OpenSilver Overview

What is OpenSilver?

OpenSilver is an open-source platform that lets .NET developers build cross-platform apps using familiar C# (or VB, F#) and XAML—no new frameworks to learn.

Originally created as a modern reimplementation of Microsoft Silverlight, OpenSilver has become “WPF Everywhere”: it brings the power and productivity of Windows Presentation Foundation (WPF) to any platform.

Your apps can run on the web (via WebAssembly), as well as on Android, iOS, Windows, macOS (via MAUI Hybrid), and Linux (via Photino)—all from a single codebase.

With OpenSilver, you can:

  • Modernize legacy WPF, Silverlight, LightSwitch, or Windows Forms apps for web and beyond
  • Build new applications that reach web, mobile, and desktop, without rewriting your code for each platform
  • Develop with C#, VB.NET, or F# and design UIs in familiar XAML
  • Use drag-and-drop WYSIWYG design in Visual Studio, VS Code, or online at XAML.io

OpenSilver combines the productivity of WPF with the reach of web and mobile technologies—so you can focus on building great applications, not learning new frameworks.

OpenSilver is professionally maintained by Userware. It is production-ready (see live samples & case studies), and open source.

Need help with migration or enterprise solutions? Contact the OpenSilver team for professional services.

Why the need to replace Silverlight?

In 2007 Microsoft released Silverlight, a plugin for Internet browsers that revolutionized both the browsing experience for users and the development experience for programmers.

On the user side, Silverlight enabled the display of rich content in web pages, much like the competing Adobe Flash plugin at the time. Thanks to Silverlight, it was possible to have very complex applications that ran entirely inside the browser, with an advanced graphical interface and performance similar to that of native Windows applications.

On the developer side, Silverlight enabled the creation of web applications using .NET, C# (or VB.NET or F#) and XAML, that is to say very powerful technologies which until then had been reserved for Windows application development.

With the advent and rapid spread of smartphones (iPhone, Android, etc.) and tablets, it has become increasingly difficult to install plugins on Internet browsers. So, little by little, even the Windows versions of the main Internet browsers decided to stop supporting plugins. That was the end for Adobe Flash and Microsoft Silverlight, which today no longer run on Safari, Chrome, Edge nor Firefox. The last notable version of Silverlight, version 5, was released in December 2011. Then, in 2012, Microsoft announced that Silverlight would only be supported for another ten years, until October 12, 2021, and would only be supported on Microsoft Internet Explorer.

This has been a major problem for many companies that had invested several years of development in their Silverlight applications, particularly in the area of ​​enterprise applications. The need has arisen to find a replacement for Silverlight so as not to throw away all these years of development, hence the birth of OpenSilver.

OpenSilver is an open-source replacement for Microsoft Silverlight and WPF. It is a development tool allowing developers to write web applications using .NET, C# (or VB.NET or F#), and XAML, like Silverlight was. A notable difference with Silverlight is that applications created with OpenSilver run on all current browsers, including Chrome, Edge, Firefox and Safari, without requiring any plugin.

The OpenSilver project was first announced in March 2020 (Technical Preview) by Userware, a French company founded in 2007 specializing in Microsoft development technologies, which had already been working since 2014 on the conversion from C # / XAML to HTML / JavaScript, as part of its other product called CSHTML5. OpenSilver can be downloaded from OpenSilver.NET. The site also contains sample applications and case studies.

The full OpenSilver source code is available under the MIT license on GitHub at github.com/OpenSilver

What are the differences between OpenSilver and Silverlight?

On the user side, Silverlight requires the installation of a plugin in order to work in Internet browsers. OpenSilver on the contrary exploits modern and standardized technologies integrated into browsers, in particular HTML5, CSS3 and WebAssembly, to be able to run on all current browsers: Chrome, Edge, Firefox, Safari.

On the developer side, OpenSilver supports .NET Standard, the latest version of the C# (or VB.NET or F#) languages (whereas Silverlight was stuck with C# 6 and was not compatible with .NET Standard) and the latest version of Visual Studio. Only a few features of Silverlight are not supported in OpenSilver yet, such as XNA and Smooth Streaming. While OpenSilver lets developers directly reference .NET Standard assemblies without recompiling them, it is not possible to reference a Silverlight assembly directly from an OpenSilver project: it must be recompiled with OpenSilver from its source code.

Internally, the implementation of OpenSilver is completely different from that of Silverlight. For example, Silverlight uses Windows technologies for rendering, while OpenSilver creates an HTML element and applies CSS styles for each element of the XAML. The end result is almost identical, but the approach is very different.

How to migrate an existing Silverlight or WPF application?

The general principle for migrating a Silverlight or WPF application to OpenSilver consists in creating an OpenSilver-type project for each of the original Silverlight/WPF projects, then copying / pasting all the files from the original projects to the OpenSilver projects, and finally compiling the solution.

In practice some compilation errors are expected, since OpenSilver currently supports a (fairly large) subset of Silverlight functionality, and a growing subset of the WPF UI API. Particularly in the area of third-party libraries, some manual work may be required if a particular library is not yet supported.

Userware, the company behind the open-source project OpenSilver, is currently working on compatibility with third-party components and has already successfully implemented many components of the Telerik UI suite. Userware has also been able to reach a good level of support for RIA Services, PRISM, MEF, MvvmLight, SharpZipLib, Newtonsoft, and other Silverlight/WPF libraries.

When a functionality is not available, the developer has several possibilities: bypass it with alternative C# (or VB.NET or F#) code and XAML, use a .NET Standard library which provides equivalent functionality, develop it in JavaScript, or import an existing JavaScript library. Soon in version 3.3, the developer will also be able to import Blazor components directly inside the XAML code, as well as mix and match XAML and Razor files within the same OpenSilver project.

In all cases, developers can contact Userware for professonal services ranging from the simple development of a functionality to the complete migration of an application. Case studies can be seen on the OpenSilver.NET web site showing applications migrated by Userware. Paid support packages are also available for guidance on migrating from Silverlight to OpenSilver.

On the OpenSilver web site there is also the possibility of submitting a XAP file, i.e. the executable of the Silverlight application to be migrated, in order to receive an analysis of the supported and unsupported features, as well as an estimate of the workload to complete the migration of the application.

The advantages of migrating with OpenSilver rather than rewriting with another language can be many. While a reduction in the time and cost of migration seems obvious, other benefits include a result that is closer to the original application both in terms of appearance and functionality, higher post-migration productivity, and fewer risks of regressions because the code has already been tested. In fact, the code of the migrated application is almost identical to the original, so developers who must maintain it have a greater knowledge and mastery of the code compared to if it was rewritten with another programming language.

The OpenSilver promise

OpenSilver addresses the need to maintain Silverlight and WPF applications which have sometimes required several years of development. But the promise of OpenSilver goes far beyond that, because it aims to revive the simplicity and the efficiency of the development of rich web applications that Silverlight proposed, by leveraging the Silverlight legacy, modernizing the Silverlight technology through WebAssembly, and pushing the boundaries of what both Silverlight and WPF can do.