System.Web.UI
Imports System.Web.UI.WebControls

	Public Class ComboBox
		Inherits DropDownList

		Private Const m_optionGroupAttribute As String = "OptionGroup"
		Private Const m_tagOptionGroup As String = "optgroup"
		Private Const m_attributeLabel As String = "label"
		Private m_viewstates As Object()

		Protected Overrides Sub RenderContents(ByVal writer As HtmlTextWriter)
			If (Not Page Is Nothing) Then
				Dim l_list As DropDownList = CType(Me, DropDownList)
				Dim l_currentOptionGroup As String
				Dim l_renderedOptionGroups As New ArrayList()

				For Each l_item As ListItem In l_list.Items
					Page.ClientScript.RegisterForEventValidation(l_list.UniqueID, l_item.Value)

					If l_item.Attributes(m_optionGroupAttribute) Is Nothing Then
						RenderListItem(l_item, writer)
					Else
						l_currentOptionGroup = l_item.Attributes(m_optionGroupAttribute)

						If (l_renderedOptionGroups.Contains(l_currentOptionGroup)) Then
							RenderListItem(l_item, writer)
						Else
							If (l_renderedOptionGroups.Count > 0) Then
								RenderOptionGroupEndTag(writer)
							End If
							RenderOptionGroupBeginTag(l_currentOptionGroup, writer)
							l_renderedOptionGroups.Add(l_currentOptionGroup)
							RenderListItem(l_item, writer)
						End If
					End If
				Next

				If (l_renderedOptionGroups.Count > 0) Then
					RenderOptionGroupEndTag(writer)
				End If
			Else
				MyBase.RenderContents(writer)
			End If
		End Sub

		Private Sub RenderOptionGroupBeginTag(ByVal name As String, ByVal writer As HtmlTextWriter)
			writer.AddAttribute(m_attributeLabel, name)
			writer.RenderBeginTag(m_tagOptionGroup)
		End Sub

		Private Sub RenderOptionGroupEndTag(ByVal writer As HtmlTextWriter)
			writer.RenderEndTag()
		End Sub

		Private Sub RenderListItem(ByVal item As ListItem, ByVal writer As HtmlTextWriter)
			For Each key As String In item.Attributes.Keys
				If (key <> m_optionGroupAttribute) Then
					writer.AddAttribute(key, item.Attributes(key))
				End If
			Next

			writer.AddAttribute(HtmlTextWriterAttribute.Value, item.Value, True)

			If (item.Selected) Then
				writer.AddAttribute(HtmlTextWriterAttribute.Selected, "selected")
			End If

			writer.RenderBeginTag(HtmlTextWriterTag.Option)
			writer.WriteEncodedText(item.Text)
			writer.RenderEndTag()
		End Sub

		Protected Overrides Function SaveViewState() As Object
			If (Not Page Is Nothing) Then
				Dim l_list As DropDownList = CType(Me, DropDownList)
				Dim l_viewState(l_list.Items.Count) As Object ' Note: Count + 1 caused eventvalidation errors.

				Dim i As Integer = 0
				For Each item As ListItem In l_list.Items

					l_viewState(i) = item.Attributes(m_optionGroupAttribute)
					i += 1
				Next

				l_viewState(i) = MyBase.SaveViewState()
				Return l_viewState
			Else
				Return MyBase.SaveViewState()
			End If
		End Function

		Protected Overrides Sub LoadViewState(ByVal state As Object)
			If (Not Page Is Nothing) Then
				m_viewstates = CType(state, Object())
				MyBase.LoadViewState(m_viewstates(m_viewstates.Length - 1))
			Else
				MyBase.LoadViewState(state)
			End If
		End Sub

		Protected Overrides Sub OnPreRender(ByVal e As System.EventArgs)
			If (Not Page Is Nothing) Then
				If (Not m_viewstates Is Nothing AndAlso m_viewstates.Length > 1) Then
					Dim l_list As DropDownList = CType(Me, DropDownList)
					If (Page.EnableEventValidation) Then
						If (m_viewstates.Length <> l_list.Items.Count + 1) Then
							Throw New ViewStateException()
						End If
					End If

					Dim l_max As Integer = m_viewstates.Length
					If (l_list.Items.Count < l_max) Then
						l_max = l_list.Items.Count
					End If

					For i As Integer = 0 To l_max - 1
						l_list.Items(i).Attributes(m_optionGroupAttribute) = CStr(m_viewstates(i))
					Next
				End If
			End If

			MyBase.OnPreRender(e)
		End Sub
	End Class