Support for optgroup in dropdownlist .NET MVC?

Continuing on from this question programmatically creating a drop down list I would like my list to have several optgroup lists too. Is this currently possible?

I know I need to pass a selectList to a dropDownList but do not know how to add text, value, optgroup to the selectList.

I want the end result to produce:

<option value="">Please select</option>
  <optgroup label="Option A">
	<option value="1">1</option>
	<option value="2">2</option>
	<option value="3">3</option>
	<option value="4">4</option>
  </optgroup>
  <optgroup label="Option B">
    <option value="a">A</option>
    <option value="b">B</option>
    <option value="c">C</option>
  </optgroup>
</option>

My extension is a bit more complicated, but it has all overloads as original DropDownList has.

Actually I've created it specially to be able to produce DropDownList with groups and to be transformed in to two joined selects with help of jDoubleSelect

using System;using System.Collections;using System.Collections.Generic;using System.Globalization; using System.Linq;using System.Linq.Expressions;using System.Text; using System.Web;using System.Web.Mvc;using System.Web.Routing;

public class GroupedSelectListItem : SelectListItem
{
	public string GroupKey { get; set; }
	public string GroupName { get; set; }
}

public static class HtmlHelpers
{
    public static MvcHtmlString DropDownGroupList(this HtmlHelper htmlHelper, string name)
    {
        return DropDownListHelper(htmlHelper, name, null, null, null);
    }

    public static MvcHtmlString DropDownGroupList(this HtmlHelper htmlHelper, string name, IEnumerable<GroupedSelectListItem> selectList)
    {
        return DropDownListHelper(htmlHelper, name, selectList, null, null);
    }

    public static MvcHtmlString DropDownGroupList(this HtmlHelper htmlHelper, string name, string optionLabel)
    {
        return DropDownListHelper(htmlHelper, name, null, optionLabel, null);
    }

    public static MvcHtmlString DropDownGroupList(this HtmlHelper htmlHelper, string name, IEnumerable<GroupedSelectListItem> selectList, IDictionary<string, object> htmlAttributes)
    {
        return DropDownListHelper(htmlHelper, name, selectList, null, htmlAttributes);
    }

    public static MvcHtmlString DropDownGroupList(this HtmlHelper htmlHelper, string name, IEnumerable<GroupedSelectListItem> selectList, object htmlAttributes)
    {
        return DropDownListHelper(htmlHelper, name, selectList, null, HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes));
    }

    public static MvcHtmlString DropDownGroupList(this HtmlHelper htmlHelper, string name, IEnumerable<GroupedSelectListItem> selectList, string optionLabel)
    {
        return DropDownListHelper(htmlHelper, name, selectList, optionLabel, null);
    }

    public static MvcHtmlString DropDownGroupList(this HtmlHelper htmlHelper, string name, IEnumerable<GroupedSelectListItem> selectList, string optionLabel, IDictionary<string, object> htmlAttributes)
    {
        return DropDownListHelper(htmlHelper, name, selectList, optionLabel, htmlAttributes);
    }

    public static MvcHtmlString DropDownGroupList(this HtmlHelper htmlHelper, string name, IEnumerable<GroupedSelectListItem> selectList, string optionLabel, object htmlAttributes)
    {
        return DropDownListHelper(htmlHelper, name, selectList, optionLabel, HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes));
    }

    public static MvcHtmlString DropDownGroupListFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, IEnumerable<GroupedSelectListItem> selectList)
    {
        return DropDownGroupListFor(htmlHelper, expression, selectList, null /* optionLabel */, null /* htmlAttributes */);
    }

    public static MvcHtmlString DropDownGroupListFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, IEnumerable<GroupedSelectListItem> selectList, object htmlAttributes)
    {
        return DropDownGroupListFor(htmlHelper, expression, selectList, null /* optionLabel */, HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes));
    }

    public static MvcHtmlString DropDownGroupListFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, IEnumerable<GroupedSelectListItem> selectList, IDictionary<string, object> htmlAttributes)
    {
        return DropDownGroupListFor(htmlHelper, expression, selectList, null /* optionLabel */, htmlAttributes);
    }

    public static MvcHtmlString DropDownGroupListFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, IEnumerable<GroupedSelectListItem> selectList, string optionLabel)
    {
        return DropDownGroupListFor(htmlHelper, expression, selectList, optionLabel, null /* htmlAttributes */);
    }

