Question

Can anyone recommend a dropdownlist control for asp.net (3.5) that can render option groups? Thanks

Answer 1

I’ve used the standard control in the past, and just added a simple ControlAdapter for it that would override the default behavior so it could render <optgroup>s in certain places. This works great even if you have controls that don’t need the special behavior, because the additional feature doesn’t get in the way.

Note that this was for a specific purpose and written in .Net 2.0, so it may not suit you as well, but it should at least give you a starting point. Also, you have to hook it up using a .browserfile in your project (see the end of the post for an example).

'This codes makes the dropdownlist control recognize items with '--' 'for the label or items with an OptionGroup attribute and render them 'as <optgroup> instead of <option>. Public Class DropDownListAdapter     Inherits System.Web.UI.WebControls.Adapters.WebControlAdapter      Protected Overrides Sub RenderContents(ByVal writer As HtmlTextWriter)         Dim list As DropDownList = Me.Control         Dim currentOptionGroup As String         Dim renderedOptionGroups As New Generic.List(Of String)          For Each item As ListItem In list.Items             Page.ClientScript.RegisterForEventValidation(list.UniqueID, item.Value)             If item.Attributes('OptionGroup') IsNot Nothing Then                 'The item is part of an option group                 currentOptionGroup = item.Attributes('OptionGroup')                 If Not renderedOptionGroups.Contains(currentOptionGroup) Then                     'the header was not written- do that first                     'TODO: make this stack-based, so the same option group can be used more than once in longer select element (check the most-recent stack item instead of anything in the list)                     If (renderedOptionGroups.Count > 0) Then                         RenderOptionGroupEndTag(writer) 'need to close previous group                     End If                     RenderOptionGroupBeginTag(currentOptionGroup, writer)                     renderedOptionGroups.Add(currentOptionGroup)                 End If                 RenderListItem(item, writer)             ElseIf item.Text = '--' Then 'simple separator                 RenderOptionGroupBeginTag('--', writer)                 RenderOptionGroupEndTag(writer)             Else                 'default behavior: render the list item as normal                 RenderListItem(item, writer)             End If         Next item          If renderedOptionGroups.Count > 0 Then             RenderOptionGroupEndTag(writer)         End If     End Sub      Private Sub RenderOptionGroupBeginTag(ByVal name As String, ByVal writer As HtmlTextWriter)         writer.WriteBeginTag('optgroup')         writer.WriteAttribute('label', name)         writer.Write(HtmlTextWriter.TagRightChar)         writer.WriteLine()     End Sub      Private Sub RenderOptionGroupEndTag(ByVal writer As HtmlTextWriter)         writer.WriteEndTag('optgroup')         writer.WriteLine()     End Sub      Private Sub RenderListItem(ByVal item As ListItem, ByVal writer As HtmlTextWriter)         writer.WriteBeginTag('option')         writer.WriteAttribute('value', item.Value, True)         If item.Selected Then             writer.WriteAttribute('selected', 'selected', False)         End If          For Each key As String In item.Attributes.Keys             writer.WriteAttribute(key, item.Attributes(key))         Next key          writer.Write(HtmlTextWriter.TagRightChar)         HttpUtility.HtmlEncode(item.Text, writer)         writer.WriteEndTag('option')         writer.WriteLine()     End Sub End Class 

Here’s a C# implementation of the same Class:

/* This codes makes the dropdownlist control recognize items with '--'  * for the label or items with an OptionGroup attribute and render them  * as <optgroup> instead of <option>.  */ public class DropDownListAdapter : WebControlAdapter {     protected override void RenderContents(HtmlTextWriter writer)     {         //System.Web.HttpContext.Current.Response.Write('here');         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 (item.Attributes['OptionGroup'] != null)             {                 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();     } } 

My browser file was named ‘App_Browsers\BrowserFile.browser’ and looked like this:

<!--     You can find existing browser definitions at     <windir>\Microsoft.NET\Framework\<ver>\CONFIG\Browsers --> <browsers>    <browser refID='Default'>       <controlAdapters>         <adapter controlType='System.Web.UI.WebControls.DropDownList'                 adapterType='DropDownListAdapter' />       </controlAdapters>    </browser> </browsers>