Xamarin was a cross-platform mobile development framework that let developers build native iOS, Android, and Windows applications using the C# language and the .NET ecosystem. Founded in 2011 by engineers who had previously worked on the open-source Mono project, Xamarin built on Mono, an open implementation of the .NET runtime, to execute C# code on mobile operating systems while binding to each platform’s native APIs. This allowed developers to share business logic across platforms while delivering fully native app experiences.
On February 24, 2016, Microsoft announced a definitive agreement to acquire Xamarin in a post on the official Microsoft Blog by executive vice president Scott Guthrie. The announcement described Xamarin as “a leading platform provider for mobile app development” whose offering enabled developers “to build mobile apps using C# and deliver fully native mobile app experiences to all major devices including iOS, Android, and Windows.” Following the acquisition, Microsoft moved to open-source the Xamarin SDK and bundle it into Visual Studio at no additional cost, broadly lowering the barrier to C# mobile development.
Xamarin offered two main approaches. Xamarin.iOS and Xamarin.Android gave direct, platform-specific access to native APIs from C#, while Xamarin.Forms provided a higher-level, cross-platform UI abstraction that mapped shared layouts onto each platform’s native controls. Xamarin.Forms in particular enabled a large fraction of UI code to be written once and rendered natively on multiple platforms.
The framework’s runtime model reflected its Mono heritage and the underlying platforms. On Android, C# is compiled to an intermediate language and then just-in-time compiled to native code at app launch; on iOS, where dynamic code generation is restricted, code is fully ahead-of-time compiled to native ARM assembly. This split between JIT and AOT compilation, inherited by its successor, allowed managed C# code to run within the constraints of each mobile operating system.
Xamarin.Forms ultimately evolved into .NET Multi-platform App UI (.NET MAUI), which Microsoft’s documentation describes as “the evolution of Xamarin.Forms, extended from mobile to desktop scenarios, with UI controls rebuilt from the ground up for performance and extensibility.” .NET MAUI unified Android, iOS, macOS, and Windows development into a single project and API while retaining the C# and XAML model and the Mono-based runtime on mobile platforms, carrying Xamarin’s core idea forward into the modern .NET platform.