Develop Reference

Checking multiple XML files

I am re-wording this from my original post:
I have two XML files, and they are related to a given year each. For example, 18/19 and 17/18. They conform to the same structure and below is small sample from one of these files. What I want is, in C#, to compare all records in these files where the Given Name, the Family Name, the NI Number and the Date of birth are the same, BUT the Learner Ref Number is different. I need to be able to compare, then push only these records into a data table so I can then push them into a spreadsheet (the spreadsheet bit I can do). I currently have the below as a starting block, but am still very much stuck.
Firstly, I have my Import button press for which:
private void Btn_Import_Click(object sender, RoutedEventArgs e)
{
ILRChecks.ILRReport.CrossYear();}
Then this goes to look at the Class of which eventually pushes the file to my location:
using System.Data;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using ILRValidation;
using InfExcelExtension;
namespace ILRChecks
{
internal static partial class ILRReport
{
internal static void CrossYear()
{
DataSet ds_CrossYearChecks =
ILRValidation.Validation.CrossYearChecks(Global.fileNames);
string output = Path.Combine(Global.foldername, "ULIN_Issues" +
".xlsx");
ds_CrossYearChecks.ToWorkBook(output);
}
}
}
And this is the bit I'm stuck on, which is the production of finding the differences:
using System;
using System.Collections.Generic;
using System.Data;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace ILRValidation
{
public static partial class Validation
{
public static DataSet CrossYearChecks(DataSet ds_CrossYearChecks)
{
return CrossYearChecks(ds_CrossYearChecks);
}
public static DataSet CrossYearChecks(string[] xmlPath)
{
DataSet ds_xmlCrossYear = new DataSet();
return CrossYearChecks(ds_xmlCrossYear);
}
}
}
XML:
<Learner>
<LearnRefNumber></LearnRefNumber>
<ULN></ULN>
<FamilyName></FamilyName>
<GivenNames></GivenNames>
<DateOfBirth></DateOfBirth>
<Ethnicity></Ethnicity>
<Sex></Sex>
<LLDDHealthProb></LLDDHealthProb>
<NINumber></NINumber>
<PriorAttain></PriorAttain>
<MathGrade></MathGrade>
<EngGrade></EngGrade>
<PostcodePrior></PostcodePrior>
<Postcode></Postcode>
<AddLine1></AddLine1>
<AddLine3></AddLine3>
<Email></Email>

Well, you can traverse both XML files recursively and write down all the encountered changes. Something like should be helpful:
static string AppendPrefix(string oldPrefix, string addition) =>
oldPrefix == "" ? addition : $"{oldPrefix}.{addition}";
static void CompareElements(string prefix, XElement d1, XElement d2)
{
// 1. compare names
var newPrefix = AppendPrefix(prefix, d1.Name.ToString());
if (d1.Name != d2.Name)
{
Console.WriteLine(
$"Name mismatch: {newPrefix} != {AppendPrefix(prefix, d2.Name.ToString())}");
return;
}
// 2. compare attributes
var attrs = d1.Attributes().OrderBy(a => a.Name);
var unpairedAttributes = new HashSet<XAttribute>(d2.Attributes());
foreach (var attr in attrs)
{
var otherAttr = d2.Attributes(attr.Name).SingleOrDefault();
if (otherAttr == null)
{
Console.WriteLine($"No new attr: {newPrefix}/{attr.Name}");
continue;
}
unpairedAttributes.Remove(otherAttr);
if (attr.Value != otherAttr.Value)
Console.WriteLine(
$"Attr value mismatch: {newPrefix}/{attr.Name}: {attr.Value} != {otherAttr.Value}");
}
foreach (var attr in unpairedAttributes)
Console.WriteLine($"No old attr: {newPrefix}/{attr.Name}");
// 3. compare subelements
var leftNodes = d1.Nodes().ToList();
var rightNodes = d2.Nodes().ToList();
var smallerCount = Math.Min(leftNodes.Count, rightNodes.Count);
for (int i = 0; i < smallerCount; i++)
CompareNodes(newPrefix, i, leftNodes[i], rightNodes[i]);
if (leftNodes.Count > smallerCount)
Console.WriteLine($"Extra {leftNodes.Count - smallerCount} nodes at old file");
if (rightNodes.Count > smallerCount)
Console.WriteLine($"Extra {rightNodes.Count - smallerCount} nodes at new file");
}
static void CompareNodes(string prefix, int index, XNode n1, XNode n2)
{
if (n1.NodeType != n2.NodeType)
{
Console.WriteLine($"Node type mismatch: {prefix}/[{index}]");
return;
}
switch (n1.NodeType)
{
case XmlNodeType.Element:
CompareElements(prefix, (XElement)n1, (XElement)n2);
break;
case XmlNodeType.Text:
CompareText(prefix, index, (XText)n1, (XText)n2);
break;
}
}
static void CompareText(string prefix, int index, XText t1, XText t2)
{
if (t1.Value != t2.Value)
Console.WriteLine($"Text mismatch at {prefix}[{index}]");
}
Usage:
XDocument d1 = <get document #1 from somewhere>,
d2 = <get document #2 from somewhere>;
CompareNodes("", 0, d1.Root, d2.Root);
Obviously, instead of writing to console you should write to the appropriate spreadsheet.
Note that I'm ignoring the attribute reorder but not subnode reorder (which seems to be right).

