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OpenSilver, a ‘plugin-free’ replacement for retired Silverlight browser technology, has released an update that claims to be able to reuse 99% of existing Silverlight application code and run it in a modern browser with WebAssembly and HTML5.

Silverlight was a rich internet application (RIA) technology using Microsoft XAML (extensible application markup language) for UI and supported by a custom .NET Framework runtime that ran inside the browser and provided the developers with capabilities to make desktop-like applications that were mainly targetting business scenarios. Launched in 2007, it required that the users install a browser plugin to run Silverlight applications, similar to Adobe Flash, another popular browser plugin. At its peak in 2011, Silverlight was installed in more than 60% of web users’ browsers.

However, the mainstream technology for browser applications shifted from plugins to HTML5 standards and the major browsers started blocking plugins to reduce vulnerabilities. Silverlight was eventually blocked by all major browsers except for Internet Explorer 11. Microsoft announced in May 2013 that Silverlight would reach the end of support in October 2021.

The existing Silverlight developers reacted angrily and vocally to Microsoft’s abandonment of the technology. The migration of the existing Silverlight applications was not clearly defined, as HTML5 and JS frameworks were radically different to their programming model. That made the owners of existing Silverlight applications decide to keep them running, as the rewrite would have been costly.

As early as 2014 a French company called Userware, which specialises in providing support for migrating existing XAML applications into modern web platforms, started working on CSHTML5, a solution that compiled .NET and XAML code into JavaScript and HTML5. In 2020, CSHTML5 evolved into OpenSilver (the name stands for ‘Open-source reimplementation of Silverlight’). OpenSilver compiles .NET and XAML code into HTML5 and WebAssembly and reimplements standard Silverlight controls together with some of the most popular third-party controls such as Telerik UI suite of components, allowing the developers to reuse instead of rewriting their XAML applications. OpenSilver 1.0 was released in October 2021.

One year later, in October 2022, OpenSilver 1.1 came out with new features and performance improvements. The developers claim that more than 99% of the existing Silverlight applications’ code is reusable in this release. Specifically, popular controls such as DataGrid, Silverlight toolkit library controls or the mentioned Telerik RadGridViewsilve ‘work flawlessly’ now and are ‘pixel-perfect’. The compatibility extends to some popular Silverlight libraries such as Caliburn.Micro or Microsoft Enterprise Library.

The plans for OpenSilver 2.0, slated for the first quarter of 2023, foresee adding support for VB.NET Silverlight applications. Later in the roadmap, OpenSilver developers are eyeing migrating WPF applications and supporting Visual Studio LightSwitch applications.

The reactions to OpenSilver release have been generally positive. Some developers have stated that it would solve their Silverlight migration problems while others are cautious about the alleged compatibility. Even in 2022, with only 0.02% of the browsers running Silverlight, there are still new Silverlight migration projects, apparently justifying the existence of the OpenSilver initiative.