Is Html Agility Pack thread-safe?

The Html Agility Pack (HAP) is a .NET library used to parse and manipulate HTML documents. It is particularly useful for web scraping as it allows you to navigate the document tree and select nodes using XPath queries.

Regarding thread safety, the Html Agility Pack itself is not inherently thread-safe. This is common with many object-oriented libraries, as thread safety can impose performance penalties and is not always required by the consumers of the library.

When working with HAP in a multi-threaded environment, you have to ensure that you're not modifying the same HtmlDocument or HtmlNode objects from multiple threads simultaneously. If you need to process multiple documents in parallel, the best practice is to create separate instances of HtmlDocument for each thread or task.

Here's a simple example of how you might use the Task Parallel Library (TPL) in C# to parse multiple HTML documents in parallel, each with its own instance of HtmlDocument to ensure thread safety:

using HtmlAgilityPack;
using System.Collections.Generic;
using System.Threading.Tasks;

public class HtmlParser
{
    public void ParseMultipleDocuments(IEnumerable<string> htmlContents)
    {
        Parallel.ForEach(htmlContents, htmlContent =>
        {
            // Each thread gets its own HtmlDocument instance
            var htmlDoc = new HtmlDocument();
            htmlDoc.LoadHtml(htmlContent);

            // Perform your parsing operation here
            // Example: Getting all the paragraph tags
            var paragraphNodes = htmlDoc.DocumentNode.SelectNodes("//p");
            foreach (var pNode in paragraphNodes)
            {
                // Process pNode as needed
            }
        });
    }
}

In the above code, each HTML content string in htmlContents is processed in parallel, and each is loaded into its own HtmlDocument instance, avoiding any concurrency issues.

If you need to access shared data structures during your processing (for example, if you're aggregating results from multiple documents), you will need to synchronize access to those shared structures using locks or other concurrency mechanisms provided by .NET, such as ConcurrentBag, ConcurrentDictionary, or using lock statements where appropriate.

Remember, the thread safety of your code when using HAP is your responsibility as a developer. Always consider the implications of concurrent modifications and access to shared data in a multi-threaded context.