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Introduction to Developing HTTP Modules in ASP.NET

Faisal Khan — Sat, 18 Jul 2009 09:00:00 GMT

Introduction
In ASP.NET, an HTTP Module is a class that implements the IHttpModule interface. By doing that it can listen to and handle events in the web request handling pipeline. They are the equivalent of 'ISAPI Filters' in IIS and 'Filters' in JSP. They are used for multiple purposes from authenticating a request to modifying the response and logging the request. In this tutorial, we will learn:

Common uses of HTTP Modules
Web Request Handling Pipeline
Structure of an HTTP Module
Creating your first HTTP Module

Common Uses of HTTP Modules
Following is a noncomprehensive list of common uses of HTTP Modules:

Displaying Emails from Mailbox on Gmail using POP3 and ASP.NET

Faisal Khan — Fri, 10 Jul 2009 05:52:00 GMT

Introduction
In this tutorial, we will create the missing ASP.NET page, that was left in the previous tutorial (Connecting to Mailbox on Gmail and Fetching List of Emails using POP3 and ASP.NET), that takes an individual email's sort number on the POP3 server to retrieve and display it to the user after properly decoding content-transfer-encodings like base64 and quoted-printable. You will learn how to access individual message parts in MIME encoded emails including fetching and enlisting attachments.

Note: It is mandatory that you read the previous tutorial (Connecting to Mailbox on Gmail and Fetching List of Emails using POP3 and ASP.NET) to download and place the essential Pop3.cs file in the /App_Code folder of your ASP.NET web application. Without this code, you will not be able to connect and retrieve emails using POP3.

In the previous tutorial, we limited ourselves to only listing the emails in our mailbox on Gmail. That in itself was quite a bit of a job since we had to develop all the baseline code that connects, sends commands, receives data, parses and instantiates objects. The classes that we created like Pop3Client, Email, and MessagePart made the task a lot more easier for us. Only few lines of code were needed to retrieve the list of emails in the mailbox and display it on the ASP.NET page. We will build on that base in this tutorial.

Following is the screen shot of the ASP.NET page that we developed in the previous tutorial:

Connecting to Mailbox on Gmail and Fetching List of Emails using POP3 and ASP.NET

Faisal Khan — Wed, 08 Jul 2009 10:13:00 GMT

Introduction
In this tutorial, we will learn about Post Office Protocol (POP3). It is a protocol (a set of commands) used by an email client to connect to and retrieve email(s) from the mailbox on the remote server. We will develop the code that connects to the POP3 server and sends commands to retrieve the list of emails on the server, and then later, using an ASP.NET page, displays the list to the user. To demonstrate the code, we will be connecting to Gmail's POP3 server using SSL (Secure Sockets Layer).

To connect to your own POP3 mail server, you can either use the secure connection (SSL) if your mail server supports it or connect in unsecure mode. The code is simple to understand and by looking at it you will learn a lot about how network programming is done in the .NET environment.

Note: In this tutorial, we will limit ourselves to retrieving and displaying a list of emails from user's mailbox which the user can browse through using 'Previous Page' and 'Next Page' links. In the next tutorial (Displaying Emails from Mailbox on Gmail using POP3 and ASP.NET), we will learn how to retrieve and view an individual email.

Following screen shot shows an ASP.NET page listing emails in my Gmail mailbox:

Basic POP3 Commands
Following is a list of commonly used POP3 commands:

Displaying Twitter Updates in a Page by Page fashion with ASP.NET

Faisal Khan — Sat, 20 Jun 2009 06:49:00 GMT

Introduction
In this tutorial, we will develop a technique to retrieve and display a page full of Twitter updates to the user, giving him the ability to move to the next page. The user will be able to browse all the pages using 'Previous Page' and 'Next Page' links. Our technique will be highly efficient as we will be retrieving only enough number of Twitter updates from a given screen name, to display on a single page. Only when the user browses to the next page will another set of Twitter updates be retrieved. At the same time we will cache the results, page by page, giving seamless and smooth performance to the end user.