    public static MvcHtmlString DropDownGroupListFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, IEnumerable<GroupedSelectListItem> selectList, string optionLabel, object htmlAttributes)
    {
        return DropDownGroupListFor(htmlHelper, expression, selectList, optionLabel, HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes));
    }

    public static MvcHtmlString DropDownGroupListFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, IEnumerable<GroupedSelectListItem> selectList, string optionLabel, IDictionary<string, object> htmlAttributes)
    {
        if (expression == null)
        {
            throw new ArgumentNullException("expression");
        }

        return DropDownListHelper(htmlHelper, ExpressionHelper.GetExpressionText(expression), selectList, optionLabel, htmlAttributes);
    }

    private static MvcHtmlString DropDownListHelper(HtmlHelper htmlHelper, string expression, IEnumerable<GroupedSelectListItem> selectList, string optionLabel, IDictionary<string, object> htmlAttributes)
    {
        return SelectInternal(htmlHelper, optionLabel, expression, selectList, false /* allowMultiple */, htmlAttributes);
    }


    // Helper methods

    private static IEnumerable<GroupedSelectListItem> GetSelectData(this HtmlHelper htmlHelper, string name)
    {
        object o = null;
        if (htmlHelper.ViewData != null)
        {
            o = htmlHelper.ViewData.Eval(name);
        }
        if (o == null)
        {
            throw new InvalidOperationException(
                String.Format(
                    CultureInfo.CurrentCulture,
                    "Missing Select Data"));
        }
        var selectList = o as IEnumerable<GroupedSelectListItem>;
        if (selectList == null)
        {
            throw new InvalidOperationException(
                String.Format(
                    CultureInfo.CurrentCulture,
                    "Wrong Select DataType"));
        }
        return selectList;
    }

    internal static string ListItemToOption(GroupedSelectListItem item)
    {
        var builder = new TagBuilder("option")
        {
            InnerHtml = HttpUtility.HtmlEncode(item.Text)
        };
        if (item.Value != null)
        {
            builder.Attributes["value"] = item.Value;
        }
        if (item.Selected)
        {
            builder.Attributes["selected"] = "selected";
        }
        return builder.ToString(TagRenderMode.Normal);
    }

    private static MvcHtmlString SelectInternal(this HtmlHelper htmlHelper, string optionLabel, string name, IEnumerable<GroupedSelectListItem> selectList, bool allowMultiple, IDictionary<string, object> htmlAttributes)
    {
        name = htmlHelper.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldName(name);
        if (String.IsNullOrEmpty(name))
        {
            throw new ArgumentException("Null Or Empty", "name");
        }

        bool usedViewData = false;

        // If we got a null selectList, try to use ViewData to get the list of items.
        if (selectList == null)
        {
            selectList = htmlHelper.GetSelectData(name);
            usedViewData = true;
        }

        object defaultValue = (allowMultiple) ? htmlHelper.GetModelStateValue(name, typeof(string[])) : htmlHelper.GetModelStateValue(name, typeof(string));

        // If we haven't already used ViewData to get the entire list of items then we need to
        // use the ViewData-supplied value before using the parameter-supplied value.
        if (!usedViewData)
        {
            if (defaultValue == null)
            {
                defaultValue = htmlHelper.ViewData.Eval(name);
            }
        }

        if (defaultValue != null)
        {
            var defaultValues = (allowMultiple) ? defaultValue as IEnumerable : new[] { defaultValue };
            var values = from object value in defaultValues select Convert.ToString(value, CultureInfo.CurrentCulture);
            var selectedValues = new HashSet<string>(values, StringComparer.OrdinalIgnoreCase);
            var newSelectList = new List<GroupedSelectListItem>();

            foreach (var item in selectList)
            {
                item.Selected = (item.Value != null) ? selectedValues.Contains(item.Value) : selectedValues.Contains(item.Text);
                newSelectList.Add(item);
            }
            selectList = newSelectList;
        }

        // Convert each ListItem to an <option> tag
        var listItemBuilder = new StringBuilder();