Seems to me you're having trouble extracting the values you want from the xml, correct?
As the others have mentioned in the comments, without knowing the layout of your xml its impossible to give a specific example for your case. If you edit your question to include an example of your xml, we can help more.
Here are some general examples of how to extract values from xml:
private static bool CheckXmlDocument(string xmlPathCheck)
{
// if you have multiple files from which you need to extract values, pass in an array or List<string> and loop over it, fetching the values
// XmlDocument will allow you to edit the document as well as read it
// there's another option to use XPathDocument and XPathNavigator but it's read-only
var doc = new XmlDocument();
// this can throw various exceptions so might want to add some handling
doc.Load(xmlPathCheck);
// getting the elements, you have some options depending on the layout of the document
// if the nodes you want are identified by 'id' use this:
var nameElement = doc.GetElementById("name");
// if the nodes you want are identified by 'tag', use this:
var nameNodeList = doc.GetElementsByTagName("name");
// if you know the xpath to the specific node you want, use this:
var selectNameNode = doc.SelectSingleNode("the/xpath/to/the/node");
// if there are several nodes that have the same xpaths, use this:
var selectNameList = doc.SelectNodes("the/xpath/that/may/match/many/nodes");
// getting the value depends on the if you have an XmlNode, XmlElement or XmlNodeList
// if you have a single XmlElement or XmlNode you can get the value one of these ways depending on the layout of your document:
var name = nameElement.InnerText;
name = nameElement.InnerXml;
// if you have an XmlNodeList, you'll have to iterate through the nodes to find the one you want, like this:
foreach (var node in nameNodeList)
{
// here use some condition that will determine if its the element/node you want or not (depends on your xml layout)
if (node is XmlNode n)
{
name = n.InnerText;
}
}
// do that for all the values you want to compare, then compare them
return CheckValues(/*the values to compare*/);
}
XmlDocument
XmlNode
XmlElement

Is there a better way to remove span elements but leave child nodes?

I want to remove all span elements (without attributes) but leave the inner html. I have created the following code snippet which appears to work but I can't help thinking this is overly complicated for such a task. Is there a better way?
var config = Configuration.Default.WithDefaultLoader().WithCss();
var parser = new HtmlParser(config);
var document = parser.Parse("<p><span><span><em>span text</em></span> </span> span text</p>");
foreach (var element in document.Descendents())
{
var parent = element.Parent;
while (parent != null)
{
var span = parent as IHtmlSpanElement;
if (span != null && !span.Attributes.Any())
{
span.Replace(span.ChildNodes.ToArray());
}
parent = parent.Parent;
}
}
document.Body.InnerHtml.Dump();
// outputs: <p><em>span text</em> span text</p>

What you want is a replacement. Luckily, such an API exists, which you already use (Replace). However, most of your boilerplate code can also be replaced with standard APIs (like QuerySelectorAll):
var config = Configuration.Default.WithDefaultLoader().WithCss();
var parser = new HtmlParser(config);
var document = parser.Parse("<p><span><span><em>span text</em></span> </span> span text</p>");
foreach (var element in document.QuerySelectorAll("span").Where(m => m.Attributes.Length == 0))
{
element.Replace(element.ChildNodes.ToArray());
}
document.Body.InnerHtml.Dump();
Note: I've only placed the Where to have the same condition as you placed in your code - namely no attribute should be found on these span elements.
Hope this helps!

