System.InvalidCastException Iterating Through Viewdata

System.InvalidCastException iterating through Viewdata

I need to replace the code
"<%=Html.DropDownList("Part", (SelectList)ViewData["Parts"])%>"
for dropdown in the following manner for some reason.
<% foreach (Hexsolve.Data.BusinessObjects.HSPartList item in (IEnumerable)ViewData["Parts"])
{ %>
">
<%=item.PartName %>
<%=item.IssueNo %>
<% } %>

I am getting error converting SelectedList to IEnumerable) Error: Unable to cast object of type 'System.Web.Mvc.SelectList' to type 'System.Collections.Generic.IEnumerable`1[Hexsolve.Data.BusinessObjects.HSPartList]'. Is this the right way to iterate through viewdata[].