The code in this tutorial makes use of the code we developed in the previous tutorial: Retrieving Twitter Updates with ASP.NET. We will only be updating TwitterClient.cs a little bit to enable retrieval of Twitter updates page by page. Rest of the files will remain the same. We will develop an ASP.NET page to demonstrate the technique.

I advise that you read this series of tutorials from the beginning and create/place the files as advised in the previous tutorial.

This tutorial is 3rd in the series of tutorials regarding Twitter and ASP.NET:

Introduction to Twitter for ASP.NET Developers
Retrieving Twitter Updates with ASP.NET
Displaying Twitter Updates in a Page by Page fashion with ASP.NET

Demonstration of Twitter Updates being Displayed Page by Page
In this tutorial, we will develop an ASP.NET page that retrieves 5 status updates for a given screen name and displays them on a page. When 'Next Page' link is clicked, next 5 status updates are retrieved and displayed to the user.

Retrieving Twitter Updates with ASP.NET

Faisal Khan — Wed, 17 Jun 2009 07:33:00 GMT

Introduction
In this tutorial, we will learn how to retrieve Twitter status updates using Twitter API and ASP.NET. We will develop code that encapsulates calls to Twitter API and provides easy to use .NET classes to retrieve Twitter status updates, programmatically. We will demonstrate the code by developing an ASP.NET control that uses these .NET classes to retrieve and display Twitter status updates on our website.

This tutorial is 2nd in the series of tutorials regarding Twitter and ASP.NET:

Introduction to Twitter for ASP.NET Developers
Retrieving Twitter Updates with ASP.NET
Displaying Twitter Updates in a Page by Page fashion with ASP.NET

Things to know about Twitter API
Before getting started, it is prudent to know a few things about Twitter API.

Twitter API is entirely HTTP based. This means that all you need to do to retrieve or post status updates is to make simple HTTP GET and POST calls, respectively, over the internet.

Twitter API conforms to REST (Representational State Transfer) style of software architecture. You can use URL like http://twitter.com/statuses/user_timeline.format to retrieve status updates of a user. You can choose the desired format of the response by specifying the format in the URL. For instance, to retrieve the status updates in XML, the complete URL will be http://twitter.com/statuses/user_timeline.xml. You have the option of choosing the format between XML, JSON, RSS and Atom.

Introduction to Twitter for ASP.NET Developers

Faisal Khan — Sat, 13 Jun 2009 12:49:00 GMT

Introduction
Twitter is a free social networking service that lets its users communicate with each other through short text messages. There is a limit of 140 characters per message. Users can send messages using online website, mobile devices or by sending SMS from their cell phones. These messages, known as &apostweets' are then delivered, by a variety of means, to the users who have subscribed to receive them; known as 'followers'.

Guidelines for sending Messages
In text messages, username of the person you are replying to should start with &apos@' (at) sign e.g., @Stardeveloper. Twitter will convert these usernames in the text messages to clickable links e.g., "Hello @Stardeveloper!".

This tutorial is 1st in the series of tutorials regarding Twitter and ASP.NET:

Introduction to Twitter for ASP.NET Developers
Retrieving Twitter Updates with ASP.NET
Displaying Twitter Updates in a Page by Page fashion with ASP.NET

Connecting with your Audience
Twitter provides another opportunity for ASP.NET developers to connect with their website audience. ASP.NET developers can create accounts with screen names of their websites e.g., Stardeveloper (Faisal Khan). This account can then be used as a micro-blog for the website. Not only should the status updates include important website updates, but should also focus on the interesting things you, as a developer, are working on or can benefit your audience. A link is placed on the website linking to the Twitter profile, giving users the opportunity to view the profile, and if he/she chooses so, to start following this account.

Validating an XML Document using DTD in ASP.NET

Faisal Khan — Fri, 05 Jun 2009 03:36:00 GMT

Introduction
In this tutorial, we will learn how to validate an XML document using DTD. We will develop an ASP.NET page to demonstrate the code. After you are finished reading this tutorial, you will be proficient in writing C# code that validates XML documents using DTD.

