Introducing the Windows App SDK and WinUI 3.0

Now that we have learned a bit of the history of the Microsoft development platform for Windows, we can better understand how the Windows App SDK fits into the story. We've learned that, over time, Microsoft has released new UI platforms with the goal of being state of the art at the time of release. However, UWP introduced a few challenges, especially for developers building line-of-business applications. UWP is a great fit for many consumer scenarios – the sandbox enables an application to run more safely, since it has limited access to critical Windows features, such as a registry or a full filesystem; thanks to a rich capabilities system, the user is always in control of which features of the device (a webcam, a microphone, location, and so on) an application can access; and thanks to a modern life cycle, applications are more respectful of the CPU, memory, and battery life of a device. In many cases, however, enterprises require deeper control and flexibility – applications need to always run, even when they are minimized in the taskbar; they need to interact with custom hardware devices and retrieve information from the Windows Registry.

Also, Windows Runtime, the application architecture behind UWP, introduced a few challenges. This modern runtime offers a lot of benefits: it's built in C++, which means it offers great performance; it's based on asynchronous patterns, which help developers to build applications that are fast and responsive; and through a feature called projections, it enables developers to consume Windows Runtime APIs from multiple languages, such as C++, C#, and Rust. However, since it's a significantly different platform compared to the ones already on the market (such as .NET), developers who have heavily invested in C++, Windows Forms, and WPF are able to reuse little or no code when porting their applications to UWP. This requirement created a lot of friction, since in the enterprise ecosystem, it's easy to find desktop applications with decades of development that are very hard, if not impossible, to port to UWP without rewriting them from scratch. The outcome is that even if developers loved Fluent Design and the new features introduced in Windows 10, most of them weren't able to take advantage of them.

In late 2019, Microsoft announced the first milestone to overcome these challenges – WinUI 3.0. Earlier versions of WinUI already started to lift most of the UI controls and features from the operating system, but the library was still targeting only UWP. With the 3.0 release, instead, Microsoft lifted the whole UI framework from the operating system, enabling other development platforms such as .NET to start taking advantage of the new UI library.

Thanks to WinUI 3.0, developers can now build modern applications and experiences but, at the same time, leverage familiar development platforms such as .NET or C++; reuse most of the investments they did building Windows Forms, WPF, or C++ applications; and use popular NuGet packages that they had already adopted in Windows Forms and WPF applications. Additionally, since applications using WinUI 3.0 are no longer dependent on UWP, but instead run as classic desktop applications, many of the features that enterprise developers saw as a constraint (the sandbox, the life cycle optimized to reduce CPU, and memory usage) don't apply anymore.

Microsoft, during the 2020 edition of the Build conference, officially shared the next step of the journey by announcing Project Reunion, a new development platform with the goal to provide developers with the best of both worlds – familiar developer frameworks and languages (such as .NET, C#, and C++) and the ability to use all the Windows features. WinUI 3.0 was incorporated into this new platform, and it became its first and most important building block.

Project Reunion continued to evolve over time by gradually bringing new features that were exclusively a part of UWP, such as push notifications and activation contracts. On June 24, 2021, when Microsoft revealed to the world Windows 11, the Windows development team announced the official name of Project Reunion – the Windows App SDK.

The Windows App SDK and WinUI 3.0 have the ambitious goal of becoming the new reference UI platform for all Windows developers, regardless of their background. In fact, the Windows App SDK targets both C++ developers (who, before the Windows App SDK, were stuck on MFC as the UI framework offered by Microsoft) and .NET developers (who can reuse their existing skills but target a more modern UI framework than Windows Forms and WPF).

The Windows App SDK introduced the following advantages for developers:

• Consistent support: Windows and its development platform continue to evolve over time, and developers must rely on techniques such as adaptive coding to keep a single code base and use a specific feature only if the application is running on a version of the operating system that supports it. Over time, this approach can add complexity, so Microsoft decided to create the Windows App SDK with built-in down-level support. An application that uses the Windows App SDK can run from Windows 11 down to Windows 10, version 1809. Newer features will be automatically enabled if the application is running on the proper version of the operating system. An example of this feature is Mica, a new material introduced in Windows 11 to enrich the background of an application. If you configure your application to use it, Mica will automatically light up if the application is running on Windows 11, while it will automatically fall back to a solid color on Windows 10.
• A faster release cadence: Since the Windows App SDK is a library, Microsoft can ship updates faster, without being aligned to the release cycle of Windows.
• A unified API surface: Historically, the number of Windows features you could use in your application depended on your framework of choice. For example, many Windows 10 features were supported only by UWP apps. The Windows App SDK unifies access to Windows APIs so that, regardless of the app model you choose, you will be able to use the same set of Windows features.

The Windows App SDK and WinUI are considered the future of the Windows developer platform. UWP is now considered a stable and mature platform that will continue to receive security updates and be supported. However, all the investments for new features and scenarios will be focused on the Windows App SDK, making it the best choice to build future-proof Windows applications.

This book is dedicated to C# developers, and it will cover the usage of the Windows App SDK with classic desktop applications based on the .NET runtime. However, every concept you are going to learn can also be applied to C++ applications.