Better way to add a style attribute to Html using HtmlAgilityPack

I am using the HtmlAgilityPack. I am searching through all P tags and adding a "margin-top: 0px" to the style within the P tag.
As you can see it is kinda "brute forcing" the margin-top attribute. It seems there has to be a better way to do this using the
HtmlAgilityPack but I could not find it, and the HtmlAgilityPack documentation is non-existent.
Anybody know a better way?
HtmlNodeCollection pTagNodes = node.SelectNodes("//p[not(contains(#style,'margin-top'))]");
if (pTagNodes != null && pTagNodes.Any())
{
foreach (HtmlNode pTagNode in pTagNodes)
{
if (pTagNode.Attributes.Contains("style"))
{
string styles = pTagNode.Attributes["style"].Value;
pTagNode.SetAttributeValue("style", styles + "; margin-top: 0px");
}
else
{
pTagNode.Attributes.Add("style", "margin-top: 0px");
}
}
}
UPDATE: I have modified the code based on Alex's suggestions. Would still like to know if there is a some built-in
functionality in HtmlAgilityPack that will handle the style attributes in a more "DOM" manner.
const string margin = "; margin-top: 0px";
HtmlNodeCollection pTagNodes = node.SelectNodes("//p[not(contains(#style,'margin-top'))]");
if (pTagNodes != null && pTagNodes.Any())
{
foreach (var pTagNode in pTagNodes)
{
string styles = pTagNode.GetAttributeValue("style", "");
pTagNode.SetAttributeValue("style", styles + margin);
}
}

You could simplify your code a little bit by using HtmlNode.GetAttributeValue method, and making your "margin-top" magic string as constant:
const string margin = "margin-top: 0";
foreach (var pTagNode in pTagNodes)
{
var styles = pTagNode.GetAttributeValue("style", null);
var separator = (styles == null ? null : "; ");
pTagNode.SetAttributeValue("style", styles + separator + margin);
}
Not a very significant improvement, but this code is simpler as for me.

First of all, are you sure you need more than what you asked for? Alex solution should just work fine for your current problem, if it's always that "simple" why bother and add more complexity to it?
Anway, the AgilityPack doesn't have that kind of function, but surely the .Net Framework has. Note this is all for .Net 4, if you're using an earlier version things might be a bit different.
First of, System.Web.dll comes with the CssStyleCollection Class, this class already has everything build in that you could want for parsing inline css, there's just one catch, it's constructor is internal so the solution is a bit "hacky".
First off, for construction an instance of the class all you need is a bit of reflection, the code for that has already been done here. Just keep in mind that this works now, but could break in a future version of .Net.
All that's left is really easy
CssStyleCollection css = CssStyleTools.Create();
css.Value = "border-top:1px dotted #BBB;margin-top: 0px;font-size:12px";
Console.WriteLine(css["margin-top"]); //prints "0px"
IF you can't for some reason add a reference to System.Web (would be the case if you're using .Net 4 Client Profile) there's always the possibility to use Reflector.
Personally i'd go with Alex's solution, but it's up to you to decide. :)

