public static StringBuilder RenderTree(ICollection<ItemType> items, int? parentId = null, StringBuilder builder = null)
{
   if (builder == null) 
      builder = new StringBuilder();
   var list = items // получаем всех "детей"
      .Where(x => x.ParentId == parentId)
      .ToList();
   if(list.Count == 0)  // нечего рендерить
      return builder;
   builder.Append("<ul>");
   foreach(var item in items.Where(x => x.ParentId == parentId))
   {
      builder.Append("<li>").Append(item.Name);
      RenderTree(items, item.Id, builder); // рекурсия
      builder.Append("</li>");
   }
   return builder.Append("</ul>");
}


// пример:

@Html.Raw(RenderHelper.RenderTree(items).ToString())