        // Make optionLabel the first item that gets rendered.
        if (optionLabel != null)
        {
            listItemBuilder.AppendLine(ListItemToOption(new GroupedSelectListItem { Text = optionLabel, Value = String.Empty, Selected = false }));
        }

        foreach (var group in selectList.GroupBy(i => i.GroupKey))
        {
            string groupName = selectList.Where(i => i.GroupKey == group.Key).Select(it => it.GroupName).FirstOrDefault();
            listItemBuilder.AppendLine(string.Format("<optgroup label=\"{0}\" value=\"{1}\">", groupName, group.Key));
            foreach (GroupedSelectListItem item in group)
            {
                listItemBuilder.AppendLine(ListItemToOption(item));
            }
            listItemBuilder.AppendLine("</optgroup>");
        }

        var tagBuilder = new TagBuilder("select")
        {
            InnerHtml = listItemBuilder.ToString()
        };
        tagBuilder.MergeAttributes(htmlAttributes);
        tagBuilder.MergeAttribute("name", name, true /* replaceExisting */);
        tagBuilder.GenerateId(name);
        if (allowMultiple)
        {
            tagBuilder.MergeAttribute("multiple", "multiple");
        }

        // If there are any errors for a named field, we add the css attribute.
        ModelState modelState;
        if (htmlHelper.ViewData.ModelState.TryGetValue(name, out modelState))
        {
            if (modelState.Errors.Count > 0)
            {
                tagBuilder.AddCssClass(HtmlHelper.ValidationInputCssClassName);
            }
        }

        return MvcHtmlString.Create(tagBuilder.ToString());
    }

    internal static object GetModelStateValue(this HtmlHelper helper, string key, Type destinationType)
    {
        ModelState modelState;
        if (helper.ViewData.ModelState.TryGetValue(key, out modelState))
        {
            if (modelState.Value != null)
            {
                return modelState.Value.ConvertTo(destinationType, null /* culture */);
            }
        }
        return null;
    }

}

This was added to ASP.Net MVC at version 5.2 and is now built-in.

The Group property on SelectListItem allows you to specify a group for each item:

New SelectList constructors also allow you to provide the name of the field that contains the group title on the supplied list of items.

The HtmlHelper DropDownList and DropDownListFor methods now generate optgroup elements based on the groups included on the list of items.

Easy!

I just write a extensions for do this, see it:

using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Web.Routing;

namespace System.Web.Mvc.Html
{
    public static class GroupDropListExtensions
    {
        public static string GroupDropList(this HtmlHelper helper, string name, IEnumerable<GroupDropListItem> data, string SelectedValue, object htmlAttributes)
        {
            if (data == null && helper.ViewData != null)
                data = helper.ViewData.Eval(name) as IEnumerable<GroupDropListItem>;
            if (data == null) return string.Empty;

            var select = new TagBuilder("select");

            if (htmlAttributes != null)
                select.MergeAttributes(new RouteValueDictionary(htmlAttributes));

            select.GenerateId(name);

            var optgroupHtml = new StringBuilder();
            var groups = data.ToList();
            foreach (var group in data)
            {
                var groupTag = new TagBuilder("optgroup");
                groupTag.Attributes.Add("label", helper.Encode( group.Name));
                var optHtml = new StringBuilder();
                foreach (var item in group.Items)
                {
                    var option = new TagBuilder("option");
                    option.Attributes.Add("value", helper.Encode(item.Value));
                    if (SelectedValue != null && item.Value == SelectedValue)
                        option.Attributes.Add("selected", "selected");
                    option.InnerHtml = helper.Encode(item.Text);
                    optHtml.AppendLine(option.ToString(TagRenderMode.Normal));
                }
                groupTag.InnerHtml = optHtml.ToString();
                optgroupHtml.AppendLine(groupTag.ToString(TagRenderMode.Normal));
            }
            select.InnerHtml = optgroupHtml.ToString();
            return select.ToString(TagRenderMode.Normal);
        }
}

    public class GroupDropListItem
    {
        public string Name { get; set; }
        public List<OptionItem> Items { get; set; }
    }

    public class OptionItem
    {
        public string Text { get; set; }
        public string Value { get; set; }
    }
}

Looking through the code on www.codeplex.com/aspnet, it doesn't appear that neither the SelectList nor the DropDownList extension method supports the use of OptGroup in a select. Looks like you'll need to write your own extension method and extend SelectListItem to contain the grouping or generate the select manually in markup.

Data annotations for client validations missing?

