title: selectList.SelectedValue is null
body: <p>I keep getting this error when I implement my code for a dropdown list.
  Here is my code:</p>
  
  <pre><code>String sort = "";
  String day_hire = "Daily hire rate";
  String man_date = "Manufacturing date";
  string man_n = "Manufacturer's name";
  
  if (sl2.SelectedValue.ToString()== man_n)
  {
      sort = "man.manufacturer_name";
  
  }
  else if (sl2.SelectedValue.ToString() == man_date)
  {
      sort = "veh.manufacturing_date";
  }
  else if (sl2.SelectedValue.ToString() == day_hire) 
  {
      sort = "veh.daily_hire_rate";
  }
  else
  {
      sort = "veh.daily_hire_rate"; 
  }
  </code></pre>
  
  <p>this is in constructor. In view I've got:</p>
  
  <pre><code>&lt;%=Html.DropDownList ("sort",((SelectList)ViewData["selectOptions2"]))%&gt;
  </code></pre>
  
  <p>it's a runtime error that says <code>sl2.SelectedValue.ToString()</code> is null. Please help</p>