/// <summary>
/// Allows setting of a value in a UrlEncoded string. If the key doesn't exist
/// a new one is set, if it exists it's replaced with the new value.
/// </summary>
/// <param name="urlEncoded">A UrlEncoded string of key value pairs</param>
/// <param name="key"></param>
/// <param name="value"></param>
/// <returns></returns>
public static string SetUrlEncodedKey(string urlEncoded, string key, string value)
{
    if (!urlEncoded.EndsWith("?") && !urlEncoded.EndsWith("&"))
        urlEncoded += "&";
 
    Match match = Regex.Match(urlEncoded, "[?|&]" + key + "=.*?&");
 
    if (match == null || string.IsNullOrEmpty(match.Value))
        urlEncoded = urlEncoded + key + "=" + UrlEncode(value) + "&";
    else
        urlEncoded = urlEncoded.Replace(match.Value, match.Value.Substring(0, 1) + key + "=" + UrlEncode(value) + "&");
 
    return urlEncoded.TrimEnd('&');
}