What are the main features of Html Agility Pack?

Html Agility Pack (HAP) is a .NET code library that is designed to provide an efficient way to parse HTML documents and manipulate them as required. It is particularly useful for web scraping because it can deal with malformed HTML that is commonly found on the web. Here are the main features of Html Agility Pack:

1. Robust Parsing: HAP can parse HTML that might not be well-formed or that is invalid according to W3C standards. This is critical for web scraping because real-world HTML often contains errors.

2. XPath and LINQ Support: It supports selecting nodes using XPath queries. This makes it easy to navigate through the document and select elements, attributes, and text. HAP also supports LINQ to XML, which allows for querying the document in a more .NET idiomatic way.

3. Document Manipulation: HAP allows you to easily create, modify, and remove nodes from the document. You can also manipulate attributes and merge documents.

4. HtmlDocument and HtmlNode Classes: The library provides a HtmlDocument class that represents the parsed HTML document and HtmlNode class for each element in the HTML document. This is similar to the DOM (Document Object Model) in a web browser.

5. Encoding Support: HAP automatically detects and handles character encodings, which is essential for correctly scraping and displaying text from various web pages.

6. Flexibility and Extensibility: The library is highly customizable. For instance, you can extend it with custom features by inheriting from the provided classes.

7. Support for HTML5: While originally created in the days when HTML4 was prevalent, the library has evolved to handle HTML5 tags and attributes.

8. Performance: The library is optimized for performance, making it a good choice for scraping tasks that need to process large volumes of HTML data.

9. Cross-Platform: With the advent of .NET Core, HAP can be used in a cross-platform environment, allowing it to run on Windows, Linux, and macOS.

Here's a basic example of how to use Html Agility Pack in a C# .NET project to load an HTML document and select nodes using XPath:

// You will need to add the HtmlAgilityPack via NuGet to your project to use it.
using HtmlAgilityPack;

// Load an HTML document from a file, a web address, or a string.
var htmlDoc = new HtmlDocument();
htmlDoc.Load("path_to_html_file.html");

// Alternatively, load the document from the web
var web = new HtmlWeb();
htmlDoc = web.Load("http://example.com");

// Use XPath to select specific nodes.
var nodes = htmlDoc.DocumentNode.SelectNodes("//a[@href]");

// Iterate over the selected nodes.
foreach (var node in nodes)
{
    // Do something with the node, like retrieving the href attribute.
    string hrefValue = node.Attributes["href"].Value;
    Console.WriteLine(hrefValue);
}

Html Agility Pack is available as a NuGet package and can be easily added to a .NET project using the NuGet Package Manager or the Package Manager Console:

Install-Package HtmlAgilityPack

Or using the .NET CLI:

dotnet add package HtmlAgilityPack

Remember that Html Agility Pack is specifically for .NET applications. For web scraping in other languages like Python or JavaScript, different libraries are used, such as Beautiful Soup or Cheerio, respectively.