C#实现将HTML转换成纯文本的方法

 /// 
 /// Converts HTML to plain text. /// 
 class HtmlToText { // Static data tables protected static Dictionary _tags; protected static HashSet _ignoreTags; // Instance variables protected TextBuilder _text; protected string _html; protected int _pos; // Static constructor (one time only) static HtmlToText() { _tags = new Dictionary(); _tags.Add("address", "\n"); _tags.Add("blockquote", "\n"); _tags.Add("div", "\n"); _tags.Add("dl", "\n"); _tags.Add("fieldset", "\n"); _tags.Add("form", "\n"); _tags.Add("h1", "\n"); _tags.Add("/h1", "\n"); _tags.Add("h2", "\n"); _tags.Add("/h2", "\n"); _tags.Add("h3", "\n"); _tags.Add("/h3", "\n"); _tags.Add("h4", "\n"); _tags.Add("/h4", "\n"); _tags.Add("h5", "\n"); _tags.Add("/h5", "\n"); _tags.Add("h6", "\n"); _tags.Add("/h6", "\n"); _tags.Add("p", "\n"); _tags.Add("/p", "\n"); _tags.Add("table", "\n"); _tags.Add("/table", "\n"); _tags.Add("ul", "\n"); _tags.Add("/ul", "\n"); _tags.Add("ol", "\n"); _tags.Add("/ol", "\n"); _tags.Add("/li", "\n"); _tags.Add("br", "\n"); _tags.Add("/td", "\t"); _tags.Add("/tr", "\n"); _tags.Add("/pre", "\n"); _ignoreTags = new HashSet(); _ignoreTags.Add("script"); _ignoreTags.Add("noscript"); _ignoreTags.Add("style"); _ignoreTags.Add("object"); } /// 
 /// Converts the given HTML to plain text and returns the result. /// 
 /// HTML to be converted /// Resulting plain text public string Convert(string html) { // Initialize state variables _text = new TextBuilder(); _html = html; _pos = 0; // Process input while (!EndOfText) { if (Peek() == '<') { // HTML tag bool selfClosing; string tag = ParseTag(out selfClosing); // Handle special tag cases if (tag == "body") { // Discard content before  _text.Clear(); } else if (tag == "/body") { // Discard content after  _pos = _html.Length; } else if (tag == "pre") { // Enter preformatted mode _text.Preformatted = true; EatWhitespaceToNextLine(); } else if (tag == "/pre") { // Exit preformatted mode _text.Preformatted = false; } string value; if (_tags.TryGetValue(tag, out value)) _text.Write(value); if (_ignoreTags.Contains(tag)) EatInnerContent(tag); } else if (Char.IsWhiteSpace(Peek())) { // Whitespace (treat all as space) _text.Write(_text.Preformatted ? Peek() : ' '); MoveAhead(); } else { // Other text _text.Write(Peek()); MoveAhead(); } } // Return result return HttpUtility.HtmlDecode(_text.ToString()); } // Eats all characters that are part of the current tag // and returns information about that tag protected string ParseTag(out bool selfClosing) { string tag = String.Empty; selfClosing = false; if (Peek() == '<') { MoveAhead(); // Parse tag name EatWhitespace(); int start = _pos; if (Peek() == '/') MoveAhead(); while (!EndOfText && !Char.IsWhiteSpace(Peek()) && Peek() != '/' && Peek() != '>') MoveAhead(); tag = _html.Substring(start, _pos - start).ToLower(); // Parse rest of tag while (!EndOfText && Peek() != '>') { if (Peek() == '"' || Peek() == '\'') EatQuotedValue(); else { if (Peek() == '/') selfClosing = true; MoveAhead(); } } MoveAhead(); } return tag; } // Consumes inner content from the current tag protected void EatInnerContent(string tag) { string endTag = "/" + tag; while (!EndOfText) { if (Peek() == '<') { // Consume a tag bool selfClosing; if (ParseTag(out selfClosing) == endTag) return; // Use recursion to consume nested tags if (!selfClosing && !tag.StartsWith("/")) EatInnerContent(tag); } else MoveAhead(); } } // Returns true if the current position is at the end of // the string protected bool EndOfText { get { return (_pos >= _html.Length); } } // Safely returns the character at the current position protected char Peek() { return (_pos <_html.Length) ? _html[_pos] : (char)0; } // Safely advances to current position to the next character protected void MoveAhead() { _pos = Math.Min(_pos + 1, _html.Length); } // Moves the current position to the next non-whitespace // character. protected void EatWhitespace() { while (Char.IsWhiteSpace(Peek())) MoveAhead(); } // Moves the current position to the next non-whitespace // character or the start of the next line, whichever // comes first protected void EatWhitespaceToNextLine() { while (Char.IsWhiteSpace(Peek())) { char c = Peek(); MoveAhead(); if (c == '\n') break; } } // Moves the current position past a quoted value protected void EatQuotedValue() { char c = Peek(); if (c == '"' || c == '\'') { // Opening quote MoveAhead(); // Find end of value int start = _pos; _pos = _html.IndexOfAny(new char[] { c, '\r', '\n' }, _pos); if (_pos <0) _pos = _html.Length; else MoveAhead();  // Closing quote } } /// 
 /// A StringBuilder class that helps eliminate excess whitespace. /// 
 protected class TextBuilder { private StringBuilder _text; private StringBuilder _currLine; private int _emptyLines; private bool _preformatted; // Construction public TextBuilder() { _text = new StringBuilder(); _currLine = new StringBuilder(); _emptyLines = 0; _preformatted = false; } /// 
 /// Normally, extra whitespace characters are discarded. /// If this property is set to true, they are passed /// through unchanged. /// 
 public bool Preformatted { get { return _preformatted; } set { if (value) { // Clear line buffer if changing to // preformatted mode if (_currLine.Length > 0) FlushCurrLine(); _emptyLines = 0; } _preformatted = value; } } /// 
 /// Clears all current text. /// 
 public void Clear() { _text.Length = 0; _currLine.Length = 0; _emptyLines = 0; } /// 
 /// Writes the given string to the output buffer. /// 
 ///  public void Write(string s) { foreach (char c in s) Write(c); } /// 
 /// Writes the given character to the output buffer. /// 
 /// Character to write public void Write(char c) { if (_preformatted) { // Write preformatted character _text.Append(c); } else { if (c == '\r') { // Ignore carriage returns. We'll process // '\n' if it comes next } else if (c == '\n') { // Flush current line FlushCurrLine(); } else if (Char.IsWhiteSpace(c)) { // Write single space character int len = _currLine.Length; if (len == 0 || !Char.IsWhiteSpace(_currLine[len - 1])) _currLine.Append(' '); } else { // Add character to current line _currLine.Append(c); } } } // Appends the current line to output buffer protected void FlushCurrLine() { // Get current line string line = _currLine.ToString().Trim(); // Determine if line contains non-space characters string tmp = line.Replace(" ", String.Empty); if (tmp.Length == 0) { // An empty line _emptyLines++; if (_emptyLines <2 && _text.length> 0) _text.AppendLine(line); } else { // A non-empty line _emptyLines = 0; _text.AppendLine(line); } // Reset current line _currLine.Length = 0; } /// 
 /// Returns the current output as a string. /// 
 public override string ToString() { if (_currLine.Length > 0) FlushCurrLine(); return _text.ToString(); } } }