public static string Serialize(this CookieContainer cookieContainer) { var listCookieCollection = cookieContainer.GetAllCookieCollections(); var listBase64String = new List <string>(); foreach (var cookieCollection in listCookieCollection) { var cookieCollectionSerializer = new DataContractSerializer(typeof(CookieCollection)); using var memoryStream = new MemoryStream(); cookieCollectionSerializer.WriteObject(memoryStream, cookieCollection); listBase64String.Add(Convert.ToBase64String(memoryStream.ToArray())); } var listSerializer = new DataContractSerializer(typeof(List <string>)); using var stream = new MemoryStream(); listSerializer.WriteObject(stream, listBase64String); return(Convert.ToBase64String(stream.ToArray())); }