Sub Main()
    Dim excelFile As String = "C:\Temp\PartsForDxf.xlsx"
    Dim excelSheet As String = "List1"
    Dim dataFromExcel = ReadDataFromExcel(excelFile, excelSheet)
    Dim fileNames = CreateFileNames(dataFromExcel)
    For Each fileName As String In fileNames
        'Check file exists
        If Not System.IO.File.Exists(fileName) Then
            MsgBox("File not found" & vbCrLf & fileName)
            Continue For
        End If
        'Open file
        Dim partDocument As PartDocument = ThisApplication.Documents.Open(fileName)
        'Export to DXF
        ExportToDxf(partDocument)
        'Close without changes
        partDocument.Close(True)
    Next
End Sub
Private Sub ExportToDxf(partDocument As PartDocument)
    'Implement your export to DXF here
    MsgBox(String.Format("Exporting to dxf...{1}{0}", partDocument.FullFileName, vbCrLf))
End Sub
Private Function CreateFileNames(dataFromExcel As List(Of String)) As List(Of String)
    Dim fileNames As New List(Of String)
    For Each fileName As String In dataFromExcel
        fileNames.Add(String.Format("C:\Temp\{0}.ipt", fileName))
    Next
    Return fileNames
End Function
Private Function ReadDataFromExcel(excelFile As String, excelSheet As String) As List(Of String)
    Dim cellValues As New List(Of String)
    Dim column As String = "A"
    Dim row As Integer = 1
    GoExcel.Open(excelFile, excelSheet)
    Do
        Dim cellAddress As String = String.Format("{0}{1}", column, row)
        Dim cellValue = GoExcel.CellValue(cellAddress)
        If (cellValue Is Nothing OrElse cellValue.ToString() = "") Then
            Exit Do
        End If
        cellValues.Add(cellValue)
        row += 1
    Loop
    GoExcel.Close()
    Return cellValues
End Function

	Dim fName = System.IO.Path.ChangeExtension(partDocument.FullFileName, ".dxf")
	MsgBox(fName)