Screen scraping is the process of programmatically accessing and processing information from an external website. For example, a price comparison website might screen scrape a variety of online retailers to build a database of products and what various retailers are selling them for. Typically, screen scraping is performed by mimicking the behavior of a browser - namely, by making an HTTP request from code and then parsing and analyzing the returned HTML.
The .NET Framework offers a variety of classes for accessing data from a remote website, namely the
WebClient class and the
HttpWebRequest class. These classes are useful for making an HTTP request to a
remote website and pulling down the markup from a particular URL, but they offer no assistance in parsing the returned HTML. Instead,
developers commonly rely on string parsing methods like
String.Substring, and the like, or
through the use of regular expressions.
Another option for parsing HTML documents is to use the HTML Agility Pack, a free, open-source library designed to
simplify reading from and writing to HTML documents. The HTML Agility Pack constructs a Document Object Model
(DOM) view of the HTML document being parsed. With a few lines of code, developers can walk through the DOM, moving from a
node to its children, or vice versa. Also, the HTML Agility Pack can return specific nodes in the DOM through the use
XPath expressions. (The HTML Agility Pack also includes a class for downloading an HTML document from a remote
website; this means you can both download and parse an external web page using the HTML Agility Pack.)