Visual Basic

Public Class Form1
    Dim index As Long = -1
    Const file As String = "c:\test.txt"

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        'FileInfo object so we can get the file size
        Dim info As New IO.FileInfo(file)

        'How many new bytes since we last checked
        Dim count As Long = info.Length - index

        'File hasn't changed so return
        If count <= 0 Then Return

        'File stream to read file
        Dim stream As IO.FileStream = IO.File.Open(file, IO.FileMode.Open, IO.FileAccess.Read)

        'Buffer to retrieve bytes
        Dim buf(count) As Byte

        'Set the streams position to where we last left off
        stream.Position = index

        'Read the new bytes
        stream.Read(buf, 0, count)

        'clean-up so we don't get in the way of the other program
        stream.Close()
        stream.Dispose()

        'Decoder to convert byte array to string
        Dim textDecoder As New System.Text.ASCIIEncoding
        'The new text
        Dim str As String = textDecoder.GetString(buf)
        'Display new text
        MsgBox(str)

        'Set index to next position
        index = info.Length
    End Sub

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        'Initialize index to the last position in the file
        Dim info As New IO.FileInfo(file)
        index = info.Length
    End Sub
End Class

Do
    'Get the most up to date information
    info = New IO.FileInfo(file)
    'If file doesn't exist leave
    If Not info.Exists Then Return
    'info.length throws an error if the file doesn't exist, thus the needed line above.
    count = info.Length - index
    Try
        stream = IO.File.Open(file, IO.FileMode.Open, IO.FileAccess.Read)
    Catch ex As Exception
        'Sleep a little and try again later
        Threading.Thread.Sleep(5)
    End Try
Loop While stream Is Nothing