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Library tutorials & articles

How to POP3 in C#

Introduction

This is the second in a series of articles on Internet programming with Microsoft's new C# programming language. In

the first article, I wrote a simple SMTP class. In this article, I'm going to write a simple POP3 class. The SMTP class

that I wrote was not very useful, except maybe as an exercise, as there already exists a similar SMTP class in theWeb.Mail namespace of the .NET framework called SmtpMail. Our POP3 class in this article will be a little more

useful as it doesn't already exist in the .NET framework. I have encountered many POP3 C# classes in my searches

of the Internet and most were sufficient to begin programming email clients.

Getting Started

I usually begin writing new classes by introducing an exception class that I can use to throw and catch all exceptions

of the class.

I will not explain the exception class, but rather I expect the reader have enough expertise with C# to understand this

exception class before reading the rest of the article.

Next I created a small class that defines a POP3 message.

When you retrieve lists of POP3 messages from a POP3 server, the list includes a message number and number of

bytes. You can then use the message number to retrieve the message content. You'll see this later when we define

our List and Retrieve methods. We derive our Pop3 class from the System. Net.Sockets.TcpClient class in

the .NET framework.

The TcpClient class and the other classes in the System.Net.Sockets namespace of the .NET framework are

great encapsulations of the familiar function-oriented socket library.

Connecting and Disconnecting

The first method of our Pop3 class is the Connect method. This method takes a server name, username and

password parameter to connect to a remote (sometimes local) POP3 server.

We begin by calling the TcpClient.Connect method passing the server name and the 110 port. The 110 port

number is the well known port number for POP3 operations. What that means is that POP3 servers by default should

listen for connections on port 110. When the POP3 server connects to a client, it should immediately respond with the

+OK acknowledgement message. Next we send two messages, USER and PASS, back to the server. The POP3 server

should acknowledge a successful login by acknowledging both messages. If the POP3 server returns anything but

public class Pop3Exception : System. ApplicationException{ public Pop3Exception( string str) : base( str) { }}

public class Pop3Message{ public long number; public long bytes; public bool retrieved; public string message;}

public class Pop3 : System.Net.Sockets.TcpClient {

public void Connect(string server, string username, string password){ string message; string response;

Connect(server, 110); response = Response(); if (response.Substring(0, 3) != "+OK") { throw new Pop3Exception(response); }

message = "USER " + username + "\r\n"; Write(message); response = Response(); if (response.Substring(0, 3) != "+OK") { throw new Pop3Exception(response);

}

message = "PASS " + password + "\r\n"; Write(message); response = Response(); if (response.Substring(0, 3) != "+OK") { throw new Pop3Exception(response); }}

By Randy Charles Morin, published on 01 Oct 2003 Comments (5) PDF

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+OK, then the message will contain the reason for the failure. In the advent of a failure, I attach that failure message to

our exception class and throw it back to the client. It should be noted that some POP3 servers don't require

authentication and may reject the calls to USER and PASS. I haven't encountered such a POP3 server, but the protocol

allows it. In those cases, you'll have to slightly modify the class to make things work.

Any use of our Pop3 class should begin with a call to Connect and end with a class to Disconnect .

The Disconnect method sends a QUIT message to the POP3 server.

Retrieving Messages

Between the calls to Connect and Disconnect , the client may call three other methods, List, Retrieve and

Delete, any number of times. The client will usually begin by calling our List method to retrieve an array of

messages that are queued on the POP3 server.

After sending the LIST message to the POP3 server, the server will respond with a +OK acknowledgement, followed

by several lines representing one message each and finally by a line with a single period indicating the end of the

messages. Each message line has two numbers, the first indicating the unique number of the message and the

second indicating the message size in bytes.

Our List method will return a list of Pop3Message objects. The objects will only contain the message number and

size of each message. In order to retrieve the full message, you can pass the message object to the Retrieve

method. The Retrieve method will then respond with another Pop3Message containing the message content.

