remove html node from htmldocument :HTMLAgilityPack

It seems you're modifying the collection during the enumeration by using HtmlNode.RemoveChild method.

To fix this you need is to copy your nodes to a separate list/array by calling e.g. Enumerable.ToList<T>() or Enumerable.ToArray<T>().

var nodesToRemove = doc.DocumentNode
    .SelectNodes("//img[not(string-length(normalize-space(@src)))]")
    .ToList();

foreach (var node in nodesToRemove)
    node.Remove();

If I'm right, the problem will disappear.

What I have done is:

    List<string> xpaths = new List<string>();
    foreach (HtmlNode node in doc.DocumentNode.DescendantNodes())
    {
                        if (node.Name.ToLower() == "img")
                        {
                            string src = node.Attributes["src"].Value;
                            if (string.IsNullOrEmpty(src))
                            {
                                xpaths.Add(node.XPath);
                                continue;
                            }
                        }
    }

    foreach (string xpath in xpaths)
    {
            doc.DocumentNode.SelectSingleNode(xpath).Remove();
    }