ASP.NET DropDownList with OptionGroup support

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

public class DropDownListAdapter : WebControlAdapter
{
    private const string OptionGroupAttribute = "OptionGroup";
    private const string TagOptionGroup = "optgroup";
    private const string AttributeLabel = "label";
    protected override void RenderContents(HtmlTextWriter writer)
    {
        DropDownList list = Control as DropDownList;
        string currentOptionGroup;
        List<string> renderedOptionGroups = new List<string>();
        foreach (ListItem item in list.Items)
        {
            Page.ClientScript.RegisterForEventValidation(list.UniqueID, item.Value);
            if (item.Attributes[OptionGroupAttribute] == null)
            {
                RenderListItem(item, writer);
            }
            else
            {
                currentOptionGroup = item.Attributes[OptionGroupAttribute];
                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.AddAttribute(AttributeLabel, name);
        writer.RenderBeginTag(TagOptionGroup);
    }

    private void RenderOptionGroupEndTag(HtmlTextWriter writer)
    {
        writer.RenderEndTag();
    }

    private void RenderListItem(ListItem item, HtmlTextWriter writer)
    {
        foreach (string key in item.Attributes.Keys)
        {
            if (key != OptionGroupAttribute)
            {
                writer.AddAttribute(key, item.Attributes[key]);
            }
        }
        writer.AddAttribute(HtmlTextWriterAttribute.Value, item.Value, true);
        if (item.Selected)
        {
            writer.AddAttribute(HtmlTextWriterAttribute.Selected, "selected");
        }
        writer.RenderBeginTag(HtmlTextWriterTag.Option);
        writer.WriteEncodedText(item.Text);
        writer.RenderEndTag();
    }
    protected override object SaveAdapterViewState()
    {
        DropDownList list = Control as DropDownList;
        object[] viewStates = new object[list.Items.Count + 2];

        int i = 0;
        foreach (ListItem item in list.Items)
            viewStates[i++] = item.Attributes[OptionGroupAttribute];

        viewStates[i++] = base.SaveAdapterViewState();
        viewStates[i] = Hash(list.Items);
        
        return viewStates;
    }

    private static int Hash(ListItemCollection listItems)
    {
        int hash = 0;
        foreach (ListItem listItem in listItems)
            hash += listItem.GetHashCode();

        return hash;
    }

    object[] viewStates;

    protected override void LoadAdapterViewState(object state)
    {
        viewStates = (object[])state;
        base.LoadAdapterViewState(viewStates[viewStates.Length - 1]);

    }
    protected override void OnPreRender(System.EventArgs e)
    {
        if (viewStates != null && viewStates.Length > 1)
        {
            DropDownList list = Control as DropDownList;
            if (Page.EnableEventValidation)
            {
                if (viewStates.Length != list.Items.Count + 1)
                {
                    throw new ViewStateException();
                }
            }
            if (Hash(list.Items) == (int)viewStates[viewStates.Length - 1])
            {
                int max = viewStates.Length - 2;
                if (list.Items.Count < max)
                {
                    max = list.Items.Count;
                }
                for (int i = 0; i < max; i++)
                {
                    list.Items[i].Attributes[OptionGroupAttribute] = (string)viewStates[i];
                }
            }
        }
        base.OnPreRender(e);
    }
}

public class DropDownListAdapter : WebControlAdapter
{
    const string AttributeLabel = "label";
    const string TagOptionGroup = "optgroup";

    private DropDownList ctl = null;

    protected override void OnInit(EventArgs e)
    {
        base.OnInit(e);
        ctl = base.Control as DropDownList;
    }

    protected override void RenderContents(HtmlTextWriter writer)
    {
        if (ctl == null)
            return;

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

        foreach(ListItem item in ctl.Items)
        {
            if (item.Enabled)
                RenderListItem(item, writer);
            else
            {
                if (renderedOptionGroups.Contains(item.Text))
                    RenderListItem(item, writer);
                else
                {
                    if (renderedOptionGroups.Count > 0)
                        RenderOptionGroupEndTag(writer);

                    RenderOptionGroupBeginTag(item.Text, writer);
                    renderedOptionGroups.Add(item.Text);
                }
            }
        }

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

    private void RenderOptionGroupBeginTag(string name, HtmlTextWriter writer)
    {
        writer.AddAttribute(AttributeLabel, name);
        writer.RenderBeginTag(TagOptionGroup);
    }

    private void RenderOptionGroupEndTag(HtmlTextWriter writer)
    {
        writer.RenderEndTag();
    }

    private void RenderListItem(ListItem item, HtmlTextWriter writer)
    {
        if (Page != null)
            Page.ClientScript.RegisterForEventValidation(ctl.UniqueID, item.Value);

        writer.AddAttribute(HtmlTextWriterAttribute.Value, item.Value, true);

        if (item.Selected)
            writer.AddAttribute(HtmlTextWriterAttribute.Selected, "selected");

        writer.RenderBeginTag(HtmlTextWriterTag.Option);
        writer.WriteEncodedText(item.Text);
        writer.RenderEndTag();
    }
}