Just use the following extension method of the HtmlNode:
public static void AddOrUpdateCssValue(this HtmlNode htmlNode, string cssKey, string cssValue)
{
string style = htmlNode.GetAttributeValue("style", "");
string newStyle = addOrUpdateCssStyleKeyValue(style: style, newKey: cssKey, newValue: cssValue);
htmlNode.SetAttributeValue(name: "style", value: newStyle);
}
private static string addOrUpdateCssStyleKeyValue(string style, string newKey, string newValue)
{
if (String.IsNullOrEmpty(style)) return style;
if (String.IsNullOrEmpty(newKey)) return style;
if (String.IsNullOrEmpty(newValue)) return style;
style = style.Clone() as string;
List<string> keyValue = style.Split(';').Where(x => String.IsNullOrEmpty(x) == false).Select(x => x.Trim()).ToList();
bool found = false;
List<string> updatedStyles = new List<string>();
foreach (string keyValuePair in keyValue)
{
if (String.IsNullOrEmpty(keyValuePair) == true) continue;
if (keyValuePair.Contains(':') == false) continue;
List<string> splitted = keyValuePair.Split(':').Where(x => String.IsNullOrEmpty(x) == false).Select(x => x.Trim()).ToList();
if (splitted == null) continue;
if (splitted.Count < 2) continue;
string key = splitted[0];
string value = splitted[1];
if (key == newKey)
{
value = newValue;
found = true;
}
updatedStyles.Add(String.Format("{0}: {1}", key, value));
}
if (found == false)
{
updatedStyles.Add(String.Format("{0}: {1}", newKey, newValue));
}
string result = String.Join("; ", updatedStyles);
return result;
}

speed up parsing in html agility pack

This is a method I use to grab certain tags with the html agility pack. I use this method to do rankings with google local. It seems to take quite a bit of time and be memory intensive, does anyone have any suggestions to make it better?
private void findGoogleLocal(HtmlNode node) {
String name = String.Empty;
//
// ----------------------------------------
if (node.Attributes["id"] != null) {
if (node.Attributes["id"].Value.ToString().Contains("panel_") && node.Attributes["id"].Value.ToString() != "panel__")
{
GoogleLocalResults.Add(new Result(URLGoogleLocal, Listing, node, SearchEngine.Google, SearchType.Local, ResultType.GooglePlaces));
}
}
if (node.HasChildNodes) {
foreach (HtmlNode children in node.ChildNodes) {
findGoogleLocal(children);
}
}
}

Why does this method have to be recursive? Just get all the nodes in one go (example using the Linq support in HAP):
var results = node.Descendants()
.Where(x=> x.Attributes["id"]!= null &&
x.Attributes["id"].Value.Contains("panel_") &&
x.Attributes["id"].Value!= "panel__")
.Select( x=> new Result(URLGoogleLocal, Listing, x, SearchEngine.Google, SearchType.Local, ResultType.GooglePlaces));

I just want to add another clean, simple and fast solution: using XPath.
var results = node
.SelectNodes(#"//*[contains(#id, 'panel_') and #id != 'panel__']")
.Select(x => new Result(URLGoogleLocal, Listing, x, SearchEngine.Google, SearchType.Local, ResultType.GooglePlaces));
foreach (var result in results)
GoogleLocalResults.Add(result);

Fizzler: CSS selector engine for HAP
http://code.google.com/p/fizzler/

What is the best way to parse html in C#? [closed]

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I'm looking for a library/method to parse an html file with more html specific features than generic xml parsing libraries.

Html Agility Pack
This is an agile HTML parser that builds a read/write DOM and supports plain XPATH or XSLT (you actually don't HAVE to understand XPATH nor XSLT to use it, don't worry...). It is a .NET code library that allows you to parse "out of the web" HTML files. The parser is very tolerant with "real world" malformed HTML. The object model is very similar to what proposes System.Xml, but for HTML documents (or streams).

You could use TidyNet.Tidy to convert the HTML to XHTML, and then use an XML parser.
Another alternative would be to use the builtin engine mshtml:
using mshtml;
...
object[] oPageText = { html };
HTMLDocument doc = new HTMLDocumentClass();
IHTMLDocument2 doc2 = (IHTMLDocument2)doc;
doc2.write(oPageText);
This allows you to use javascript-like functions like getElementById()

I found a project called Fizzler that takes a jQuery/Sizzler approach to selecting HTML elements. It's based on HTML Agility Pack. It's currently in beta and only supports a subset of CSS selectors, but it's pretty damn cool and refreshing to use CSS selectors over nasty XPath.
http://code.google.com/p/fizzler/

