ASP.NET - drop downlist with optiongroup

public class DropDownListAdapter : 
       System.Web.UI.WebControls.Adapters.WebControlAdapter {
    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) {
            if(item.Attributes["OptionGroup"] == null) {
                RenderListItem(item, writer);
            } else {
                currentOptionGroup = item.Attributes["OptionGroup"];
                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) {
        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();
    }
}

<ddlb:optiongroupselect id="OptionGroupSelect1" runat="server" enableviewstate="true">
   <ddlb:OptionGroupItem ID="OptionGroupItem1" 
      runat="server" Value="1" Text="ONE" OptionGroup="Odd" />
   <ddlb:OptionGroupItem ID="OptionGroupItem2" 
      runat="server" Value="2" Text="TWO" OptionGroup="Even" />
   <ddlb:OptionGroupItem ID="OptionGroupItem3" 
      runat="server" Value="3" Text="TREE" OptionGroup="Odd" />
   <ddlb:OptionGroupItem ID="OptionGroupItem4" 
      runat="server" Value="4" Text="FOUR" OptionGroup="Even" />
</ddlb:optiongroupselect>

protected void Page_Load(object sender, EventArgs e)
{
  if (!this.IsPostBack)
  {
      this.OptionGroupSelect1.Items.Add(new OptionGroupItem("A", 
                              "Letter A", "Letters"));
      this.OptionGroupSelect1.Items.Add(new OptionGroupItem("B", 
                              "Letter B", "Letters"));
      this.OptionGroupSelect1.Items.Add(new OptionGroupItem("C", 
                              "Letter C", "Letters"));
  }
}

<ddlb:optiongroupselect id="OptionGroupSelect1" runat="server" enableviewstate="true">
   <ddlb:OptionGroupItem ID="OptionGroupItem1" 
      runat="server" Value="1" Text="ONE" OptionGroup="Odd" />
   <ddlb:OptionGroupItem ID="OptionGroupItem2" 
      runat="server" Value="2" Text="TWO" OptionGroup="Even" />
   <ddlb:OptionGroupItem ID="OptionGroupItem3" 
      runat="server" Value="3" Text="TREE" OptionGroup="Odd" />
   <ddlb:OptionGroupItem ID="OptionGroupItem4" 
      runat="server" Value="4" Text="FOUR" OptionGroup="Even" />
</ddlb:optiongroupselect>

Hi,

No built-in support <optgroup> in ASP.NET, so you will have to render the tags yourself. Best way to do this is via a control adapter, that enables you to manipulate the outputted (X)HTML of an existing control (in this case the drop down). Could show you an example, but the following article seems to be pretty good:

http://www.codeproject.com/aspnet/DropDownListOptionGroup.asp

Hope this helps,