assign value to dropdownlist.selectvalue

Dear All,

 I don't know why the assignment below to the dropdownlist.selectvalue does not work? Do you know?

 

if (!IsPostBack)
            {
                IList<InterfaceFileTypeDto> fileType = GetInterfaceFileType();
                ddlFileType.DataTextField = "Description";
                ddlFileType.DataValueField = "Code";
                ddlFileType.DataSource = fileType;
                ddlFileType.DataBind();
       &n ...