Basic HTML Helper class for C# to Convert To and From HTML providing Extensions to test if a String contains HTML.

HtmlHelper.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Web;

namespace MyApp.Helpers
{
    public static class HtmlHelper
    {
        private static string _paraBreak = "\r\n\r\n";
        private static string _link = "<a href=\"{0}\">{1}</a>";
        private static string _linkNoFollow = "<a href=\"{0}\" rel=\"nofollow\">{1}</a>";

        /// <summary>
        /// Remove HTML tags from the given string.
        /// </summary>
        public static string RemoveHtmlTags(string source)
        {
            char[] array = new char[source.Length];
            int arrayIndex = 0;
            bool inside = false;

            for (int i = 0; i < source.Length; i++)
            {
                char let = source[i];
                if (let == '<')
                {
                    inside = true;
                    continue;
                }
                if (let == '>')
                {
                    inside = false;
                    continue;
                }
                if (!inside)
                {
                    array[arrayIndex] = let;
                    arrayIndex++;
                }
            }

            string plainText = new string(array, 0, arrayIndex).Replace("&nbsp;", string.Empty);

            return plainText;
        }

        /// <summary>
        /// Convert a Plain Text String to its HTML equivalent.
        /// </summary>
        /// <param name="source">Source Plain Text String</param>
        /// <param name="nofollow">Boolean Flag to follow through Paragraphs.</param>
        /// <returns>HTML String</returns>
        public static string ConvertToHtml(string source, bool nofollow)
        {
            StringBuilder sb = new StringBuilder();

            int pos = 0;
            while (pos < source.Length)
            {
                // Extract next paragraph
                int start = pos;
                pos = source.IndexOf(_paraBreak, start);
                if (pos < 0)
                    pos = source.Length;
                string para = source.Substring(start, pos - start).Trim();

                // Encode non-empty paragraph
                if (para.Length > 0)
                    EncodeParagraph(para, sb, nofollow);

                // Skip over paragraph break
                pos += _paraBreak.Length;
            }

            // Get HTML Version
            string html = sb.ToString();

            // Convert URL Addresses to HTML Anchor Tags
            html = Regex.Replace(html,
                @"((http|ftp|https):\/\/[\w\-_]+(\.[\w\-_]+)+([\w\-\.,@?^=%&amp;:/~\+#]*[\w\-\@?^=%&amp;/~\+#])?)",
                "<a target='_blank' href='$1'>$1</a>");

            // Return HTML Version
            return html;
        }

        /// <summary>
        /// Encodes a single paragraph to HTML.
        /// </summary>
        /// <param name="s">Text to encode</param>
        /// <param name="sb">StringBuilder to write results</param>
        /// <param name="nofollow">If true, links are given "nofollow"
        /// attribute</param>
        private static void EncodeParagraph(string s, StringBuilder sb, bool nofollow)
        {
            // Start new paragraph
            sb.AppendLine("<p>");

            // HTML encode text
            s = HttpUtility.HtmlEncode(s);

            // Convert single newlines to <br>
            s = s.Replace(Environment.NewLine, "<br />\r\n");

            // Encode any hyperlinks
            EncodeLinks(s, sb, nofollow);

            // Close paragraph
            sb.AppendLine("\r\n</p>");
        }

        /// <summary>
        /// Encodes [[URL]] and [[Text][URL]] links to HTML.
        /// </summary>
        /// <param name="text">Text to encode</param>
        /// <param name="sb">StringBuilder to write results</param>
        /// <param name="nofollow">If true, links are given "nofollow"
        /// attribute</param>
        private static void EncodeLinks(string s, StringBuilder sb, bool nofollow)
        {
            // Parse and encode any hyperlinks
            int pos = 0;
            while (pos < s.Length)
            {
                // Look for next link
                int start = pos;
                pos = s.IndexOf("[[", pos);
                if (pos < 0)
                    pos = s.Length;
                // Copy text before link
                sb.Append(s.Substring(start, pos - start));
                if (pos < s.Length)
                {
                    string label, link;

                    start = pos + 2;
                    pos = s.IndexOf("]]", start);
                    if (pos < 0)
                        pos = s.Length;
                    label = s.Substring(start, pos - start);
                    int i = label.IndexOf("][");
                    if (i >= 0)
                    {
                        link = label.Substring(i + 2);
                        label = label.Substring(0, i);
                    }
                    else
                    {
                        link = label;
                    }
                    // Append link
                    sb.Append(String.Format(nofollow ? _linkNoFollow : _link, link, label));

                    // Skip over closing "]]"
                    pos += 2;
                }
            }
        }

        #region Extension Methods
        /// <summary>
        ///  Extension Method to Remove HTML Tags from a given String.
        /// </summary>
        /// <param name="source">Sourch HTML String</param>
        /// <returns>Plain Text</returns>
        public static string RemoveHtml(this string source)
        {
            return RemoveHtmlTags(source);
        }

        /// <summary>
        /// Extension Method to Return a copy of this string converted to HTML markup.
        /// </summary>
        public static string ToHtml(this string source)
        {
            return ToHtml(source, false);
        }

        /// <summary>
        /// Extension Method to Return a copy of this string converted to HTML markup.
        /// </summary>
        /// <param name="source">Non-HTML Text Source String</param>
        /// <param name="nofollow">If true, links are given "nofollow"
        /// attribute</param>
        public static string ToHtml(this string source, bool nofollow)
        {
            // Return HTML Version
            return ConvertToHtml(source, nofollow);
        }

        /// <summary>
        /// Extension Method to convert a String to an HtmlString to prevent markup being escaped.
        /// </summary>
        /// <param name="source">Sourch HTML String</param>
        /// <returns></returns>
        public static HtmlString ToHtmlString(this string source)
        {
            return new HtmlString(source);
        }
        #endregion
    }
}