You can do a lot without going nuts on 3rd-party products and mshtml (i.e. interop). use the System.Windows.Forms.WebBrowser. From there, you can do such things as "GetElementById" on an HtmlDocument or "GetElementsByTagName" on HtmlElements. If you want to actually inteface with the browser (simulate button clicks for example), you can use a little reflection (imo a lesser evil than Interop) to do it:
var wb = new WebBrowser()
... tell the browser to navigate (tangential to this question). Then on the Document_Completed event you can simulate clicks like this.
var doc = wb.Browser.Document
var elem = doc.GetElementById(elementId);
object obj = elem.DomElement;
System.Reflection.MethodInfo mi = obj.GetType().GetMethod("click");
mi.Invoke(obj, new object[0]);
you can do similar reflection stuff to submit forms, etc.
Enjoy.

I've written some code that provides "LINQ to HTML" functionality. I thought I would share it here. It is based on Majestic 12. It takes the Majestic-12 results and produces LINQ XML elements. At that point you can use all your LINQ to XML tools against the HTML. As an example:
IEnumerable<XNode> auctionNodes = Majestic12ToXml.Majestic12ToXml.ConvertNodesToXml(byteArrayOfAuctionHtml);
foreach (XElement anchorTag in auctionNodes.OfType<XElement>().DescendantsAndSelf("a")) {
if (anchorTag.Attribute("href") == null)
continue;
Console.WriteLine(anchorTag.Attribute("href").Value);
}
I wanted to use Majestic-12 because I know it has a lot of built-in knowledge with regards to HTML that is found in the wild. What I've found though is that to map the Majestic-12 results to something that LINQ will accept as XML requires additional work. The code I'm including does a lot of this cleansing, but as you use this you will find pages that are rejected. You'll need to fix up the code to address that. When an exception is thrown, check exception.Data["source"] as it is likely set to the HTML tag that caused the exception. Handling the HTML in a nice manner is at times not trivial...
So now that expectations are realistically low, here's the code :)
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Majestic12;
using System.IO;
using System.Xml.Linq;
using System.Diagnostics;
using System.Text.RegularExpressions;
namespace Majestic12ToXml {
public class Majestic12ToXml {
static public IEnumerable<XNode> ConvertNodesToXml(byte[] htmlAsBytes) {
HTMLparser parser = OpenParser();
parser.Init(htmlAsBytes);
XElement currentNode = new XElement("document");
HTMLchunk m12chunk = null;
int xmlnsAttributeIndex = 0;
string originalHtml = "";
while ((m12chunk = parser.ParseNext()) != null) {
try {
Debug.Assert(!m12chunk.bHashMode); // popular default for Majestic-12 setting
XNode newNode = null;
XElement newNodesParent = null;
switch (m12chunk.oType) {
case HTMLchunkType.OpenTag:
// Tags are added as a child to the current tag,
// except when the new tag implies the closure of
// some number of ancestor tags.
newNode = ParseTagNode(m12chunk, originalHtml, ref xmlnsAttributeIndex);
if (newNode != null) {
currentNode = FindParentOfNewNode(m12chunk, originalHtml, currentNode);
newNodesParent = currentNode;
newNodesParent.Add(newNode);
currentNode = newNode as XElement;
}
break;
case HTMLchunkType.CloseTag:
if (m12chunk.bEndClosure) {
newNode = ParseTagNode(m12chunk, originalHtml, ref xmlnsAttributeIndex);
if (newNode != null) {
currentNode = FindParentOfNewNode(m12chunk, originalHtml, currentNode);
newNodesParent = currentNode;
newNodesParent.Add(newNode);
}
}
else {
XElement nodeToClose = currentNode;
string m12chunkCleanedTag = CleanupTagName(m12chunk.sTag, originalHtml);
while (nodeToClose != null && nodeToClose.Name.LocalName != m12chunkCleanedTag)
nodeToClose = nodeToClose.Parent;
if (nodeToClose != null)
currentNode = nodeToClose.Parent;
Debug.Assert(currentNode != null);
}
break;
case HTMLchunkType.Script:
newNode = new XElement("script", "REMOVED");
newNodesParent = currentNode;
newNodesParent.Add(newNode);
break;
case HTMLchunkType.Comment:
newNodesParent = currentNode;
if (m12chunk.sTag == "!--")
newNode = new XComment(m12chunk.oHTML);
else if (m12chunk.sTag == "![CDATA[")
