Private Shared Function FileFromBase64(ByVal sFileIn As String, 
        sFileOut As String) As Boolean
 
        If Not IO.File.Exists(sFileIn) Then Return False
 
        Try
            Dim str64 As String = IO.File.ReadAllText(sFileIn)
            Dim chars() As Char = str64.ToCharArray
 
            Dim bytes As Byte() = 
              System.Convert.FromBase64CharArray(chars, 0, chars.Length)
 
            Dim stb As New Text.StringBuilder
            For i As Integer = 0 To bytes.Length - 1 Step 1
                stb.Append(ChrW(bytes(i)))
            Next i
 
            IO.File.WriteAllText(sFileOut, stb.ToString)
            Return True
        Catch
            Return False
        End Try
 
End Function

   Dim str64 As String = IO.File.ReadAllText(sFileIn)
 
   Dim bytes() As Byte = System.Convert.FromBase64String(str64)
 
 
   IO.File.WriteAllText(sFileOut, System.Text.Encoding.UTF8.GetString(bytes))

Private Function FileToBase64(ByVal sFileIn As String, ByVal sFileOut As _
  String) As Boolean
 
        Dim bResult As Boolean = False
        Dim oReader As IO.FileStream = Nothing
        Dim oWriter As IO.StreamWriter = Nothing
 
        Try
            ' Falls Output-Datei bereits existiert, löschen
            If IO.File.Exists(sFileOut) Then IO.File.Delete(sFileOut)
 
            ' Input-Datei zum Lesen öffnen
            oReader = New IO.FileStream(sFileIn, IO.FileMode.Open, _
              IO.FileAccess.Read)
 
            ' Output-Datei zum Schreiben öffnen
            oWriter = New IO.StreamWriter(sFileOut, False, _
              System.Text.Encoding.UTF8)
 
            ' Datei blockweise einlesen
            Dim nBytes As Integer
            Dim Buffer(56) As Byte
            Do
                nBytes = oReader.Read(Buffer, 0, Buffer.Length)
                If nBytes > 0 Then
                    ' Inhalt nach Base64 codieren und in Output-Datei schreiben
                    oWriter.WriteLine(System.Convert.ToBase64String(Buffer, 0, _
                      nBytes))
                End If
            Loop Until nBytes = 0
 
            ' Streams (Dateien) schließen
            oReader.Close()
            oWriter.Close()
 
            bResult = True
 
        Catch ex As Exception
            If oReader IsNot Nothing Then oReader.Close()
            If oWriter IsNot Nothing Then oWriter.Close()
        End Try
 
        Return bResult
End Function
 
Private Function FileFromBase64(ByVal sFileIn As String, sFileOut As String) As _
  Boolean
 
        If Not IO.File.Exists(sFileIn) Then Return False
 
        Try
            Dim str64 As String = IO.File.ReadAllText(sFileIn)
 
            Dim bytes() As Byte = System.Convert.FromBase64String(str64)
 
            IO.File.WriteAllText(sFileOut, System.Text.Encoding.UTF8.GetString( _
              bytes))
 
            Return True
        Catch
            Return False
        End Try
 
End Function

'-------------------------------------------------------------
FileToBase64("Alt.docx", "Alt.docx.dat")
FileToBase64("Alt.pdf", "Alt.pdf.dat")
'-------------------------------------------------------------
FileFromBase64("Alt.docx.dat", "Neu.docx")
FileFromBase64("Alt.pdf.dat", "Neu.pdf")
'-------------------------------------------------------------

Dim bytes() As Byte
 
bytes = IO.File.ReadAllBytes("G:\daten\irgendeine Datei")
Dim str As String = Convert.ToBase64String(bytes, 0, bytes.Length)
 
Dim bytes2() As Byte = Convert.FromBase64String(str)
IO.File.WriteAllBytes("G:\daten\Datei decodiert", bytes2)
 
For i As Integer = 0 To bytes.Length - 1
    If bytes2(i) <> bytes(i) Then Stop
Next i

  Private Function FileToB64(FileIn As String, Fileout As String) As Boolean
 
        Try
            IO.File.Delete(Fileout)
            IO.File.WriteAllText(Fileout, 
            Convert.ToBase64String(IO.File.ReadAllBytes(FileIn)), _
              System.Text.Encoding.UTF8)
            Return True
        Catch ex As Exception
            Return False
        End Try
 
    End Function
 
 
 
    Private Function FileFromB64(FileIn As String, Fileout As String) As Boolean
 
        Try
            IO.File.Delete(Fileout)
            IO.File.WriteAllBytes(Fileout, 
            Convert.FromBase64String(IO.File.ReadAllText(FileIn, _
              System.Text.Encoding.UTF8)))
            Return True
        Catch
            Return False
        End Try
 
    End Function

Zitat:

ich möchte unterschiedliche Dateianhänge in eine XML Datei abspeichern. Das soll einfacher sein, wenn diese als Base64 vorliegen.

System.Xml.XmlReader
ReadContentAsBase64(Byte()) As Integer
 
System.Xml.XmlWriter
WriteBase64(Byte())