.NET Standard vs .NET Core

265
votes

I have read about the difference between .NET Standard and .NET Core, but I really don't know what the difference is, or when to choose a .NET Standard library project and when to choose a .NET Core library project.

I have read that .NET Standard is to ensure that a set of APIs are always available, no matter the platform used (as long as that platform is compatible with the .NET Standard version that I have chosen). If I'm not mistaken, this means that I can create a class library of .NET Standard and then use it on any platform that is compatible with the .NET Standard version that I have chosen.

With .NET Core, I have read that it is intended for cross-platform use too, so if I choose a .NET Core library it seems that I can use it on many platforms too, just like .NET Standard.

So at the end, I don't see the difference. When should I use which? What is the difference between them?

7
.net.net-core.net-standard
In code terms: .net standard = interface, .net core = class; if you code against the class you nay get more methods (etc), but you are restricted to that concrete type (and descendants); if you use the interface you might get a smaller surface, but it will work against arbitrary implementations... as long as those implementations do what is expected :) yes, .net core targets multiple platforms, but there are other implementations of .net standard – Marc Gravell♦
.NETStandard is a replacement for PCL. A Portable Class Library helped you write a library that could run on more than one platform (phone, desktop, store, browser, xbox, etc). It did not scale very well, suffering badly from the n! problem, so they abandoned it. .NETCore is just the first framework they got done, it was the easiest, the rest has to catch up. Do keep in mind that this is very much work-in-progress, big changes ahead with .NETStandard v2.0. The standard to rule them all, for now :) – Hans Passant
Please don't add extra questions to an existing one. Your question in the edit is separate to this. – Jon Skeet
@JonSkeet Then I should open a new question? Thanks for the advise. because at first I have opened a new quesetion about .net Core multi target and I have been downvoted because they said it is a duplicate question. – Álvaro García
@ÁlvaroGarcía: Yes, but make it clear - your current one-sentence question isn't clear to me at all. – Jon Skeet

7 Answers

209
votes

I will try to further clarify your doubts and extend Jon Skeet answer.

.NET Standard is a specification, so a library compiled for a specific .NET Standard version can be used in different .NET Standard implementations.

As said in my other comment, a good analogy for the relationship between .NET Standard and other .NET Standard Implementations (.NET Core, .NET Framework, etc) is this gist by David Fowler: .NET Standard versions are Interfaces, while frameworks are implementations of those interfaces.

This simplified diagram may help to understand this relationship:

NET Standard Interfaces analogy

Anything targetting NetCore10 has access to INetStandard15 APIs and NetCore10 specific APIs (such as DotNetHostPolicy).

Of course this library cannot be used in different INetStandard15 implementations (NetCore10 is not convertible to NetFramework462 or Mono46).

If you, instead, need access only to INetStandard15 APIs (and target that specification instead of a concrete framework) your library may be used by any framework which implements it (NetCore10, NetFramework462, etc.)

Note: in the original analogy David Fowler used interfaces for both .NET Standard versions and frameworks implementations. I believe that using interfaces and classes is, instead, more intuitive and better represents the relationship between specifications and concrete implementations.

192
votes

.NET Core is an implementation of .NET Standard. It's available on multiple operating systems, but that's not the same thing - there are other implementations of .NET Standard as well.

So if you create a .NET Core library, it will have access to things that are implemented in .NET Core, but aren't part of .NET Standard, and your library won't be compatible with other implementations of .NET Standard, such as Xamarin, Tizen, full .NET desktop framework etc.

In short: to achieve maximum portability, make your library target .NET Standard.

9
votes

.NET Standard is a specification of .NET APIs intended to be available on .NET implementations. This enables to define uniform set of BCL APIs for all .NET implementations.

.NET Core is one such implementation of .NET Standard. .NET Framework is another implementation of .NET Standard.

Image from .NET Blog

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Federicos answer gives you a graphical overview of how each framework evolve with versions. Take a look at below diagram from Microsoft Docs.

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Targeting .NET Standard increases your platform support whereas targeting a particular .NET platform such as .NET Core (or .NET Framework) will allow you to use all the platform features for that platform.

6
votes

.NET Core Class library is basically a subset of .NET Framework library, which just contains fewer APIs. Sticking to .NET Core Class library makes it difficult to share code between runtimes. This code might not work for a different runtime (Mono for Xamarin), because it doesn't have the API that you need. To solve this there is .NET Standard, which is just set of specification that tells you which APIs you can use. The main purpose of .NET Standard is to share code between runtimes. And it's important that this specification is implemented by all runtimes. (.NET Framework, .NET Core and Mono for Xamarin).

So if you are sure that you will use your library only for .NET Core projects, you can ignore .NET Standard, but if there is even a tiny chance that your code will be used by .NET Framework or Mono for Xamarin then it's better to stick to .NET Standard

Also note that higher versions of .NET Standard contain more APIs, but lower versions are supported by more platforms. Therefore if you create a .NET Standard library that you want to share between runtimes then target the lowest version you can, which helps you reach the most platforms. For example, if you want to run on .NET Framework 4.5 and .NET Core 1.0, the highest .NET Standard version you can use is .NET Standard 1.1. Refer to this great table from the documentation for more info about it.

PS: Also if you want to convert your library to .NET Standard, the .NET Portability Analyzer could help you with that.

2
votes

.NET Standard is a specification of APIs that all .NET implementations must provide. It brings consistency to the .NET family and enables you to build libraries you can use from any .NET implementation. It replaces PCLs for building shared components.

.NET Core is an implementation of the .NET Standard that’s optimized for building console applications, Web apps and cloud services using ASP.NET Core. Its SDK comes with a powerful tooling that in addition to Visual Studio development supports a full command line-based development workflow. You can learn more about them at aka.ms/netstandardfaq and aka.ms/netcore.

The above, together with a very clear explanation of most of the stuff discussed in this question can be found in the following extremely helpful article by Microsoft (MSDN - September 2017): .NET Standard - Demystifying .NET Core and .NET Standard

1
votes

In simple terms, .NET standard is used for writing class library projects which compiles to dll. .NET Core can be used for developing actual web applications which can run on all operating systems (Windows, Linux, MacOS). (In .NET Core 3 Microsoft has provide the functionality to develop desktop apps using WPF, but uptil now these apps will not be cross platform and will only run on windows system. In future Microsoft might make them cross-platform too) .NET standard libraries/dlls can be used in any application which uses .NET (.NET framework, .NET Core) which means that you can use .NET standard with both .NET framework and .NET core.

0
votes

Did you mean .NET Framework? Because .NET standard is an implementations, such as .NET Framework, .NET Core and Xamarin.

I love .NET Core because we can host it on Linux (use nginx in my experience). It's different than .NET framework which is you can only host on IIS. You can consider about hosting budget in this case (Because windows server is expensive for me).

In the development environment perspective, .Net core is lightweight. So, you can use VSCode, Sublime, for IDE (not only visual studio).