Xamarin is a developer platform that is used for developing native applications for iOS (Xamarin.iOS), Android (Xamarin.Android), and macOS (Xamarin.Mac). It is technically a binding layer on top of these platforms. Binding to platform APIs enables .NET developers to use C# (and F#) to develop native applications with the full capacity of each platform. The C# APIs we use when we develop applications with Xamarin are more or less identical to the platform APIs, but they are .NETified. For example, APIs are often customized to follow .NET naming conventions, and Android set and get methods are often replaced by properties. The reason for this is that APIs should be easier to use for .NET developers.

Mono (https://www.mono-project.com) is an open source implementation of the Microsoft .NET framework, which is based on the European Computer Manufacturers Association (ECMA) standards for C# and the common language runtime (CLR). Mono was created to bring the .NET framework to platforms other than Windows. It is part of the .NET foundation (http://www.dotnetfoundation.org), an independent organization that supports open development and collaboration involving the .NET ecosystem.

With the combination of Xamarin platforms and Mono, we will be able to use both all platform-specific APIs and all platform-independent parts of .NET, including, for example, namespaces, systems, System.Linq, System.IO, System.Net, and System.Threading.Tasks.

There are several reasons to use Xamarin for mobile application development, as we will see in the following sections.

If there is one common programming language for multiple mobile platforms, and even server platforms, then we can share a lot of code between our target platforms, as illustrated in the following diagram. All code that isn't related to the target platform can be shared with other .NET platforms. Code that is typically shared in this way includes business logic, network calls, and data models:

There is also a large community based around the .NET platforms and a wide range of third-party libraries and components that can be downloaded from NuGet (https://nuget.org) and used across the .NET platforms.

Code sharing across platforms will lead to shorter development times. It will also lead to applications of a higher quality because we only need to write the code for business logic once. There will be a lower risk of bugs, and we will also be able to guarantee that a calculation will return the same result, no matter what platform our users are using.

For .NET developers who want to start building native mobile applications, it is easier to just learn the APIs for the new platforms than it is to learn programming languages and APIs for both old and new platforms.

Similarly, organizations that want to build native mobile applications could use their existing developers with their knowledge of .NET to develop applications. Because there are more .NET developers than Objective-C and Swift developers, it would be easier to find new developers for mobile application development projects.