[natfriedman]

You have two options:

1. You can bind the native iOS or Android library into C#, which is relatively straightforward:

http://developer.xamarin.com/guides/ios/advanced_topics/bind...

http://developer.xamarin.com/guides/android/advanced_topics/...(.jar)/

2. Or you could create a single, managed library in C#, which would also make it easier to get Windows support.

Option 2 is more work.