QueryString helper class

QueryStringHelper.cs

/// <summary>
    /// A chainable query string helper class.
    /// Example usage :
    /// string strQuery = QueryString.Current.Add("id", "179").ToString();
    /// string strQuery = new QueryString().Add("id", "179").ToString();
    /// </summary>
    public class QueryString : NameValueCollection
    {
        public QueryString()
        {
        }

        public QueryString(string queryString)
        {
            FillFromString(queryString);
        }

        public static QueryString Current
        {
            get { return new QueryString().FromCurrent(); }
        }

        /// <summary>
        /// extracts a querystring from a full URL
        /// </summary>
        /// <param name="s">the string to extract the querystring from</param>
        /// <returns>a string representing only the querystring</returns>
        public string ExtractQuerystring(string s)
        {
            if (!string.IsNullOrEmpty(s))
            {
                if (s.Contains("?"))
                    return s.Substring(s.IndexOf("?") + 1);
            }
            return s;
        }

        /// <summary>
        /// returns a querystring object based on a string
        /// </summary>
        /// <param name="s">the string to parse</param>
        /// <returns>the QueryString object </returns>
        public QueryString FillFromString(string s)
        {
            base.Clear();
            if (string.IsNullOrEmpty(s)) return this;
            foreach (string keyValuePair in ExtractQuerystring(s).Split('&'))
            {
                if (string.IsNullOrEmpty(keyValuePair)) continue;
                string[] split = keyValuePair.Split('=');
                base.Add(split[0],
                         split.Length == 2 ? split[1] : "");
            }
            return this;
        }

        /// <summary>
        /// returns a QueryString object based on the current querystring of the request
        /// </summary>
        /// <returns>the QueryString object </returns>
        public QueryString FromCurrent()
        {
            if (HttpContext.Current != null)
            {
                return FillFromString(HttpContext.Current.Request.QueryString.ToString());
            }
            base.Clear();
            return this;
        }

        /// <summary>
        /// add a name value pair to the collection
        /// </summary>
        /// <param name="name">the name</param>
        /// <param name="value">the value associated to the name</param>
        /// <returns>the QueryString object </returns>
        public new QueryString Add(string name, string value)
        {
            return Add(name, value, false);
        }

        /// <summary>
        /// adds a name value pair to the collection
        /// </summary>
        /// <param name="name">the name</param>
        /// <param name="value">the value associated to the name</param>
        /// <param name="isUnique">true if the name is unique within the querystring. This allows us to override existing values</param>
        /// <returns>the QueryString object </returns>
        public QueryString Add(string name, string value, bool isUnique)
        {
            string existingValue = base[name];
            if (string.IsNullOrEmpty(existingValue))
                base.Add(name, HttpUtility.UrlEncodeUnicode(value));
            else if (isUnique)
                base[name] = HttpUtility.UrlEncodeUnicode(value);
            else
                base[name] += "," + HttpUtility.UrlEncodeUnicode(value);
            return this;
        }

        /// <summary>
        /// removes a name value pair from the querystring collection
        /// </summary>
        /// <param name="name">name of the querystring value to remove</param>
        /// <returns>the QueryString object</returns>
        public new QueryString Remove(string name)
        {
            string existingValue = base[name];
            if (!string.IsNullOrEmpty(existingValue))
                base.Remove(name);
            return this;
        }

        /// <summary>
        /// clears the collection
        /// </summary>
        /// <returns>the QueryString object </returns>
        public QueryString Reset()
        {
            base.Clear();
            return this;
        }

        /// <summary>
        /// overrides the default
        /// </summary>
        /// <param name="name"></param>
        /// <returns>the associated decoded value for the specified name</returns>
        public new string this[string name]
        {
            get { return HttpUtility.UrlDecode(base[name]); }
        }

        /// <summary>
        /// overrides the default indexer
        /// </summary>
        /// <param name="index"></param>
        /// <returns>the associated decoded value for the specified index</returns>
        public new string this[int index]
        {
            get { return HttpUtility.UrlDecode(base[index]); }
        }

        /// <summary>
        /// checks if a name already exists within the query string collection
        /// </summary>
        /// <param name="name">the name to check</param>
        /// <returns>a boolean if the name exists</returns>
        public bool Contains(string name)
        {
            string existingValue = base[name];
            return !string.IsNullOrEmpty(existingValue);
        }

        /// <summary>
        /// outputs the querystring object to a string
        /// </summary>
        /// <returns>the encoded querystring as it would appear in a browser</returns>
        public override string ToString()
        {
            var builder = new StringBuilder();
            for (var i = 0; i < base.Keys.Count; i++)
            {
                if (!string.IsNullOrEmpty(base.Keys[i]))
                {
                    foreach (string val in base[base.Keys[i]].Split(','))
                        builder.Append((builder.Length == 0) ? "?" : "&").Append(
                            HttpUtility.UrlEncodeUnicode(base.Keys[i])).Append("=").Append(val);
                }
            }
            return builder.ToString();
        }
    }

There's a bug in one of the Add methods, "public QueryString Add(string name, string value, bool isUnique)"
If you call the above Add multiple times, and "value" is null and "isUnique" is true, "name" is added multiple times.
A proposed fix would be like this:

public QueryString Add(string name, string value, bool isUnique)
{
    if (base.BaseGet(name) == null)
        base.Add(name, HttpUtility.UrlEncodeUnicode(value));
    else if (isUnique)
        base[name] = HttpUtility.UrlEncodeUnicode(value);
    else
        base[name] += "," + HttpUtility.UrlEncodeUnicode(value);
    return this;
}