DropDownList with group wise in ASP.NET

Introduction:
This article explains how to to bind with OptionGroup.  ASP.NET 2.0, for all its bells and whistles, lacks the odd bit of functionality for reasons completely unknown. One such notable omission is that of OptionGroup (<optgroup>) support in the DropDownList control. For those unfamiliar with the <optgroup> element, it is part of the XHTML standard, and has the effect of categorising items in a <select>, as the following image shows

With HTML Code:
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<select id="Select1">

                <optgroup label="-------Andhra Pradesh-----" title="Andhra Pradesh">

                <option>Hyderabad</option>

                <option>Visakhaptanam</option>

                <option>Tirupathi</option>

                <option>Vijayawada</option>

                </optgroup>

                <optgroup label="------Maharastra-----" title="Maharastra">

                <option>Mumbai</option>

                <option>Pune</option>

                <option>Nagapur</option>

                <option>Thane</option>

                </optgroup>

  </select> 

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With C# Code:
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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();
    }
}

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