ASP.NET Native Select with option groups

It is a well known fact that the asp.net DropdownList control is lacking a very specific feature: OPTGROUP element support.

Here's a little something to add that functionality, with a drop-in assembly and a .browser file.

1. Create a project named DropDownOptGroups (or whatever you want)
2. Create a class, named DropDownListAdapter, inheriting from System.Web.UI.WebControls.Adapters.WebControlAdapter
3. Add the following code to your class:

     public class DropDownListAdapter : WebControlAdapter    {
           protected override void RenderContents(HtmlTextWriter writer)
           {
               var list = (DropDownList)this.Control;
               string currentOptionGroup;
               var renderedOptionGroups = new List<string>();
               foreach (ListItem item in list.Items)
               {
                   Page.ClientScript.RegisterForEventValidation(list.UniqueID, item.Value);
                   //Is the item part of an option group?
                   if (!string.IsNullOrEmpty(item.Attributes["OptionGroup"]))
                   {
                       currentOptionGroup = item.Attributes["OptionGroup"];
                       //Was the option header already written, then just render the list item
                       if (renderedOptionGroups.Contains(currentOptionGroup))
                       {
                           RenderListItem(item, writer);
                       }
                       //The header was not written,do that first
                       else
                       {
                           //Close previous group
                           if (renderedOptionGroups.Count > 0)
                           {
                               RenderOptionGroupEndTag(writer);
                           }
                           RenderOptionGroupBeginTag(currentOptionGroup, writer);
                           renderedOptionGroups.Add(currentOptionGroup);
                           RenderListItem(item, writer);
                       }
                   }
                   //Simple separator
                   else if (item.Text == "--")
                   {
                       RenderOptionGroupBeginTag("--", writer);
                       RenderOptionGroupEndTag(writer);
                   }
                   //Default behavior, render the list item as normal
                   else
                   {
                       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();
           }
       }

4. Build this project, get the resulting dll, put it in your main project's bin folder.
5. Add the App_Browsers folder to your main project (if you don't already have it in place)
6. Add the following file, named OptGroup.browser in the App_Browser folder:

   <browsers>
     <browser refID="Default">
       <controlAdapters>
         <adapter controlType="System.Web.UI.WebControls.DropDownList" 
                adapterType="DropDownOptGroups.DropDownListAdapter, DropDownOptGroups" />
       </controlAdapters>
     </browser>
   </browsers>

7. Make sure you adjust namespaces, assembly names and class names to match what you have coded.
8. Now, in your main project, if you want to have a dropdownlist with optgroups, you use the good ole' DropdownList in your markup and add a custom "OptionGroup" attribute to all List items that you want under an option group. Something like the following:

   <asp:DropDownList ID="ddlWithOptGroup" runat="server">
       <asp:ListItem Text="Test - 0" Value="0"></asp:ListItem>
       <asp:ListItem Text="Test - 1" Value="1"></asp:ListItem>
       <asp:ListItem Text="Test - 2" Value="2" OptionGroup="Even"></asp:ListItem>
       <asp:ListItem Text="Test - 4" Value="4" OptionGroup="Even"></asp:ListItem>
       <asp:ListItem Text="Test - 6" Value="6" OptionGroup="Even"></asp:ListItem>
       <asp:ListItem Text="Test - 3" Value="3" OptionGroup="Odd"></asp:ListItem>
       <asp:ListItem Text="Test - 5" Value="5" OptionGroup="Odd"></asp:ListItem>
   </asp:DropDownList>

9. The result is as follows:
   
   

Happy coding!