mvc cshtml 字符串操作

@using System.Text;
@{
    int js = 1;
    string  str="";
    StringBuilder sb = new StringBuilder();
}

        @foreach (var item in ViewBag.Doctypes)
                                    {
                                        if (item.DocTypeNo.Length == 5)
                                        {
                                            sb.AppendFormat("<optgroup label='{0}'>", item.DocTypeName);

                                            foreach (var item1 in ViewBag.Doctypes)
                                            {
                                                if (item1.DocTypeNo.Length < 6)//字符串小于6跳过
                                                {
                                                    continue;
                                                }

                                                if (item.DocTypeNo == item1.DocTypeNo.Substring(0, 5)) //判断前5个是否相同
                                                {
                                                    if (item1.DocTypeName == ViewBag.Current)
                                                    {
                                                        sb.AppendFormat("<option value='{0}' selected>{1}</option>", @item1.DocTypeName, @item1.DocTypeName);
                                                        //<option value="@item1.DocTypeName" selected>@item1.DocTypeName</option>
                                                        continue;
                                                    }
                                                    sb.AppendFormat("<option value='{0}'>{1}</option>", @item1.DocTypeName, @item1.DocTypeName);
                                                
                                                }
                                            }

                                            sb.Append("</optgroup>" );
                                        }
                                    }
 @Html.Raw(sb.ToString())