In answer to Chrno Love's question above, adding to Serge Zab's solution - Milimetric's fix for MVC3 clientside validation attributes when using a collection of dropdowns in the same view:

        public static MvcHtmlString DropDownGroupListFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper,
                                                                        Expression<Func<TModel, TProperty>>
                                                                            expression,
                                                                        IEnumerable<GroupedSelectListItem>
                                                                            selectList, string optionLabel,
                                                                        IDictionary<string, object> htmlAttributes)
    {
        if (expression == null)
        {
            throw new ArgumentNullException("expression");
        }

        // fixing clientside validation attributes
        // http://stackoverflow.com/questions/4799958/asp-net-mvc-3-unobtrusive-client-validation-does-not-work-with-drop-down-lists/8102022#8102022
        var metadata = ModelMetadata.FromLambdaExpression(expression, htmlHelper.ViewData);
            var mergedAttributes =
                htmlHelper.GetUnobtrusiveValidationAttributes(ExpressionHelper.GetExpressionText(expression), metadata);

            if (htmlAttributes != null)
            {
                foreach (PropertyDescriptor descriptor in TypeDescriptor.GetProperties(htmlAttributes))
                {
                    object value = descriptor.GetValue(htmlAttributes);
                    mergedAttributes.Add(descriptor.Name, value);
                }
            }

        //return DropDownListHelper(htmlHelper, ExpressionHelper.GetExpressionText(expression), selectList, optionLabel, htmlAttributes);
        return DropDownListHelper(htmlHelper, ExpressionHelper.GetExpressionText(expression), selectList, optionLabel, mergedAttributes);
    }

Serge Zab's answer was exactly what I was looking for. Being a die hard VB programmer I ported it to this VB module:

'based on Serge Zab's answer on http://stackoverflow.com/questions/607188/support-for-optgroup-in-dropdownlist-net-mvc

Imports System.Collections
Imports System.Collections.Generic
Imports System.Globalization
Imports System.Linq
Imports System.Linq.Expressions
Imports System.Text
Imports System.Web
Imports System.Web.Mvc
Imports System.Web.Routing

Public Class GroupedSelectListItem
    Inherits SelectListItem
    Public Property GroupKey() As String
        Get
            Return m_GroupKey
        End Get
        Set(value As String)
            m_GroupKey = Value
        End Set
    End Property
    Private m_GroupKey As String
    Public Property GroupName() As String
        Get
            Return m_GroupName
        End Get
        Set(value As String)
            m_GroupName = Value
        End Set
    End Property
    Private m_GroupName As String
End Class

Public Module HtmlHelpers
    <System.Runtime.CompilerServices.Extension> _
    Public Function DropDownGroupList(htmlHelper As HtmlHelper, name As String) As MvcHtmlString
        Return DropDownListHelper(htmlHelper, name, Nothing, Nothing, Nothing)
    End Function

    <System.Runtime.CompilerServices.Extension> _
    Public Function DropDownGroupList(htmlHelper As HtmlHelper, name As String, selectList As IEnumerable(Of GroupedSelectListItem)) As MvcHtmlString
        Return DropDownListHelper(htmlHelper, name, selectList, Nothing, Nothing)
    End Function

    <System.Runtime.CompilerServices.Extension> _
    Public Function DropDownGroupList(htmlHelper As HtmlHelper, name As String, optionLabel As String) As MvcHtmlString
        Return DropDownListHelper(htmlHelper, name, Nothing, optionLabel, Nothing)
    End Function

    <System.Runtime.CompilerServices.Extension> _
    Public Function DropDownGroupList(htmlHelper As HtmlHelper, name As String, selectList As IEnumerable(Of GroupedSelectListItem), htmlAttributes As IDictionary(Of String, Object)) As MvcHtmlString
        Return DropDownListHelper(htmlHelper, name, selectList, Nothing, htmlAttributes)
    End Function

    <System.Runtime.CompilerServices.Extension> _
    Public Function DropDownGroupList(htmlHelper As HtmlHelper, name As String, selectList As IEnumerable(Of GroupedSelectListItem), htmlAttributes As Object) As MvcHtmlString
        Return DropDownListHelper(htmlHelper, name, selectList, Nothing, New RouteValueDictionary(htmlAttributes))
    End Function

