Home > Article > Backend Development > C# implements method of converting HTML into plain text
C# implements method of converting HTML into plain text
- 高洛峰Original
- 2017-01-14 13:19:412037browse
The example in this article describes the method of converting HTML into plain text in C#. Share it with everyone for your reference. details as follows:
Instructions:
HtmlToText convert = new HtmlToText(); textBox2.Text = convert.Convert(textBox1.Text);
The C# code is as follows:
/// <summary>
/// Converts HTML to plain text.
/// </summary>
class HtmlToText
{
// Static data tables
protected static Dictionary<string, string> _tags;
protected static HashSet<string> _ignoreTags;
// Instance variables
protected TextBuilder _text;
protected string _html;
protected int _pos;
// Static constructor (one time only)
static HtmlToText()
{
_tags = new Dictionary<string, string>();
_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<string>();
_ignoreTags.Add("script");
_ignoreTags.Add("noscript");
_ignoreTags.Add("style");
_ignoreTags.Add("object");
}
/// <summary>
/// Converts the given HTML to plain text and returns the result.
/// </summary>
/// <param name="html">HTML to be converted</param>
/// <returns>Resulting plain text</returns>
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 <body>
_text.Clear();
}
else if (tag == "/body")
{
// Discard content after </body>
_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
}
}
/// <summary>
/// A StringBuilder class that helps eliminate excess whitespace.
/// </summary>
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;
}
/// <summary>
/// Normally, extra whitespace characters are discarded.
/// If this property is set to true, they are passed
/// through unchanged.
/// </summary>
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;
}
}
/// <summary>
/// Clears all current text.
/// </summary>
public void Clear()
{
_text.Length = 0;
_currLine.Length = 0;
_emptyLines = 0;
}
/// <summary>
/// Writes the given string to the output buffer.
/// </summary>
/// <param name="s"></param>
public void Write(string s)
{
foreach (char c in s)
Write(c);
}
/// <summary>
/// Writes the given character to the output buffer.
/// </summary>
/// <param name="c">Character to write</param>
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;
}
/// <summary>
/// Returns the current output as a string.
/// </summary>
public override string ToString()
{
if (_currLine.Length > 0)
FlushCurrLine();
return _text.ToString();
}
}
}
I hope this article will be helpful to everyone’s C# programming.
For more C# methods to convert HTML into plain text, please pay attention to the PHP Chinese website!
Statement：
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn
Previous article：C# exports html table to excel instanceNext article：C# exports html table to excel instance