How to check iOS Device Generation ?

How to check iOS device generation ?

I'm trying to use UnityEngine.iOS.Device.generation but it doesn't work.

My code is ...

 switch (UnityEngine.iOS.Device.generation) {
     case UnityEngine.iOS.DeviceGeneration.iPhone5 :
      //---- Do some stuff but it doesn't work --//
     break;
 }

I want to generate button size create by GUI Button for multi iOS Device, Coding in different device case.

What is the best way to resolve this issue ?

Thank you very much.