    <System.Runtime.CompilerServices.Extension> _
    Public Function DropDownGroupList(htmlHelper As HtmlHelper, name As String, selectList As IEnumerable(Of GroupedSelectListItem), optionLabel As String) As MvcHtmlString
        Return DropDownListHelper(htmlHelper, name, selectList, optionLabel, Nothing)
    End Function

    <System.Runtime.CompilerServices.Extension> _
    Public Function DropDownGroupList(htmlHelper As HtmlHelper, name As String, selectList As IEnumerable(Of GroupedSelectListItem), optionLabel As String, htmlAttributes As IDictionary(Of String, Object)) As MvcHtmlString
        Return DropDownListHelper(htmlHelper, name, selectList, optionLabel, htmlAttributes)
    End Function

    <System.Runtime.CompilerServices.Extension> _
    Public Function DropDownGroupList(htmlHelper As HtmlHelper, name As String, selectList As IEnumerable(Of GroupedSelectListItem), optionLabel As String, htmlAttributes As Object) As MvcHtmlString
        Return DropDownListHelper(htmlHelper, name, selectList, optionLabel, New RouteValueDictionary(htmlAttributes))
    End Function

    <System.Runtime.CompilerServices.Extension> _
    Public Function DropDownGroupListFor(Of TModel, TProperty)(htmlHelper As HtmlHelper(Of TModel), expression As Expression(Of Func(Of TModel, TProperty)), selectList As IEnumerable(Of GroupedSelectListItem)) As MvcHtmlString
        ' optionLabel 
        ' htmlAttributes 
        Return DropDownGroupListFor(htmlHelper, expression, selectList, Nothing, Nothing)
    End Function

    <System.Runtime.CompilerServices.Extension> _
    Public Function DropDownGroupListFor(Of TModel, TProperty)(htmlHelper As HtmlHelper(Of TModel), expression As Expression(Of Func(Of TModel, TProperty)), selectList As IEnumerable(Of GroupedSelectListItem), htmlAttributes As Object) As MvcHtmlString
        ' optionLabel 
        Return DropDownGroupListFor(htmlHelper, expression, selectList, Nothing, New RouteValueDictionary(htmlAttributes))
    End Function

    <System.Runtime.CompilerServices.Extension> _
    Public Function DropDownGroupListFor(Of TModel, TProperty)(htmlHelper As HtmlHelper(Of TModel), expression As Expression(Of Func(Of TModel, TProperty)), selectList As IEnumerable(Of GroupedSelectListItem), htmlAttributes As IDictionary(Of String, Object)) As MvcHtmlString
        ' optionLabel 
        Return DropDownGroupListFor(htmlHelper, expression, selectList, Nothing, htmlAttributes)
    End Function

    <System.Runtime.CompilerServices.Extension> _
    Public Function DropDownGroupListFor(Of TModel, TProperty)(htmlHelper As HtmlHelper(Of TModel), expression As Expression(Of Func(Of TModel, TProperty)), selectList As IEnumerable(Of GroupedSelectListItem), optionLabel As String) As MvcHtmlString
        ' htmlAttributes 
        Return DropDownGroupListFor(htmlHelper, expression, selectList, optionLabel, Nothing)
    End Function

    <System.Runtime.CompilerServices.Extension> _
    Public Function DropDownGroupListFor(Of TModel, TProperty)(htmlHelper As HtmlHelper(Of TModel), expression As Expression(Of Func(Of TModel, TProperty)), selectList As IEnumerable(Of GroupedSelectListItem), optionLabel As String, htmlAttributes As Object) As MvcHtmlString
        Return DropDownGroupListFor(htmlHelper, expression, selectList, optionLabel, New RouteValueDictionary(htmlAttributes))
    End Function

    <System.Runtime.CompilerServices.Extension> _
    Public Function DropDownGroupListFor(Of TModel, TProperty)(htmlHelper As HtmlHelper(Of TModel), expression As Expression(Of Func(Of TModel, TProperty)), selectList As IEnumerable(Of GroupedSelectListItem), optionLabel As String, htmlAttributes As IDictionary(Of String, Object)) As MvcHtmlString
        If expression Is Nothing Then
            Throw New ArgumentNullException("expression")
        End If

        Return DropDownListHelper(htmlHelper, ExpressionHelper.GetExpressionText(expression), selectList, optionLabel, htmlAttributes)
    End Function

