xamarin-bluetooth-le | Bluetooth LE plugin for Xamarin | iOS library

BLE communication can be a pain particularly with Xamarin.

In any case asynchronous calls from your application may return before data has actually been sent to your end device. This is due to several factors:

1. Operating system BLE driver behaviour - Any call that you make in your application code is handled by the operating system drivers. These may return before any data is actually handled by the BLE hardware.
2. BLE transport layer delays - BLE transmission of data is not instant. The data connection between two devices actually occurs within discrete time slots, between which the BLE transceiver is turned off to save power.
3. End device response time.

Most implementations of two way communication between devices using BLE use a minimum of two characteristics, one for data being sent and the other for data being received by a device e.g. device 1 receives data on the characteristic that device 2 sends on and vice versa. ​This avoids the possibility of any collision of data being sent by both parties.

You could try placing a delay in your code prior to polling for characteristic data and see if that helps, but its not an optimal solution.

I see that you are using your characteristic "anonymously", rather than using its uuid to select it. If the Bluetooth service supports more than one characteristic then you will need to use the uuid to ensure that you are using the correct one. The list of characteristics, much like the list of services and devices may not be returned in the same order whenever you request them.

You have the option of polling characteristics by reading them on a timed basis or setting up handlers for characteristic notification/indication events if the characteristic supports it and your BLE device uses that mechanism to show that data is ready.