Remember Silverlight, Microsoft’s rich Internet application plug-in that competed with Adobe Flash before HTML5 pushed aside both? The introduction of OpenSilver this week by .NET software company Userware provides an open source reimplementation of Silverlight, running on current browsers via the WebAssembly binary instruction format.

Now available as a technology preview, OpenSilver is a plug-in-free version of Silverlight, leveraging the Mono software development platform for WebAssembly and Microsoft Blazor, for building client web apps with C#.

Silverlight has enabled development of rich Internet applications with C#, XAML, and .NET. With most browsers no longer supporting plug-ins, Silverlight applications no longer work except on Microsoft’s Internet Explorer.

Filling the gap left by legacy Silverlight, OpenSilver can be used in two ways: building new apps or modernizing existing Silverlight apps. The latter could be recompiled with OpenSilver to run on browsers supporting WebAssembly including Firefox, Safari, Edge, and Chrome. Supported operating systems range from Windows and ChromeOS to the iOS and Android mobile platforms.

Developers who want to build an OpenSilver application can download a free extension to the Visual Studio 2019 IDE, which installs project templates to the “New Project” dialog. Developers using Visual Studio can select the Silverlight UWP (Universal Windows Platform) dialects. The same extension can be used to recompile apps.

Userware cautioned that not all Silverlight features are supported yet, so developers should expect some compilation errors. Developers can get around these limitations through methods such as by importing a .NET Standard or JavaScript library. The current preview covers about 60 percent of the Silverlight API, with most commonly used features supported. Features expected this year include Open RIA Services and Telerik UI for Silverlight. Userware has posted a sample application using its technology.

Microsoft still offers Silverlight 5, but it is slated to no longer receive support as of October  2021. Silverlight development has been limited currently to Internet Explorer 10 and 11.