private void SetKeyExpiry(string key, DateTime expires)
        {
            if (KeyCleaner == null)
            {
                throw new InvalidOperationException("Expiry date cannot be set if no key cleaner is present");
            }

            KeyCleaner.SetKeyExpiry(key, expires);
        }
 public DateTime?ExpiresOn(string key)
 {
     if (KeyCleaner != null)
     {
         return(KeyCleaner.GetKeyExpiry(key));
     }
     else
     {
         throw new Exception("Cannot get expiry as no KeyCleaner is available");
     }
 }