Find form fields in pdf file

I am trying to find out what the names of the fields are in a pdf file so that I can push data to them from my asp page.  I can see the file and save the fiel but I have no clue what the field names are.  Serched high and wide on web and every suggestion i use has dictionaryEntry which always errors out on me

itextsharp.text.pdf.acrofieldsItem cannot be converted to systems.collections.dictionary entry


 Private Sub ListFieldNames()
        Dim pdftemp As String = Server.MapPath("~/PDF/test.pdf")

        Dim pdfReader As PdfReader = New PdfReader(pdftemp)

        ' create and populate a string builder with each of the
        ' field names available in the subject PDF
        Dim sb As New StringBuilder()

        Dim de As New DictionaryEntry
        For Each de In pdfReader.AcroFields.Fields
            sb.Append(de.Key.ToString() + Environment.NewLine)
        Next

        ' Write the string builder's content to the form's textbox
         response.write(sb)

    End Sub