C Sharp DropDownList extender for programmatically Option Group

public class DropDownListAdapter : System.Web.UI.WebControls.Adapters.WebControlAdapter
    ...{ 
        protected override void RenderContents(HtmlTextWriter wr)
        ...{
            DropDownList list = this.Control as DropDownList; 

            string currentOptGroup;
            List<string> renderedOptGroups = new List<string>(); 

            foreach (ListItem item in list.Items)
            ...{
                if (item.Attributes["OptionGroup"] == null)
                ...{
                    RenderListItem(item, wr);
                }
                else
                ...{
                    currentOptGroup = item.Attributes["OptionGroup"]; 

                    if (renderedOptGroups.Contains(currentOptGroup))
                    ...{
                        RenderListItem(item, wr);
                    }
                    else
                    ...{
                        if (renderedOptGroups.Count > 0)
                        ...{
                            RenderOptionGroupEndTag(wr);
                        } 

                        RenderOptionGroupBeginTag(currentOptGroup, wr);
                        renderedOptGroups.Add(currentOptGroup); 

                        RenderListItem(item, wr);
                    }
                }
            } 

            if (renderedOptGroups.Count > 0)
            ...{
                RenderOptionGroupEndTag(wr);
            }
        } 

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

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

        private void RenderListItem(ListItem item, HtmlTextWriter wr)
        ...{
            wr.WriteBeginTag("option");
            wr.WriteAttribute("value", item.Value, true); 

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

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

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