The key is the CFileChopper class's CreateChoppedFiles method. It opens the input file with a StreamReader, reads the lines in the file, and writes them into the smaller files.

Public Function CreateChoppedFiles() As String
    Dim FileNamesCreated = ""
    Dim i As Integer
    Dim PartialOutputFileName As String
    PartialOutputFileName = _
        Path.GetDirectoryName(Me.m_FileNameToConvert) & "\" _
        & Path.GetFileNameWithoutExtension(Me.m_FileNameToConvert)
    For i = 1 To m_NumberOfFilesToCreate
        m_sr = CreateStreamReader(Me.m_FileNameToConvert)
        Dim mincolumnnumber As Integer = 2 + ((i - 1) * _
            (MAXFIELDSPEROUTPUTFILE - 1))
        Dim maxcolumnnumber As Integer = mincolumnnumber + _
            MAXFIELDSPEROUTPUTFILE - 2
        If maxcolumnnumber > m_Columnnames.Length Then
            maxcolumnnumber = m_Columnnames.Length
        End If
        'MessageBox.Show(mincolumnnumber & " - " &
        ' maxcolumnnumber)
        Dim newfilename As String = PartialOutputFileName & _
            Chr(65 + i - 1) & _
            Path.GetExtension(Me.m_FileNameToConvert)
        Dim sw As New StreamWriter(newfilename, False)
        Dim nextline As String
        nextline = m_sr.ReadLine()
        Do Until nextline = "" Or IsNothing(nextline)
            Dim DataFieldData() As String = _
                nextline.Split(ControlChars.Tab)
            sw.Write(DataFieldData(0))
            Dim col As Integer
            For col = mincolumnnumber - 1 To _
                maxcolumnnumber - 1
                sw.Write(ControlChars.Tab & _
                    DataFieldData(col))
            Next
            sw.Write(ControlChars.CrLf)
            nextline = m_sr.ReadLine()
        Loop
        m_sr.Close()
        sw.Close()
        If FileNamesCreated = "" Then
            FileNamesCreated = newfilename
        Else
            FileNamesCreated = FileNamesCreated & "; " & _
                ControlChars.CrLf & newfilename
        End If
    Next
    Return FileNamesCreated
End Function