/// <summary>
/// sets a persistent cookie with huge expiration date
/// </summary>
/// <param name="key">the cookie key</param>
/// <param name="value">the cookie value</param>
private static void SetCookie(string key, string value)
{
    string oldCookie = HtmlPage.Document.GetProperty("cookie") as String;
    DateTime expiration = DateTime.UtcNow + TimeSpan.FromDays(2000);
    string cookie = String.Format("{0}={1};expires={2}", key, value, expiration.ToString("R"));
    HtmlPage.Document.SetProperty("cookie", cookie);
}

/// <summary>
/// Retrieves an existing cookie
/// </summary>
/// <param name="key">cookie key</param>
/// <returns>null if the cookie does not exist, otherwise the cookie value</returns>
private static string GetCookie(string key)
{
    string[] cookies = HtmlPage.Document.Cookies.Split(';');
    key += '=';
    foreach (string cookie in cookies)
    {
        string cookieStr = cookie.Trim();
        if (cookieStr.StartsWith(key, StringComparison.OrdinalIgnoreCase))
        {
            string[] vals = cookieStr.Split('=');

            if (vals.Length >= 2)
            {
                return vals[1];
            }

            return string.Empty;
        }
    }

    return null;
}

/// <summary>
/// Deletes a specified cookie by setting its value to empty and expiration to -1 days
/// </summary>
/// <param name="key">the cookie key to delete</param>
private static void DeleteCookie(string key)
{
    string oldCookie = HtmlPage.Document.GetProperty("cookie") as String;
    DateTime expiration = DateTime.UtcNow - TimeSpan.FromDays(1);
    string cookie = String.Format("{0}=;expires={1}", key, expiration.ToString("R"));
    HtmlPage.Document.SetProperty("cookie", cookie);
}