Tutorial 07: Using TCP/IP to Check Email September 20, 2009 © NSB Corporation. All rights reserved. Contributed by Tom Newman, Mission Peak Designs

Contents:

1. Introduction
2. How is it Done?
3. Creating the Form
4. Adding the Code
5. Testing the Program

Introduction

The purpose of this tutorial is to demonstrate adding networking functionality to your NS Basic/CE programs. You should have completed Tutorial #1 before beginning this tutorial.

The program to be developed uses the NewObjects ActiveX Net Stream control to open a socket and connect with an email server. Then using the NewObjects Storage and File control to send and received data from the server.

How is it Done?

Including network support in your application starts as simply as adding NewObjects ActiveX control as a resource to your program. Select Project > Add Resource, then select newObjectsPack1.dll.

(For a complete reference on the methods above, make sure you read the Tech Note 06: Communications - TCP/IP and IRDA and Tech Note 08: File I/O Control - SFStream)

Working with TCP/IP protocols is similar to reading/writing files from the file system. There are basically three steps used to talk to a server:

1. First you have to open it (socket),
2. Then you send commands and read data to/from it,
3. and finally you close it (socket).

Because you are dealing with a remote system and not a local file, you have a "timeout" feature. It simply means that if your application does not hear from the remote machine for a specified period of time it will give up.

You must also specify to which service you want to connect in that remote machine, and that's the concept behind "port". Different ports provides different services, for instance,

• Webserver usually listens on port 80,
• Mail server accepts messages for delivery on port 25 (SMTP service), and
• Email client connects on the server's port 110 to retrieve your mail using the POP3 protocol.

Creating the Form

Start a new "Standard" project and save it as HttpPop3.nsb

• Create a TextBox and call it "txtMsg". Change Multiline to True. Change Scrollbars to 3 (both vertical and horizontal). Delete the field after the Text property.
• Create a CommandButton and call it "cmdFetch". Change the Caption to "Fetch".

The form should look like the following:

Adding the Code

In this project will simply connect to a pop3 server, send username and password, and display the result to the STAT command (it should tell how many messages you have).

Enter the following code in the Code Window (you can copy and paste this code):

Option Explicit ShowOKButton True 'for CE Dim ConnectCount AddObject "newObjects.net.NSMain", "NSMain" AddObject "newObjects.utilctls.SFStream","conn" Sub cmdFetch_Click Dim Host, addr, socket, s Host = "pop.mail.yahoo.com" Set addr = nsMain.GetHost(Host) txtMsg.Text = "Connecting to: " & Host & vbCrLf addr.Port = 110 ' POP3 SSL port (normal 110) Set socket = nsMain.NewSocket If Not socket.Socket Then txtMsg.Text= "Socket Error: " & socket.lastError & vbCrLf & txtMsg.Text Exit Sub End If If Not socket.Connect(addr) Then txtMsg.Text= "Connect Error: " & socket.lastError & vbCrLf & txtMsg.Text Exit Sub End If conn.SetStream socket s = conn.ReadText(-1) txtMsg.text = vbCrLf & "Resp: " & s & vbCrLf & txtMsg.text txtMsg.text = "Sending user name ..." & vbCrLf & txtMsg.text conn.WriteText "user myname" & vbCrLf s = conn.ReadText(-1) txtMsg.text = vbCrLf & "Resp: " & s & vbCrLf & txtMsg.text txtMsg.text = "Sending password ..." & vbCrLf & txtMsg.text conn.WriteText "pass abcxyz" & vbCrLf s = conn.ReadText(-1) txtMsg.text = vbCrLf & "Resp: " & s & vbCrLf & txtMsg.text txtMsg.text = "Sending STAT ..." & vbCrLf & txtMsg.text conn.WriteText "stat" & vbCrLf s = conn.ReadText(-1) txtMsg.text = vbCrLf & "Resp: " & s & vbCrLf & txtMsg.text txtMsg.Text= "Done." & vbCrLf & txtMsg.Text socket.Close End Sub

Now let's analyze the code, line by line:

• Lines 4 and 5 define the Net Stream object (NSMain) and the Storage and File object (SFStream).
• Lines 10 and 11 define our incoming email server address (URL) and gets the IP address of the server (calling GetHost).
• On line 15 we open the connection to "server" on port 110.
• Line 27 initializes the Socket connection for reading and writing.
• Line 29 waits for a response from the server (read a line of data)
• On line 33 we are sending the user name to the server and waiting for a response on line 35. There must be a space between "user" and the name. For Yahoo, if the user is abc© yahoo.com, the user would be "abc".
• On line 39 we are sending the password to the server and waiting for a response on line 41. There must be a space between "pass" and the password.
• On line 45 we are sending the STAT command to the server and waiting for a response on line 48.
• The Socket is closed on line 51.

This is what is called a chat-style connection. You send something and wait for a response. That's how the POP3 server works:

• you connect and it greets you
• you send your username, and it says either "ok" or that something is wrong
• you send your password and it will say if you are logged in, or if your password is not correct.

You can tell if the response is a success or an error on POP3 protocol by looking at the first character of the response line: a plus means it was accepted, a minus means there was a problem.

Of course this is a very simple example to get you started. A real application would have functions to get lines and to send lines, and also check if the password was accepted, parse the STAT result lines to display only the number of messages and not the entire ugly line, and even retrieve the header of each message and display the sender, subject and date of all the emails, and much more.

Testing the Program

Press F5 to save and start the program. Your results should look similar to the image below if you entered the proper email server address and a valid user name and password. The response from your email server may look a little different but you should see "+OK" as a response for every command sent. The message text box shows the program's progress and the responses. The "+OK 3675" in the last response is the number of emails on the server (I know, it's on my to-do list).