newNode = new XCData(m12chunk.oHTML);
else
throw new Exception("Unrecognized comment sTag");
newNodesParent.Add(newNode);
break;
case HTMLchunkType.Text:
currentNode.Add(m12chunk.oHTML);
break;
default:
break;
}
}
catch (Exception e) {
var wrappedE = new Exception("Error using Majestic12.HTMLChunk, reason: " + e.Message, e);
// the original html is copied for tracing/debugging purposes
originalHtml = new string(htmlAsBytes.Skip(m12chunk.iChunkOffset)
.Take(m12chunk.iChunkLength)
.Select(B => (char)B).ToArray());
wrappedE.Data.Add("source", originalHtml);
throw wrappedE;
}
}
while (currentNode.Parent != null)
currentNode = currentNode.Parent;
return currentNode.Nodes();
}
static XElement FindParentOfNewNode(Majestic12.HTMLchunk m12chunk, string originalHtml, XElement nextPotentialParent) {
string m12chunkCleanedTag = CleanupTagName(m12chunk.sTag, originalHtml);
XElement discoveredParent = null;
// Get a list of all ancestors
List<XElement> ancestors = new List<XElement>();
XElement ancestor = nextPotentialParent;
while (ancestor != null) {
ancestors.Add(ancestor);
ancestor = ancestor.Parent;
}
// Check if the new tag implies a previous tag was closed.
if ("form" == m12chunkCleanedTag) {
discoveredParent = ancestors
.Where(XE => m12chunkCleanedTag == XE.Name)
.Take(1)
.Select(XE => XE.Parent)
.FirstOrDefault();
}
else if ("td" == m12chunkCleanedTag) {
discoveredParent = ancestors
.TakeWhile(XE => "tr" != XE.Name)
.Where(XE => m12chunkCleanedTag == XE.Name)
.Take(1)
.Select(XE => XE.Parent)
.FirstOrDefault();
}
else if ("tr" == m12chunkCleanedTag) {
discoveredParent = ancestors
.TakeWhile(XE => !("table" == XE.Name
|| "thead" == XE.Name
|| "tbody" == XE.Name
|| "tfoot" == XE.Name))
.Where(XE => m12chunkCleanedTag == XE.Name)
.Take(1)
.Select(XE => XE.Parent)
.FirstOrDefault();
}
else if ("thead" == m12chunkCleanedTag
|| "tbody" == m12chunkCleanedTag
|| "tfoot" == m12chunkCleanedTag) {
discoveredParent = ancestors
.TakeWhile(XE => "table" != XE.Name)
.Where(XE => m12chunkCleanedTag == XE.Name)
.Take(1)
.Select(XE => XE.Parent)
.FirstOrDefault();
}
return discoveredParent ?? nextPotentialParent;
}
static string CleanupTagName(string originalName, string originalHtml) {
string tagName = originalName;
tagName = tagName.TrimStart(new char[] { '?' }); // for nodes <?xml >
if (tagName.Contains(':'))
tagName = tagName.Substring(tagName.LastIndexOf(':') + 1);
return tagName;
}
static readonly Regex _startsAsNumeric = new Regex(#"^[0-9]", RegexOptions.Compiled);
static bool TryCleanupAttributeName(string originalName, ref int xmlnsIndex, out string result) {
result = null;
string attributeName = originalName;
if (string.IsNullOrEmpty(originalName))
return false;
if (_startsAsNumeric.IsMatch(originalName))
return false;
//
// transform xmlns attributes so they don't actually create any XML namespaces
//
if (attributeName.ToLower().Equals("xmlns")) {
attributeName = "xmlns_" + xmlnsIndex.ToString(); ;
xmlnsIndex++;
}
else {
if (attributeName.ToLower().StartsWith("xmlns:")) {
attributeName = "xmlns_" + attributeName.Substring("xmlns:".Length);
}
//
// trim trailing \"
//
attributeName = attributeName.TrimEnd(new char[] { '\"' });
attributeName = attributeName.Replace(":", "_");
}
result = attributeName;
return true;
}
static Regex _weirdTag = new Regex(#"^<!\[.*\]>$"); // matches "<![if !supportEmptyParas]>"
static Regex _aspnetPrecompiled = new Regex(#"^<%.*%>$"); // matches "<%# ... %>"
static Regex _shortHtmlComment = new Regex(#"^<!-.*->$"); // matches "<!-Extra_Images->"
static XElement ParseTagNode(Majestic12.HTMLchunk m12chunk, string originalHtml, ref int xmlnsIndex) {
if (string.IsNullOrEmpty(m12chunk.sTag)) {
if (m12chunk.sParams.Length > 0 && m12chunk.sParams[0].ToLower().Equals("doctype"))
return new XElement("doctype");
if (_weirdTag.IsMatch(originalHtml))
return new XElement("REMOVED_weirdBlockParenthesisTag");
if (_aspnetPrecompiled.IsMatch(originalHtml))
return new XElement("REMOVED_ASPNET_PrecompiledDirective");
if (_shortHtmlComment.IsMatch(originalHtml))
return new XElement("REMOVED_ShortHtmlComment");
// Nodes like "<br <br>" will end up with a m12chunk.sTag==""... We discard these nodes.
