.NET Standard, .NET Core, PCL

Issue

.NET Standard has been a confusing concept. The idea of a standard .NET library format sounds great. But, it's not clear what it is exactly. There are now .NET Core, and .NET Standard Class Library project templates in Visual Studio, as well as the old PCL projects. I also found that if you go in to the settings for the project, you can switch an existing PCL library over to .NET Standard. It's not clear if .NET standard is a library? Or a DLL format.

The basic question I'm asking is which type of library should we be targeting? I'm mainly doing this for Xamarin projects, but it would be great if we could share these libraries with other platforms, especially .NET Core, and UWP. It's confusing as to why Microsoft brought out two seemingly identical Class Library types at the same time: .NET Core, and .NET Standard.

Definitions

NET Core - a .NET based runtime environment which conforms to the .NET Standard API definition.

PCL - a .NET based library format supported by several runtime environments including .NET, Mono, UWP, and so on

.NET Standard (Definition 1) - A set of standard API definitions for .NET based runtime environments

.NET Standard (Definition 2) - a .NET based library format supported by several runtime environments including .NET, Mono, UWP, and so on which is a continuation of the PCL format. In Visual Studio, a PCL library can be converted to a .NET Standard library by going in to the properties of the project and editing the target framework.

Solution

This from Samuel Englard (https://github.com/dotnet/corefx/issues/973#issuecomment-249582799):

.NET DLLs are formatted according to ECMA-335 (Better known as the Common Language Infrastructure). This format is true across all platforms we call .NET; Full Framework, Core, Xamarin, Mono, Silverlight, etc.

The reason you couldn't use a DLL compiled against one platform with another was that the format didn't specify APIs (generally speaking). So while they could all read the DLL you'd run into issues of class XYZ being in namespace A.B.C on one platform and in namespace D.E.F on another, if it existed at all.

PCLs "solved" this by doing two things:

They used Type Forwarding so that even though you wrote your code expecting class XYZ in namespace A.B.C, it could be found on platforms that had it elsewhere. It limited what APIs you could use to the lowest common set shared by all the platforms you wanted.

Taking a PCL and converting it to a .NET Standard project is not a recompile because of output format but because of the meta data included (Type Forwarding particularly).

So, I think that essentially .NET Standard libraries are no different than PCL libraries except for the fact that they put a layer over the top of a different standardized set of APIs, and those APIs may implement different pointers (Type forwarding).

.NET Core is of course a runtime environment, but I think that it is just very closely aligned with .NET Standard. I don't think that it has any special relationship to it other than the fact that it implements the .NET Standard APIs.

In my mind, it is generally better to target a .NET Standard Class library because this will be compatible across multiple platforms. If you target .NET Core, this assembly type cannot be referenced in UWP for example. You will see this:

However, if you are working in a pure .NET Core environment, you may find that there are APIs available that are specific to the .NET Core runtime environment, and therefore it will be necessary to target .NET Core directly.

Please see this glossary: https://github.com/dotnet/corefx/blob/master/Documentation/project-docs/glossary.md

Answered By - Christian Findlay