Microsoft’s new open source direction for C# and .NET (and native compilation too): Anders Hejlsberg explains

At the April 2014 Build conference Microsoft made some far-reaching announcements about its .NET platform and the C# programming language. Yes, there was talk of C# 6.0, the next version, but the real changes are more profound. Specifically:

C# and Visual Basic have a new compiler, itself written in C#, code-named Roslyn. Roslyn is not just a new compiler; Microsoft now calls it the “.NET Compiler Platform”.

There is a new commitment to open source for .NET projects. Microsoft formed the .NET Foundation to oversee existing open source projects, including  ASP.NET, Entity Framework, the Azure .NET SDK, and now Roslyn as well. “When it comes to development projects we are going to operate from the premise that open source is the default. Unless there are reasons why it does not work,” said C# lead architect Anders Hejlsberg.


Note that open source does not mean chaos. It does mean that you can fork the project if you want – the Roslyn license is Apache 2.0 – but getting Microsoft to accept new features you have contributed will not be trivial. Hejlsberg makes the point that language features are easy to add, but impossible to take away, so extreme care is necessary.

Microsoft is also supporting cross-platform C# to a greater extent than it has done in the past. The most obvious sign of this is its cooperation with Xamarin, which provides C# compilers for iOS and Android. Xamarin’s Miguel de Icaza got a top billing at Build, and is also involved in the .NET Foundation.

There is more though. The idea of standardised C# is re-emerging:

“The last ECMA standard was C# 2.0. There wasn’t a lot of demand for it, but that demand has recently risen and we have re engaged with the ECMA community to produce a standard for C# 5.0,” said Hejlsberg.

This bears some unpacking. Why was there little demand for ECMA C#? Partly I would guess from the assumption the C# was firmly in Microsoft’s grip, with Java the obvious choice for cross-platform development. The main interest was from the Mono folk (Miguel de Icaza again), which implemented .NET for Linux and the Mac with some success, but nothing to disturb Java’s momentum.

The focus now though is on mobile, and interest in C# is stronger, mainly from Microsoft-platform developers reaching beyond Windows. There is also Unity, which uses C# as a scripting language for developing games for multiple platforms, including iOS, Android, Windows, Mac, Linux, Xbox, PS3 and Wii – PS4 is coming very soon.

Microsoft has now consciously embraced multiple platforms, as evidenced by Office for iOS as well as the Xamarin collaboration. “We want C#developers to build great applications across different form factors and different device platforms,” said Jay Schmelzer Director of Program Management for Visual Studio.

You might observe that this position has been forced on the company by the rise of iOS and Android, a view which likely has some merit, but the impact it has on C# and .NET itself is still real.

I asked Hejlsberg to unpack the difference between the Roslyn project and C# 6.0, bearing in mind that both are covered on the Roslyn open source site; you can see the current status of C# 6.0 and the next Visual Basic here.

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