title: MVC dropdownlistfor add null value
body: <p>I am working in MVC 
  I added a "select all" option to a DropDownListFor as such</p>
  
  <pre><code>&lt;tr&gt;&lt;td valign="top"&gt;@Html.DropDownListFor(model =&gt; model.fk_Type, Model.ListType, "- Select All -", new { style = "width:210px" })&lt;/td&gt;&lt;/tr&gt;
  </code></pre>
  
  <p>In my Controller code the select all option is giving a value of "". Which i assume is an empty string.
  Is there a way in the html above to set the value of 'select all' to null.
  That would make the called stor proc in the model, handle it perfectly..
  many thanks</p>