    Private Function DropDownListHelper(htmlHelper As HtmlHelper, expression As String, selectList As IEnumerable(Of GroupedSelectListItem), optionLabel As String, htmlAttributes As IDictionary(Of String, Object)) As MvcHtmlString
        ' allowMultiple 
        Return SelectInternal(htmlHelper, optionLabel, expression, selectList, False, htmlAttributes)
    End Function


    ' Helper methods

    <System.Runtime.CompilerServices.Extension> _
    Private Function GetSelectData(htmlHelper As HtmlHelper, name As String) As IEnumerable(Of GroupedSelectListItem)
        Dim o As Object = Nothing
        If htmlHelper.ViewData IsNot Nothing Then
            o = htmlHelper.ViewData.Eval(name)
        End If
        If o Is Nothing Then
            Throw New InvalidOperationException([String].Format(CultureInfo.CurrentCulture, "Missing Select Data", name, "IEnumerable<GroupedSelectListItem>"))
        End If
        Dim selectList As IEnumerable(Of GroupedSelectListItem) = TryCast(o, IEnumerable(Of GroupedSelectListItem))
        If selectList Is Nothing Then
            Throw New InvalidOperationException([String].Format(CultureInfo.CurrentCulture, "Wrong Select DataType", name, o.[GetType]().FullName, "IEnumerable<GroupedSelectListItem>"))
        End If
        Return selectList
    End Function

    Friend Function ListItemToOption(item As GroupedSelectListItem) As String
        Dim builder As New TagBuilder("option") With { _
             .InnerHtml = HttpUtility.HtmlEncode(item.Text) _
        }
        If item.Value IsNot Nothing Then
            builder.Attributes("value") = item.Value
        End If
        If item.Selected Then
            builder.Attributes("selected") = "selected"
        End If
        Return builder.ToString(TagRenderMode.Normal)
    End Function

    <System.Runtime.CompilerServices.Extension> _
    Private Function SelectInternal(htmlHelper__1 As HtmlHelper, optionLabel As String, name As String, selectList As IEnumerable(Of GroupedSelectListItem), allowMultiple As Boolean, htmlAttributes As IDictionary(Of String, Object)) As MvcHtmlString
        name = htmlHelper__1.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldName(name)
        If [String].IsNullOrEmpty(name) Then
            Throw New ArgumentException("Null Or Empty", "name")
        End If

        Dim usedViewData As Boolean = False

        ' If we got a null selectList, try to use ViewData to get the list of items.
        If selectList Is Nothing Then
            selectList = htmlHelper__1.GetSelectData(name)
            usedViewData = True
        End If

        Dim defaultValue As Object = If((allowMultiple), htmlHelper__1.GetModelStateValue(name, GetType(String())), htmlHelper__1.GetModelStateValue(name, GetType(String)))

        ' If we haven't already used ViewData to get the entire list of items then we need to
        ' use the ViewData-supplied value before using the parameter-supplied value.
        If Not usedViewData Then
            If defaultValue Is Nothing Then
                defaultValue = htmlHelper__1.ViewData.Eval(name)
            End If
        End If

        If defaultValue IsNot Nothing Then
            Dim defaultValues As IEnumerable = If((allowMultiple), TryCast(defaultValue, IEnumerable), New String() {defaultValue})
            Dim values As IEnumerable(Of String) = From value In defaultValues Select (Convert.ToString(value, CultureInfo.CurrentCulture))
            Dim selectedValues As New HashSet(Of String)(values, StringComparer.OrdinalIgnoreCase)
            Dim newSelectList As New List(Of GroupedSelectListItem)()

            For Each item As GroupedSelectListItem In selectList
                item.Selected = If((item.Value IsNot Nothing), selectedValues.Contains(item.Value), selectedValues.Contains(item.Text))
                newSelectList.Add(item)
            Next
            selectList = newSelectList
        End If

        ' Convert each ListItem to an <option> tag
        Dim listItemBuilder As New StringBuilder()

        ' Make optionLabel the first item that gets rendered.
        If optionLabel IsNot Nothing Then
            listItemBuilder.AppendLine(ListItemToOption(New GroupedSelectListItem() With { _
                 .Text = optionLabel, _
                 .Value = [String].Empty, _
                 .Selected = False _
            }))
        End If

