先解码。接着转换为纯文本，用这段代码：
public static string HtmlToText(string source)
{
string result;
//remove line breaks,tabs
result = source.Replace("\r", " ");
result = result.Replace("\n", " ");
result = result.Replace("\t", " ");
//remove the header
result = Regex.Replace(result, "(<head>).*(</head>)", string.Empty, RegexOptions.IgnoreCase);
result = Regex.Replace(result, @"<( )*script([^>])*>", "<script>", RegexOptions.IgnoreCase);
result = Regex.Replace(result, @"(<script>).*(</script>)", string.Empty, RegexOptions.IgnoreCase);
//remove all styles
result = Regex.Replace(result, @"<( )*style([^>])*>", "<style>", RegexOptions.IgnoreCase); //clearing attributes
result = Regex.Replace(result, "(<style>).*(</style>)", string.Empty, RegexOptions.IgnoreCase);
//insert tabs in spaces of <td> tags
result = Regex.Replace(result, @"<( )*td([^>])*>", " ", RegexOptions.IgnoreCase);
//insert line breaks in places of <br> and <li> tags
result = Regex.Replace(result, @"<( )*br( )*>", "\r", RegexOptions.IgnoreCase);
result = Regex.Replace(result, @"<( )*li( )*>", "\r", RegexOptions.IgnoreCase);
//insert line paragraphs in places of <tr> and <p> tags
result = Regex.Replace(result, @"<( )*tr([^>])*>", "\r\r", RegexOptions.IgnoreCase);
result = Regex.Replace(result, @"<( )*p([^>])*>", "\r\r", RegexOptions.IgnoreCase);
//remove anything thats enclosed inside < >
result = Regex.Replace(result, @"<[^>]*>", string.Empty, RegexOptions.IgnoreCase);
//replace special characters:
result = Regex.Replace(result, @"&", "&", RegexOptions.IgnoreCase);
result = Regex.Replace(result, @" ", " ", RegexOptions.IgnoreCase);
result = Regex.Replace(result, @"<", "<", RegexOptions.IgnoreCase);
result = Regex.Replace(result, @">", ">", RegexOptions.IgnoreCase);
result = Regex.Replace(result, @"&(.{2,6});", string.Empty, RegexOptions.IgnoreCase);
//remove extra line breaks and tabs
result = Regex.Replace(result, @" ( )+", " ");
result = Regex.Replace(result, "(\r)( )+(\r)", "\r\r");
result = Regex.Replace(result, @"(\r\r)+", "\r\n");
return result;
}