public static class HtmlHelperExtensions
    {
        private const string Nbsp = " ";
        private const string SelectedAttribute = " selected='selected'";
 
        public static MvcHtmlString NbspIfEmpty(this HtmlHelper helper, string value)
        {
            return new MvcHtmlString(string.IsNullOrEmpty(value) ? Nbsp : value);
        }
 
        public static MvcHtmlString SelectedIfMatch(this HtmlHelper helper, object expected, object actual)
        {
            return new MvcHtmlString(Equals(expected, actual) ? SelectedAttribute : string.Empty);
        }
    }

<select>
   @foreach (var item in ViewBag.Items)
   {
      <option@Html.SelectedIfMatch((string)ViewBag.SelectedItem,

                                                           (string)item.ItemName)>@item.ItemName</option>
   }
</select>