        For Each group As Object In selectList.GroupBy(Function(i) i.GroupKey)
            Dim groupName As String = selectList.Where(Function(i) i.GroupKey = group.Key).[Select](Function(it) it.GroupName).FirstOrDefault()
            listItemBuilder.AppendLine(String.Format("<optgroup label=""{0}"" value=""{1}"">", groupName, group.Key))
            For Each item As GroupedSelectListItem In group
                listItemBuilder.AppendLine(ListItemToOption(item))
            Next
            listItemBuilder.AppendLine("</optgroup>")
        Next

        Dim tagBuilder As New TagBuilder("select") With { _
             .InnerHtml = listItemBuilder.ToString() _
        }
        TagBuilder.MergeAttributes(htmlAttributes)
        ' replaceExisting 
        TagBuilder.MergeAttribute("name", name, True)
        TagBuilder.GenerateId(name)
        If allowMultiple Then
            TagBuilder.MergeAttribute("multiple", "multiple")
        End If

        ' If there are any errors for a named field, we add the css attribute.
        Dim modelState As ModelState = Nothing
        If htmlHelper__1.ViewData.ModelState.TryGetValue(name, modelState) Then
            If modelState.Errors.Count > 0 Then
                TagBuilder.AddCssClass(HtmlHelper.ValidationInputCssClassName)
            End If
        End If

        Return MvcHtmlString.Create(TagBuilder.ToString())
    End Function

    <System.Runtime.CompilerServices.Extension> _
    Friend Function GetModelStateValue(helper As HtmlHelper, key As String, destinationType As Type) As Object
        Dim modelState As ModelState = Nothing
        If helper.ViewData.ModelState.TryGetValue(key, modelState) Then
            If modelState.Value IsNot Nothing Then
                ' culture 
                Return modelState.Value.ConvertTo(destinationType, Nothing)
            End If
        End If
        Return Nothing
    End Function

End Module

I have been trying @Serge Zab solution out which works well but was having some trouble with the unobtrusive validation, after some inspection I found the problem.

There seems to be some required attributes missing off the select element if you want the Jquery validation to fire, just after you create your TagBuilder

TagBuilder tagBuilder = new TagBuilder("select");

Add these attributes

tagBuilder.MergeAttribute("data-val", "true",true); 
tagBuilder.MergeAttribute("data-val-required", "your validation message", true)

and the unobtrusive validation should fire.

In Serge Zab answer, data attributes, like data_valuename, dont marging in correct form data-valuename.

I replaced code tagBuilder.MergeAttributes(htmlAttributes); in SelectInternal method to this

foreach (var htmlAttribute in htmlAttributes)
{
     tagBuilder.MergeAttribute(
        htmlAttribute.Key.Replace('_', '-'), 
        (string)htmlAttribute.Value
     );
}

Just a note on using Serge's extension to create more than one select list on the same form. I was having problems getting the second select list to apply the groups, and when I examined the html being generated I realized both had been given the same id. To fix this, go into the SelectInternal function in serge's extension and comment out/delete the following two lines: tagBuilder.MergeAttribute("name", name, true /* replaceExisting */); tagBuilder.GenerateId(name); Alternatively you can just pass unique id's to each (though the DropDownGroupListFor doesn't take the 'string name' parameter so you'll have to add an overload that does)

I needed a solution to handle a multiple selection with optgroup, and I used Serge Zab solution. I've just two comments about it (too long for a comment).

• Despite his claims, his solution does not support all existing overloads of DropDownListFor, since it doesn't support MultiSelectList or SelectList, oftenly used in models. But that's not hard to add these.

• His solution was not working for me for Multiple selection to initialize selected/not selected items from model : the initial values were not affected. I've just modified the following method :

  private static MvcHtmlString DropDownListHelper(...) { return SelectInternal(htmlHelper, optionLabel, expression, selectList, false /* allowMultiple */, htmlAttributes); }

To this :

private static MvcHtmlString DropDownListHelper(HtmlHelper htmlHelper, string expression, IEnumerable<GroupedSelectListItem> selectList, string optionLabel, IDictionary<string, object> htmlAttributes)
{
    bool allowMultiple = htmlAttributes.ContainsKey("multiple");
    return SelectInternal(htmlHelper, optionLabel, expression, selectList, allowMultiple, htmlAttributes);
}

And it worked as expected. Of course the multiple attribute must be defined : @Html.DropDownGroupListFor(m => m.Selected, Model.Values, new { multiple = "multiple" })

Thanks to Serge for his answer.