ASP.NET HTML Control Mapping

January 3, 2008

Ever wondered how ASP.NET takes an HTML tag, such as <head>, and maps it to an System.Web.UI.HtmlControls.HtmlHead object? Well, me neither, till today. But if you ever want to know, Reflector comes in handy. In the System.Web.UI namespace, there's an internal class called HtmlTagNameToTypeMapper. In here, there's a method called GetControlType, which looks like this:

Type ITagNameToTypeMapper.GetControlType(string tagName, IDictionary attributeBag)
{
  Type type;
  if (tagMap == null)
  {
      Hashtable hashtable = new Hashtable(10, StringComparer.OrdinalIgnoreCase);
      hashtable.Add("a", typeof(HtmlAnchor));
      hashtable.Add("button", typeof(HtmlButton));
      hashtable.Add("form", typeof(HtmlForm));
      hashtable.Add("head", typeof(HtmlHead));
      hashtable.Add("img", typeof(HtmlImage));
      hashtable.Add("textarea", typeof(HtmlTextArea));
      hashtable.Add("select", typeof(HtmlSelect));
      hashtable.Add("table", typeof(HtmlTable));
      hashtable.Add("tr", typeof(HtmlTableRow));
      hashtable.Add("td", typeof(HtmlTableCell));
      hashtable.Add("th", typeof(HtmlTableCell));
      tagMap = hashtable;
  }
  if (inputTypes == null)
  {
      Hashtable hashtable2 = new Hashtable(10, StringComparer.OrdinalIgnoreCase);
      hashtable2.Add("text", typeof(HtmlInputText));
      hashtable2.Add("password", typeof(HtmlInputPassword));
      hashtable2.Add("button", typeof(HtmlInputButton));
      hashtable2.Add("submit", typeof(HtmlInputSubmit));
      hashtable2.Add("reset", typeof(HtmlInputReset));
      hashtable2.Add("image", typeof(HtmlInputImage));
      hashtable2.Add("checkbox", typeof(HtmlInputCheckBox));
      hashtable2.Add("radio", typeof(HtmlInputRadioButton));
      hashtable2.Add("hidden", typeof(HtmlInputHidden));
      hashtable2.Add("file", typeof(HtmlInputFile));
      inputTypes = hashtable2;
  }
  if (StringUtil.EqualsIgnoreCase("input", tagName))
  {
      string str = (string) attributeBag["type"];
      if (str == null)
      {
          str = "text";
      }
      type = (Type) inputTypes[str];
      if (type == null)
      {
          throw new HttpException(SR.GetString("Invalidtypeforinputtag", new object[] { str }));
      }
      return type;
  }
  type = (Type) tagMap[tagName];
  if (type == null)
  {
      type = typeof(HtmlGenericControl);
  }
  return type;
}

Unfortunately, there's no way I can see of overriding this behaviour - for example, if you wanted to map the <li> element to a custom HtmlListItemControl, it's impossible.


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