选择使用htmltextwriter呈现的控件在Asp.net中的Request.Form中不可用

Question

我需要一个带有组的列表框，为此我在页面中放置了一个文字控件，然后使用HTMLTextWriter将一个选择控件呈现给文字控件。它完美呈现。

问题是我无法从Request.Form对象获取select控件。 Request.Form中缺少select控件的id。

为什么会如此？如何在代码中获取用户选择？代码如下。

Dictionary<string,List<string>> locations =  GetLocations();

      StringWriter stringWriter = new StringWriter();
      HtmlTextWriter html = new HtmlTextWriter(stringWriter);

      html.AddAttribute(HtmlTextWriterAttribute.Multiple, "multiple");
      html.AddAttribute(HtmlTextWriterAttribute.Id, "ddlLocations");
      html.RenderBeginTag(HtmlTextWriterTag.Select);
      foreach (string region in locations.Keys)
      {
          html.AddAttribute("label", region);
          html.RenderBeginTag("optgroup");
          //add countries
          foreach (string country in locations[region])
          {
              html.AddAttribute(HtmlTextWriterAttribute.Value, country);
              html.RenderBeginTag(HtmlTextWriterTag.Option);
              html.Write(country);

              html.RenderEndTag();//close option
          }
          html.RenderEndTag();//close optgroup

      }
      html.RenderEndTag();//close select

      literalForLocationDdl.Text = stringWriter.ToString();