return null;
}
string tagName = CleanupTagName(m12chunk.sTag, originalHtml);
XElement result = new XElement(tagName);
List<XAttribute> attributes = new List<XAttribute>();
for (int i = 0; i < m12chunk.iParams; i++) {
if (m12chunk.sParams[i] == "<!--") {
// an HTML comment was embedded within a tag. This comment and its contents
// will be interpreted as attributes by Majestic-12... skip this attributes
for (; i < m12chunk.iParams; i++) {
if (m12chunk.sTag == "--" || m12chunk.sTag == "-->")
break;
}
continue;
}
if (m12chunk.sParams[i] == "?" && string.IsNullOrEmpty(m12chunk.sValues[i]))
continue;
string attributeName = m12chunk.sParams[i];
if (!TryCleanupAttributeName(attributeName, ref xmlnsIndex, out attributeName))
continue;
attributes.Add(new XAttribute(attributeName, m12chunk.sValues[i]));
}
// If attributes are duplicated with different values, we complain.
// If attributes are duplicated with the same value, we remove all but 1.
var duplicatedAttributes = attributes.GroupBy(A => A.Name).Where(G => G.Count() > 1);
foreach (var duplicatedAttribute in duplicatedAttributes) {
if (duplicatedAttribute.GroupBy(DA => DA.Value).Count() > 1)
throw new Exception("Attribute value was given different values");
attributes.RemoveAll(A => A.Name == duplicatedAttribute.Key);
attributes.Add(duplicatedAttribute.First());
}
result.Add(attributes);
return result;
}
static HTMLparser OpenParser() {
HTMLparser oP = new HTMLparser();
// The code+comments in this function are from the Majestic-12 sample documentation.
// ...
// This is optional, but if you want high performance then you may
// want to set chunk hash mode to FALSE. This would result in tag params
// being added to string arrays in HTMLchunk object called sParams and sValues, with number
// of actual params being in iParams. See code below for details.
//
// When TRUE (and its default) tag params will be added to hashtable HTMLchunk (object).oParams
oP.SetChunkHashMode(false);
// if you set this to true then original parsed HTML for given chunk will be kept -
// this will reduce performance somewhat, but may be desireable in some cases where
// reconstruction of HTML may be necessary
oP.bKeepRawHTML = false;
// if set to true (it is false by default), then entities will be decoded: this is essential
// if you want to get strings that contain final representation of the data in HTML, however
// you should be aware that if you want to use such strings into output HTML string then you will
// need to do Entity encoding or same string may fail later
oP.bDecodeEntities = true;
// we have option to keep most entities as is - only replace stuff like
// this is called Mini Entities mode - it is handy when HTML will need
// to be re-created after it was parsed, though in this case really
// entities should not be parsed at all
oP.bDecodeMiniEntities = true;
if (!oP.bDecodeEntities && oP.bDecodeMiniEntities)
oP.InitMiniEntities();
// if set to true, then in case of Comments and SCRIPT tags the data set to oHTML will be
// extracted BETWEEN those tags, rather than include complete RAW HTML that includes tags too
// this only works if auto extraction is enabled
oP.bAutoExtractBetweenTagsOnly = true;
// if true then comments will be extracted automatically
oP.bAutoKeepComments = true;
// if true then scripts will be extracted automatically:
oP.bAutoKeepScripts = true;
// if this option is true then whitespace before start of tag will be compressed to single
// space character in string: " ", if false then full whitespace before tag will be returned (slower)
// you may only want to set it to false if you want exact whitespace between tags, otherwise it is just
// a waste of CPU cycles
oP.bCompressWhiteSpaceBeforeTag = true;
// if true (default) then tags with attributes marked as CLOSED (/ at the end) will be automatically
// forced to be considered as open tags - this is no good for XML parsing, but I keep it for backwards
// compatibility for my stuff as it makes it easier to avoid checking for same tag which is both closed
// or open
oP.bAutoMarkClosedTagsWithParamsAsOpen = false;
return oP;
}
}
}

