NullReferenceException ERROR in foreach loop using HTMLNode

Question

NullReferenceException ERROR in foreach loop using HTMLNode

c# foreach html-agility-pack nullreferenceexception

Question

How do I catch the NullReferenceException Error in the foreach loop below if 'SelectNodes' returns NULL?

I searched on stackoverflow and found mention of the null-coalescing condition (?? condition) that can be used to catch this error, however, I have no idea on what the syntax would be for HTMLNode, or if that's even possible.

foreach (HtmlNode link in imagegallery.DocumentNode.SelectNodes("//a[@href]") )
            {
                //Do Something
            }

How would you cath the NULL EXCEPTION for this loop, or is there a better way of doing this?

Here is the full code throwing the exception -

    private void TEST_button1_Click(object sender, EventArgs e)
    {
        //Declarations           
        HtmlWeb htmlWeb = new HtmlWeb();
        HtmlAgilityPack.HtmlDocument imagegallery;

            imagegallery = htmlWeb.Load(@"http://adamscreation.blogspot.com/search?updated-max=2007-06-27T10:03:00-07:00&amp;max-results=20&amp;start=18&amp;by-date=false");

            foreach (HtmlNode link in imagegallery.DocumentNode.SelectNodes("//a[@imageanchor=1 or contains(@href,'1600')]/@href"))
            {
               //do something
            }
    }

Chris L

Popular Answer

I was doing this quite a few times so made Andras' solution into an extension method:

using HtmlAgilityPack;

namespace MyExtensions {
    public static class HtmlNodeExtensions {
        /// <summary>
        ///     Selects a list of nodes matching the HtmlAgilityPack.HtmlNode.XPath expression.
        /// </summary>
        /// <param name="htmlNode">HtmlNode class to extend.</param>
        /// <param name="xpath">The XPath expression.</param>
        /// <returns>An <see cref="HtmlNodeCollection"/> containing a collection of nodes matching the <see cref="XPath"/> expression.</returns>
        public static HtmlNodeCollection SelectNodesSafe(this HtmlNode htmlNode, string xpath) {
            // Select nodes if they exist.
            HtmlNodeCollection nodes = htmlNode.SelectNodes(xpath);

            // I no matching nodes exist, return empty collection.
            if (nodes == null) {
                return new HtmlNodeCollection(HtmlNode.CreateNode(""));
            }

            // Otherwise, return matched nodes.
            return nodes;
        }
    }
}

Usage:

using MyExtensions;

foreach (HtmlNode link in imagegallery.DocumentNode.SelectNodesSafe("//a[@href]")) {
    //Do Something
}

Akaoni

View more on Stack Overflow

Accepted Answer

if(imagegallery != null && imagegallery.DocumentNode != null){
  foreach (HtmlNode link in 
    imagegallery.DocumentNode.SelectNodes("//a[@href]") 
      ?? Enumerable.Empty<HtmlNode>()) 
  {
    //do something
  }
}

NullReferenceException ERROR in foreach loop using HTMLNode

Question

Accepted Answer

Popular Answer

Performance

Expression Evaluator