The different Xamarin platforms available are Xamarin.iOS, Xamarin.Android, and Xamarin.Mac. In this section, we will take a look at each of them.
Xamarin.iOS is used to build apps for iOS with .NET and contains the bindings to the iOS APIs mentioned previously. Xamarin.iOS uses AOT compiling to compile the C# code into Advanced RISC Machine (ARM) assembly language. The Mono runtime runs alongside the Objective-C runtime. Code that uses .NET namespaces, such as System.Linq or System.Net, are executed by the Mono runtime, while code that uses iOS-specific namespaces are executed by the Objective-C runtime. Both the Mono runtime and the Objective-C runtime run on top of the X is Not Unix (XNU) Unix-like kernel (https://github.com/apple/darwin-xnu), which was developed by Apple. The following diagram shows an overview of the iOS architecture:
Xamarin.Android is used to build apps for Android with .NET and contains bindings to the Android APIs. The Mono runtime and the Android Runtime (ART) run side by side on top of a Linux kernel. Xamarin.Android apps could either be Just-In-Time (JIT)-compiled or AOT-compiled, but to AOT-compile them, we need to use Visual Studio Enterprise.
Communication between the Mono runtime and ART occurs via a Java Native Interface (JNI) bridge. There are two types of JNI bridges—Manage Callable Wrapper (MCW) and Android Callable Wrapper (ACW). An MCW is used when code needs to run in ART and an ACW is used when ART needs to run code in the Mono runtime, as shown:
Xamarin.Mac is used to build apps for macOS with .NET and contains the bindings to the macOS APIs. Xamarin.Mac has the same architecture as Xamarin.iOS—the only difference is that Xamarin.Mac apps are JIT-compiled, unlike Xamarin.iOS apps, which are AOT-compiled. This is shown in the following diagram: