If you have upgraded Xamarin.Forms from a version prior to V2.3.4, you will probably have a few warnings about the combination of Device.OS and TargetPlatform being obsolete.
 
The help text indicates that you should be using RuntimePlatform instead, but your first reaction will probably be, “But…that’s a string?” and it rather unhelpfully doesn’t give any examples of what to replace TargetPlatform with…
 
It’s an easy fix though and is a little tidier now. The Device class now has the following constants for you to use:
 

    Device.iOS
    Device.Android
    Device.WinPhone
    Device.Windows

 
Here’s a quick example.  Say you want to add a header to your rest request with a different value for iOS and Android. Just switch on Device.RuntimePlatform:

switch (Device.RuntimePlatform)
{
        case Device.Android:
             _restClient.HttpClient.DefaultRequestHeaders.Add("my-app-user-agent", "mobileapp-android");
        break;
        case Device.iOS:
             _restClient.HttpClient.DefaultRequestHeaders.Add("my-app-user-agent", "mobileapp-ios");
        break;
}