To retrieve a message from a POP3 server, we send a RETR message with the unique message number. The server

public void Disconnect(){ string message; string response; message = "QUIT\r\n"; Write(message); response = Response(); if (response.Substring(0, 3) != "+OK") { throw new Pop3Exception(response); }}

public ArrayList List(){ string message; string response;

ArrayList retval = new ArrayList(); message = "LIST\r\n"; Write(message); response = Response(); if (response.Substring(0, 3) != "+OK") { throw new Pop3Exception(response); }

while (true) { response = Response(); if (response == ".\r\n") { return retval; } else { Pop3Message msg = new Pop3Message(); char[] seps = { ' ' }; string[] values = response.Split(seps); msg.number = Int32.Parse(values[0]); msg.bytes = Int32.Parse(values[1]); msg.retrieved = false; retval.Add(msg); continue; } }}

public Pop3Message Retrieve(Pop3Message rhs){ string message; string response;

Pop3Message msg = new Pop3Message(); msg.bytes = rhs.bytes; msg.number = rhs.number;

message = "RETR " + rhs.number + "\r\n"; Write(message); response = Response(); if (response.Substring(0, 3) != "+OK") { throw new Pop3Exception(response); }

msg.retrieved = true;

while (true) { response = Response(); if (response == ".\r\n") { break; } else { msg.message += response; } }

return msg;}

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then responds with the +OK acknowledgement, the message content and finally the single period terminating line.

Removing Messages

Retrieving a message does not remove the message from the POP3 server. A further call to LIST will still return the

message. To remove a message from POP3 server, you have to call the Delete method.

The Delete method sends a DELE message with the message number to the POP3 server. The server will respond

with the +OK acknowledgment message, if successful.

Supporting Functions

The List, Retrieve and Delete methods used two private methods, Write and Response, to send and receive

messages from the POP3 server.

C# native strings, like Java native strings, are UNICODE. We therefore need to encode and decode the s trings to and

from ASCII. After encoding the string, we can then retrieve the socket stream by calling the TcpClient.GetStream

method. I finish the Write method by called the Debug.Writeline method. This sends the string to the debug

stream for help with debugging.

The Response method is similar to the Writemethod accept that we retrieve bytes from the stream before decoding

them. Again we call the Debug.WriteLine method to send the read data to the debug stream and help with

debugging. Note that we have a limitation in the Response method. We can only retrieve or send up to 1024 bytes at a

time. I'll fix this in a later release. If you intend to use this in production, then you'll have to do the same first.

Using the Class

Using the new class is pretty easy.

public void Delete(Pop3Message rhs){ string message; string response;

message = "DELE " + rhs.number + "\r\n"; Write(message); response = Response(); if (response.Substring(0, 3) != "+OK") { throw new Pop3Exception(response); }

}

private void Write(string message){ System.Text.ASCIIEncoding en = new System.Text.ASCIIEncoding() ;

byte[] WriteBuffer = new byte[1024] ; WriteBuffer = en.GetBytes(message) ;

NetworkStream stream = GetStream() ; stream.Write(WriteBuffer, 0, WriteBuffer.Length);

Debug.WriteLine("WRITE:" + message);}

private string Response(){ System.Text.ASCIIEncoding enc = new System.Text.ASCIIEncoding(); byte[] serverbuff = new Byte[1024]; NetworkStream stream = GetStream(); int count = 0; while (true) { byte[] buff = new Byte[2]; int bytes = stream.Read(buff, 0, 1 ); if (bytes == 1) { serverbuff[count] = buff[0]; count++;

if (buff[0] == '\n') {

break; } } else { break; }; };

string retval = enc.GetString(serverbuff, 0, count ); Debug.WriteLine("READ:" + retval); return retval;}

static void Main(string[] args){ try { Pop3 obj = new Pop3(); obj.Connect("mail.xxx.com", "yyy", "zzz"); ArrayList list = obj.List(); foreach (Pop3Message msg in list ) { Pop3Message msg2 = obj.Retrieve(msg); System.Console.WriteLine("Message {0}: {1}", msg2.number, msg2.message); } obj.Disconnect(); } catch (Pop3Exception e ) {

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Related tags

c++, email, pop3, sockets, tcp ip

Instantiate a new object, then call the List method. The List method will return an array of Pop3Message objects.

You can then iterate through the Pop3Message objects and retrieve each in turn. Finally, you call the Disconnect

method to release the socket. POP3 is described in RFC 1939. You can read the full specification from the IETF

website.

System.Console.WriteLine(e.ToString()); } catch (System.Exception e) { System.Console.WriteLine(e.ToString()); }}

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