At last week's .NET Connect event and two-weeks ago at the MVP Summit we got to hear about Microsoft's vision for .NET going forward. A lot of people - myself included - in recent years have wondered what the future of .NET is.

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A key component in this process is .NET Standard...

It's taken Microsoft a few years of floundering and unclear messaging about the future of .NET, but it seems Microsoft is finally nailing the message for .NET going forward and .NET Standard, with its common API specification, is a huge part in making sure that the same base library functionality of .NET is available on all .NET platforms in the future.

In this post I look at what .NET Standard is, how it works and what some of the surrounding issues and impacts are for the .NET eco system.

What is .NET Standard?

Here's my definition of what .NET Standard is:

.NET Standard is a specification, not an implementation. .NET Standard describes what a specific implementation like .NET Core, Mono, Xamarin or .NET 4.6 has to implement - at minimum - in terms of API surface in order to be compliant with a given version of .NET Standard.

The actual implementations of today's shipped .NET Standard 1.6 are .NET Core, the full .NET Framework and Mono. The current version of the standard is .NET Standard 1.6 which shipped when .NET Core 1.0 was released. Implementations can implement additional features beyond .NET Standard in their base libraries but at minimum each implementation has to implement the standard APIs of the Standard even if specific APIs end up with NotSupportedExceptions.

.NET Core as a Reference Implementation

As you might expect the API surface of .NET Standard 1.6 coincides pretty closely with the API surface of .NET Core 1.0.x and I expect that .NET Core 1.2 (or whatever the next version will be named) is likely to match very closely to what .NET Standard 2.0 specifies. In that sense it's almost like .NET Core is the reference implementation for .NET Standard at the moment. That may change in the future, but for now that's certainly holding true.

Since .NET Standard is based on full framework APIs, full framework is pretty much compatible with .NET Standard without changes, although there are a few small variations that are addressed by small update releases in .NET 4.6.1 and 4.6.2 etc.

.NET Standard corresponds to the BCL

If you're looking at .NET Standard in terms of the full version of .NET you've used for the last 15+ years, you can think of .NET Standard roughly representing the feature set of the BCL. This is the core .NET library of what used to live exclusively in mscorlib.dll and the various system dlls.

This roughly corresponds to the core .NET Base Class Library (BCL) and includes the basic type system, the runtime loading and querying operations, network and file IO, and some additional APIs like System.Data. With .NET going cross platform it's important to know that this subset has to be platform agnostic and not expect to run on any specific Operating System.

Here's a rough idea of what you can expect to see in .NET Standard 2.0:

The BCL always has been about core operating system, runtime and language services, so this OS agnostic requirement shouldn't be a huge issue. Much of the porting work that goes into moving full framework libraries to .NET Core deals with making sure that functionality works across platforms. There are lots of small issues that can trip up things like the difference in path separators between Operating Systems and the way that operating system services like threads/processes are implemented on various platforms.

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