title: asp.net mvc dropdownlist no ViewData item
body: <p>i start to study mvc, try to add dropdownlist, make construction</p>
  
  <pre><code>&lt;%= Html.DropDownList("ddl") %&gt;
  </code></pre>
  
  <p>but it show error </p>
  
  <blockquote>
    <p>There is no ViewData item of type 'IEnumerable' that has the key 'ddl'</p>
  </blockquote>
  
  <p>why? i use simple code, only pass name parameter, so why the error?</p>