Email über POP3 abfrufen

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Public Class Client

    Private Client As New Net.Sockets.TcpClient
    Private POP3Stream As Net.Sockets.NetworkStream
    Private Connected As Boolean = False

    '''<summary>
    '''Verbindungssatus abfragen.
    '''</summary>
    Public ReadOnly Property ConnectionEstablished() As Boolean

        Get
            Return Connected
        End Get

    End Property

    '''<summary>
    '''Nachrichten Klasse
    '''</summary>
    Public Class MessageList

        Public ID As Int64
        Public Size As Int64

    End Class

    '''<summary>
    '''Verfügbarer POP3 Befehlssatz.
    '''</summary>
    Private Class Commands

        Public Const Delete As String = "DELE "
        Public Const GetMessage As String = "RETR "
        Public Const List As String = "LIST"
        Public Const Password As String = "PASS "
        Public Const Quit As String = "QUIT"
        Public Const ServerConfirm As String = "+OK"
        Public Const ServerNoMoreData As String = "."
        Public Const User As String = "USER "

    End Class

    '''<summary>
    '''Stellt die Verbindung zum POP3-Server her.
    '''</summary>
    '''<param name="Server">Servername (subdomain.domain.tld)</param>
    '''<param name="User">Benutzername</param>
    '''<param name="Password">Passwort</param>
    Public Sub Connect(ByVal Server As String, ByVal User As String, ByVal Password As String)

        If Connected = True Then Me.Disconnect()
        Try
            Me.Client.Connect(Server, 110)
            Me.POP3Stream = Me.Client.GetStream
            Me.CheckResponse(Me.GetResponse)
            Me.Send(Commands.User + User)
            Me.CheckResponse(Me.GetResponse)
            Me.Send(Commands.Password + Password)
            Me.CheckResponse(Me.GetResponse)
            Me.Connected = True
        Catch ex As Exception
            Me.Disconnect()
            Throw New ApplicationException(ex.Message, ex)
        End Try

    End Sub

    '''<summary>
    '''Trennt die Verbindung zum POP3-Server her.
    '''</summary>
    Public Sub Disconnect()

        If Connected = True Then
            Me.Send(Commands.Quit)
            Me.CheckResponse(Me.GetResponse())
            Me.Connected = False
            Me.Client.Close()
        End If

    End Sub

    '''<summary>
    '''Sendet POP3 Befehl zum Server.
    '''</summary>
    '''<param name="Command">POP3 Befehl</param>
    Private Sub Send(ByVal Command As String)

        Dim data As [Byte]() = Text.Encoding.ASCII.GetBytes(Command & vbCrLf)
        Me.POP3Stream.Write(data, 0, data.Length)

    End Sub

    '''<summary>
    '''Empfängt Antwort auf Send-Befehl vom Server.
    '''</summary>
    Private Function GetResponse() As String

        Dim SR As New IO.StreamReader(Me.POP3Stream)
        Dim SBuilder As New Text.StringBuilder
        Do While SR.Peek > -1
            SBuilder.Append(SR.ReadLine + vbCr)
        Loop
        Return SBuilder.ToString

    End Function

    '''<summary>
    '''Prüft ob die erhalten Antwort von GetResponse gültig ist.
    '''</summary>
    '''<param name="Response">Antort von GetResponse</param>
    Private Sub CheckResponse(ByVal Response As String)

        If Not String.Compare(Response.Substring(0, 3), Commands.ServerConfirm, False, Globalization.CultureInfo.CurrentCulture) = 0 Then
            Me.Client.Close()
            Me.Connected = False
            Throw New ApplicationException("Response " + Response + " not expected.")
        End If

    End Sub

    '''<summary>
    '''Gibt eine List(of POP3Message) der verfügbaren Nachrichten zurück.
    '''</summary>
    Public Function GetMessageList() As List(Of MessageList)

        If Me.Connected = False Then
            Throw New InvalidOperationException("Not connected!")
        End If
        Dim Pop3l As New List(Of MessageList)
        Me.Send(Commands.List)
        Dim Response() As String = Me.GetResponse.Split(vbCrLf)
        CheckResponse(Response(0))

        For i As Integer = 1 To Response.Length - 3
            Dim POP3m As New MessageList
            POP3m.ID = i
            POP3m.Size = Convert.ToInt64(Response(i).Split(" ")(1))
            Pop3l.Add(POP3m)
        Next

        Return Pop3l

    End Function

    '''<summary>
    '''Nachricht mit angebener Nummer vom Server lesen
    '''</summary>
    '''<param name="MessageID">MessageID</param>
    Public Function GetMessage(ByVal MessageID As Integer) As String

        If Me.Connected = False Then
            Throw New InvalidOperationException("Not connected!")
        End If
        Me.Send(Commands.GetMessage & MessageID)
        Me.CheckResponse(Me.GetResponse)
        Dim Response As String = Me.GetResponse
        Return Response

    End Function

End Class