public static string CreateSign(this string timeStamp, string partnername, string key)
{
    var word = partnerName.ToLower() + timeStamp + key.ToLower();
    using (var crypt = new SHA256Managed())
    {
        var hash = new StringBuilder();
        var crypto = crypt.ComputeHash(Encoding.UTF8.GetBytes(word), 0,
            Encoding.UTF8.GetByteCount(word));
        foreach (var theByte in crypto){
            hash.Append(theByte.ToString("x2"));
        }
        return hash.ToString();
    }
}