IOSDeviceInfo.cs

 public class IOSDeviceInfo : IDeviceInfo
    {
        private string _deviceId;
        private const string DeviceIdentifier = "MyDeviceIdentifier";
        public string GetDeviceId
        {
            get
            {
                /* return UIDevice.CurrentDevice.IdentifierForVendor.ToString();*/
                _deviceId = GetRecordsFromKeychain(DeviceIdentifier);
                if (!string.IsNullOrEmpty(_deviceId)) return _deviceId;
                _deviceId = Guid.NewGuid().ToString();
                StoreKeysInKeychain(DeviceIdentifier, _deviceId);
                return _deviceId;
            }
        }


        #region Manage KeyChain
        private void StoreKeysInKeychain(string key, string value)
        {
            DeleteKeyFromKeychain(key);
            var s = new SecRecord(SecKind.GenericPassword)
            {
                AccessGroup = "kSecAttrAccessGroupToken",
                ValueData = NSData.FromString(value),
                Generic = NSData.FromString(key)
            };
            var result = SecKeyChain.Add(s);
        }

        private void DeleteKeyFromKeychain(string key)
        {
            SecStatusCode res;
            var rec = new SecRecord(SecKind.GenericPassword)
            {
                AccessGroup = "kSecAttrAccessGroupToken",
                Generic = NSData.FromString(key)
            };
            var match = SecKeyChain.QueryAsRecord(rec, out res);
            if (match != null)
                SecKeyChain.Remove(match);
        }

        private string GetRecordsFromKeychain(string key)
        {
            try
            {
                SecStatusCode res;
                var rec = new SecRecord(SecKind.GenericPassword)
                {
                    AccessGroup = "kSecAttrAccessGroupToken",
                    Generic = NSData.FromString(key)
                };
                var match = SecKeyChain.QueryAsRecord(rec, out res);

                if (match != null)
                    return match.ValueData.ToString();
            }
            catch (Exception e)
            {
                Console.WriteLine(e);
            }


            return string.Empty;
        }
        #endregion


    }