Get all attribute values of given tag with Html Agility Pack

Answer 1

try

var nodes = sourceDocument.DocumentNode.SelectNodes("//span[@id]");
List<string> ids = new List<string>(nodes.Count);

if(nodes != null)
{
    foreach(var node in nodes)
    {
        if(node.Id != null)
        ids.Add(node.Id);
    }
}

return ids;