The Html Agility Pack has been mentioned before - if you are going for speed, you might also want to check out the Majestic-12 HTML parser. Its handling is rather clunky, but it delivers a really fast parsing experience.

I think #Erlend's use of HTMLDocument is the best way to go. However, I have also had good luck using this simple library:
SgmlReader

No 3rd party lib, WebBrowser class solution that can run on Console, and Asp.net
using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Forms;
using System.Threading;
class ParseHTML
{
public ParseHTML() { }
private string ReturnString;
public string doParsing(string html)
{
Thread t = new Thread(TParseMain);
t.ApartmentState = ApartmentState.STA;
t.Start((object)html);
t.Join();
return ReturnString;
}
private void TParseMain(object html)
{
WebBrowser wbc = new WebBrowser();
wbc.DocumentText = "feces of a dummy"; //;magic words
HtmlDocument doc = wbc.Document.OpenNew(true);
doc.Write((string)html);
this.ReturnString = doc.Body.InnerHtml + " do here something";
return;
}
}
usage:
string myhtml = "<HTML><BODY>This is a new HTML document.</BODY></HTML>";
Console.WriteLine("before:" + myhtml);
myhtml = (new ParseHTML()).doParsing(myhtml);
Console.WriteLine("after:" + myhtml);

The trouble with parsing HTML is that it isn't an exact science. If it was XHTML that you were parsing, then things would be a lot easier (as you mention you could use a general XML parser). Because HTML isn't necessarily well-formed XML you will come into lots of problems trying to parse it. It almost needs to be done on a site-by-site basis.

I've used ZetaHtmlTidy in the past to load random websites and then hit against various parts of the content with xpath (eg /html/body//p[#class='textblock']). It worked well but there were some exceptional sites that it had problems with, so I don't know if it's the absolute best solution.

You could use a HTML DTD, and the generic XML parsing libraries.

Use WatiN if you need to see the impact of JS on the page [and you're prepared to start a browser]

Depending on your needs you might go for the more feature-rich libraries. I tried most/all of the solutions suggested, but what stood out head & shoulders was Html Agility Pack. It is a very forgiving and flexible parser.

Try this script.
http://www.biterscripting.com/SS_URLs.html
When I use it with this url,
script SS_URLs.txt URL("http://stackoverflow.com/questions/56107/what-is-the-best-way-to-parse-html-in-c")
It shows me all the links on the page for this thread.
http://sstatic.net/so/all.css
http://sstatic.net/so/favicon.ico
http://sstatic.net/so/apple-touch-icon.png
.
.
.
You can modify that script to check for images, variables, whatever.

I wrote some classes for parsing HTML tags in C#. They are nice and simple if they meet your particular needs.
You can read an article about them and download the source code at http://www.blackbeltcoder.com/Articles/strings/parsing-html-tags-in-c.
There's also an article about a generic parsing helper class at http://www.blackbeltcoder.com/Articles/strings/a-text-parsing-helper-class.