title: How to add additional htmlattributes in extension for DropDownListFor
body: <p>I'm trying to write an extension for <code>DropDownListFor</code>:</p>
  
  <pre><code>public static MvcHtmlString DropDownListFor&lt;TModel, TProperty&gt;(this HtmlHelper&lt;TModel&gt; htmlHelper, Expression&lt;Func&lt;TModel, TProperty&gt;&gt; expression, IEnumerable&lt;SelectListItem&gt; selectList, object htmlAttributes, bool enabled)
  {
      return htmlHelper.DropDownListFor(expression, selectList, null /* optionLabel */, HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes));
  }
  </code></pre>
  
  <p>What I want to achieve is if enabled is false no change but if enabled is true I want to add <code>@disabled="disabled"</code> to the html attributes before giving them to <code>AnonymousObjectToHtmlAttributes</code>.</p>
  
  <p>Any ideas on how to do this?</p>