GroupedDropDownList.cs

public class GroupedDropDownList
{
    public string Name { get; set; }
    public List<SelectListItem> Values { get; set; }
    public GroupedDropDownList()
    {
        Values = new List<SelectListItem>();
    }
}

public static partial class SelectExtensions
{
    
    public static MvcHtmlString GroupedDropDownListFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, string optionalLabel, IEnumerable<GroupedDropDownList> selectList, object htmlAttributes)
    {
        if (expression == null)
        {
            throw new ArgumentNullException("expression");
        }
        
        string name = ExpressionHelper.GetExpressionText(expression);
        return GroupedDropDownList(htmlHelper, name, optionalLabel, selectList, htmlAttributes);
    }
    
    public static MvcHtmlString GroupedDropDownList(this HtmlHelper htmlHelper, string name, string optionalLabel, IEnumerable<GroupedDropDownList> selectList, object htmlAttributes)
    {
        TagBuilder drop = new TagBuilder("select");
        drop.MergeAttribute("name", name);
        drop.MergeAttribute("id", name);
        if (htmlAttributes != null)
        {
            var attributes = HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes);
            drop.MergeAttributes(attributes);
        }
        
        IDictionary<string, object> validationAttributes = htmlHelper.GetUnobtrusiveValidationAttributes(name);
        drop.MergeAttributes(validationAttributes);
        
        if (optionalLabel != null && !string.IsNullOrEmpty(optionalLabel))
        {
            TagBuilder optLbl = new TagBuilder("option");
            optLbl.MergeAttribute("value", "");
            optLbl.InnerHtml = optionalLabel;
            drop.InnerHtml += optLbl.ToString();
        }
        
        foreach (var group in selectList)
        {
            TagBuilder g = new TagBuilder("optgroup");
            g.Attributes.Add("label", group.Name);
            foreach (var option in group.Values)
            {
                TagBuilder opt = new TagBuilder("option");
                if (option.Selected)
                {
                    opt.MergeAttribute("selected", "selected");
                }
                opt.MergeAttribute("value", option.Value);
                opt.InnerHtml = option.Text;
                g.InnerHtml += opt.ToString();
            }
            drop.InnerHtml += g.ToString();
        }
        
        return new MvcHtmlString(drop.ToString());
    }
}