Following topics will be covered in this tutorial:

Well-formedness vs Validation.
Validating XML Document using DTD.

This tutorial is 6th one in the series of tutorials about XML and ASP.NET:

Introduction to XML
Reading XML Files with ASP.NET
Introduction to XPath for ASP.NET Developers
Creating new XML Document in ASP.NET Programmatically
Introduction to DTD (Document Type Definition)
Validating an XML Document using DTD in ASP.NET

We will first refresh ourselves with basic XML concepts of well-formedness and validation, and later develop the code to validate XML document using DTD.

Introduction to DTD (Document Type Definition)

Faisal Khan — Tue, 26 May 2009 05:06:00 GMT

Introduction
In this tutorial, we will learn about "Document Type Definition". DTD is an XML schema language that is used to describe the structure of an XML document. It describes what elements an XML document can contain and in which order. It further specifies how these elements are arranged within one another and what attributes these elements can contain. The description of the structure and these constraints are written in a formal syntax, some of which we will learn in this tutorial. These declarations can be placed within an XML file, or more commonly in a separate file with ".dtd" extension.

Following topics will be covered in this tutorial:

Declaring elements in DTD.
Declaring attributes in DTD.
Associating DTD with XML Documents.

This tutorial is 5th one in the series of tutorials about XML and ASP.NET:

Introduction to XML
Reading XML Files with ASP.NET
Introduction to XPath for ASP.NET Developers
Creating new XML Document in ASP.NET Programmatically
Introduction to DTD (Document Type Definition)
Validating an XML Document using DTD in ASP.NET

We will now learn how elements and attributes are declared in DTD, and how DTD declarations can be associated with XML document(s).

Creating new XML Document in ASP.NET Programmatically

Faisal Khan — Thu, 19 Mar 2009 11:35:00 GMT

Introduction
In this tutorial, we will learn how to create an XML Document programmatically. We will make use of XmlDocument class from System.Xml namespace. Following topics will be covered in this tutorial:

How to create XML declaration and element nodes.
How to create attributes and append them to element nodes.
How to create CDATA sections within element nodes.
How to nest elements and attributes in one another to create a logical tree like structure.
How to send the dynamically generated XML document to the client browser with proper headers.

This tutorial is 4th one in the series of tutorials about XML and ASP.NET:

Introduction to XML
Reading XML Files with ASP.NET
Introduction to XPath for ASP.NET Developers
Creating new XML Document in ASP.NET Programmatically
Introduction to DTD (Document Type Definition)
Validating an XML Document using DTD in ASP.NET

Creating new XML Document
We will now write down the code to create new XML document.

Code for ASP.NET page
Copy and paste following code into a new text file and then save it as "CreateXmlDocument.aspx" in your ASP.NET web application:

Introduction to XPath for ASP.NET Developers

Faisal Khan — Tue, 10 Mar 2009 05:26:00 GMT

Introduction
XPath is a language for selecting nodes (parts or segments) within an XML document. As you are already familiar with XML, XML is a markup language which uses elements and attributes to encapsulate data in a logical manner. XPath furthers builds on it and provides navigational abilities in an XML document. For example, you can use an XPath query to select one or more nodes within an XML document that match a certain criteria. That criteria can by anything from an element matching a given name to an element whose attribute matches a specific value.

This tutorial is 3rd one in the series of tutorials about XML and ASP.NET:

Introduction to XML
Reading XML Files with ASP.NET
Introduction to XPath for ASP.NET Developers
Creating new XML Document in ASP.NET Programmatically
Introduction to DTD (Document Type Definition)
Validating an XML Document using DTD in ASP.NET

Tree like representation of an XML Document
In XPath, an XML document is represented as a tree of nodes. There is a parent node with one or more child nodes. While a node in XPath can be of 7 types; for practical purposes, a node in XPath corresponds to an element or attribute within an XML document. For example, have a look at following XML document: