@Html.DropdownListがSelectedを消してしまう

しかも"たまに" (# ﾟДﾟ)ふぁーーっく!!!

こんな状況

//---model---
public class StayType{
  public int id{get;set;}
  public string name{get;set;}
}

public class Reserve{
   [いろいろ]
   public ICollection<ReserveItem> items{get;set;}
}

public class ReserveItem{
   [いろいろ]
   public int stayTypeId{get;set;}
}
  
//---controller---

public ActionResult Edit(int id){
   ViewBag.StayTypes = entity.StayType.ToList();
   return View();
}

//---View----

@foreach (item in Model.items){
<table> 
  <tr>
    <td>@Html.DropdownList("StayType",new SelectList(ViewBag.StayTypes,"id","name"item.StayTypeId),new{}) </td>
   ...

}

これで出力すると、StayTypeId=3にも関わらず

<select class="staytypeselect" name="StayType"><option value="1">1泊2食</option>
<option value="2">1泊3食</option>
<option value="4">1泊朝食</option>
<option value="3">1泊夕食</option>
<option value="5">素泊まり</option>
</select>

こうなる。(# ﾟДﾟ)

SelectListをダンプするとValue=3のSelectListItemがSelectedになってる。

(# ﾟДﾟ) ファック

もういい作る

public static MvcHtmlString GLDropDownList(this HtmlHelper htmlHelper, string name, string id ,IEnumerable<SelectListItem> selectList, object htmlAttributes) {
    if (id == null)
        id = name;
    StringBuilder options = new StringBuilder();
    options.Append(string.Concat<string>(selectList.Select(sl => GLRenderListItem(sl))));

    TagBuilder tagBuilder = new TagBuilder("select") {
        InnerHtml = options.ToString()
    };
    tagBuilder.MergeAttributes<string, object>(HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes));
    tagBuilder.MergeAttribute("name", name, true);
    tagBuilder.MergeAttribute("id", id, true);
    tagBuilder.MergeAttributes<string, object>(htmlHelper.GetUnobtrusiveValidationAttributes(name));
    return new MvcHtmlString(tagBuilder.ToString(TagRenderMode.Normal));

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

( ﾟДﾟ) できた。

しかし不可解な現象だ。かんべんしてよもう。