What is the difference between NET Core and NET Standard Class Library project types

Question

In Visual Studio  there are at least three different types of class libraries you can create   Class Library   NET Framework  Class Library   NET Standard  Class Library   NET Core   While the first is what we ve been using for years  a major point of confusion I ve been having is when to use the  NET Standard and  NET Core class library types  I ve been bitten by this recently when attempting to multi-target different framework versions  and creating a unit test project  So  what is the difference between Class Library   NET Standard  and Class Library   NET Core   why do both exist  and when should we use one over the other

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NET Framework Windows Forms  ASP NET and WPF application must be developed using  NET Framework library   NET Standard Xamarin  iOS and Mac  OS  X application must be developed using  NET Standard library  NET Core Universal Windows Platform  UWP  and Linux application must be developed using  NET Core library  The API is implemented in C   and you can using the C    VB NET  C   F  and JavaScript languages NET

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NET and  NET Core are two different implementations of the  NET runtime  Both Core and Framework  but especially Framework  have different profiles that include larger or smaller  or just plain different  selections of the many APIs and assemblies Microsoft has created for  NET  depending on where they are installed and in what profile   For example  there are some different APIs available in Universal Windows apps than in the  normal  Windows profile  Even on Windows  you might have the  Client  profile vs  the  Full  profile  Additionally  and there are other implementations  like Mono  that have their own sets of libraries    NET Standard is a specification for which sets of API libraries and assemblies must be available  An app written for  NET Standard 1 0 should be able to compile and run with any version of Framework  Core  Mono  etc   that advertises support for the  NET Standard 1 0 collection of libraries  Similar is true for  NET Standard 1 1  1 5  1 6  2 0  etc  As long as the runtime provides support for the version of Standard targeted by your program  your program should run there   A project targeted at a version of Standard will not be able to make use of features that are not included in that revision of the standard  This doesn t mean you can t take dependencies on other assemblies  or APIs published by other vendors  i e   items on NuGet   But it does mean that any dependencies you take must also include support for your version of  NET Standard   NET Standard is evolving quickly  but it s still new enough  and cares enough about some of the smaller runtime profiles  that this limitation can feel stifling   Note a year and a half later  this is starting to change  and recent  NET Standard versions are much nicer and more full-featured    On the other hand  an app targeted at Standard should be able to be used in more deployment situations  since in theory it can run with Core  Framework  Mono  etc  For a class library project looking for wide distribution  that s an attractive promise  For a class library project used mainly for internal purposes  it may not be as much of a concern    NET Standard can also be useful in situations where the system administrator team is wanting to move from ASP NET on Windows to ASP NET for  NET Core on Linux for philosophical or cost reasons  but the Development team wants to continue working against  NET Framework in Visual Studio on Windows

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NET Standard exists mainly to improve code sharing and make the APIs available in each  NET implementation more consistent   While creating libraries we can have the target as  NET Standard 2 0 so that the library created would be compatible with different versions of  NET Framework including  NET Core  Mono  etc

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NET Framework and  NET Core are both frameworks    NET Standard is a standard  in other words  a specification    You can make an executable project  like a console application  or ASP NET application  with  NET Framework and  NET Core  but not with  NET Standard   With  NET Standard you can make only a class library project that cannot be executed standalone and should be referenced by another  NET Core or  NET Framework executable project

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A  NET Core Class Library is built upon the  NET Standard  If you want to implement a library that is portable to the  NET Framework   NET Core and Xamarin  choose a  NET Standard Library  NET Core will ultimately implement  NET Standard 2  as will Xamarin and  NET Framework   NET Core  Xamarin and  NET Framework can  therefore  be identified as flavours of  NET Standard To future-proof your applications for code sharing and reuse  you would rather implement  NET Standard libraries  Microsoft also recommends that you use  NET Standard instead of Portable Class Libraries  To quote MSDN as an authoritative source   NET Standard is intended to be One Library to Rule Them All  As pictures are worth a thousand words  the following will make things very clear  1  Your current application scenario  fragmented  Like most of us  you are probably in the situation below    NET Framework  Xamarin and now  NET Core flavoured applications   2  What the  NET Standard Library will enable for you  cross-framework compatibility  Implementing a  NET Standard Library allows code sharing across all these different flavours   For the impatient    NET Standard solves the code sharing problem for  NET developers across all platforms by bringing all the APIs that you expect and love across the environments that you need  desktop applications  mobile apps  amp  games  and cloud services   NET Standard is a set of APIs that all  NET platforms have to implement  This unifies the  NET platforms and prevents future fragmentation   NET Standard 2 0 will be implemented by  NET Framework   NET Core  and Xamarin  For  NET Core  this will add many of the existing APIs that have been requested   NET Standard 2 0 includes a compatibility shim for  NET Framework binaries  significantly increasing the set of libraries that you can reference from your  NET Standard libraries   NET Standard will replace Portable Class Libraries  PCLs  as the tooling story for building multi-platform  NET libraries   For a table to help understand what the highest version of  NET Standard that you can target  based on which  NET platforms you intend to run on  head over here  Sources  MSDN  Introducing  NET Standard

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NET Standard  Think of it as a big standard library  When using this as a dependency you can only make libraries   DLLs   not executables  A library made with  NET standard as a dependency can be added to a Xamarin Android  a Xamarin iOS  a  NET Core Windows OS nbsp X Linux project    NET Core  Think of it as the continuation of the old  NET framework  just it s opensource and some stuff is not yet implemented and others got deprecated  It extends the  NET standard with extra functions  but it only runs on desktops  When adding this as a dependency you can make runnable applications on Windows  Linux and OS nbsp X   Although console only for now  no GUIs   So  NET Core    NET Standard   desktop specific stuff   Also UWP uses it and the new ASP NET Core uses it as a dependency too

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The short answer would be  IAnimal     NetStandard  General  ICat     NetCore  Less general  IDog     NetFramework  Specific   oldest and has the most features

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The previous answers may describe the best understanding about the difference between  NET Core   NET Standard and  NET Framework  so I just want to share my experience when choosing this over that  In the project that you need to mix between  NET Framework   NET Core and  NET Standard  For example  at the time we build the system with  NET Core 1 0  there is no support for Window Services hosting with  NET Core  The next reason is we were using Active Report which doesn t support  NET Core  So we want to build an infrastructure library that can be used for both  NET Core  ASP NET Core  and Windows Service and Reporting   NET Framework  - gt  That s why we chose  NET Standard for this kind of library  Choosing  NET standard means you need to carefully consider every class in the library should be simple and cross  NET  Core  Framework  and Standard   Conclusion    NET Standard for the infrastructure library and shared common  This library can be referenced by  NET Framework and  NET Core   NET Framework for unsupported technologies like Active Report  Window Services  now with  NET 3 0 it supports    NET Core for ASP NET Core of course   Microsoft just announced  NET 5  Introducing  NET 5

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When should we use one over the other   The decision is a trade-off between compatibility and API access  Use a  NET Standard library when you want to increase the number of applications that will be compatible with your library  and you are okay with a decrease in the  NET API surface area your library can access  Use a  NET Core library when you want to increase the  NET API surface area your library can access  and you are okay with allowing only  NET Core applications to be compatible with your library  For example  a library that targets  NET Standard 1 3 will be compatible with applications that target  NET Framework 4 6   NET Core 1 0  Universal Windows Platform 10 0  and any other platform that supports  NET Standard 1 3  The library will not have access to some parts of the  NET API  though  For instance  the  Microsoft NETCore CoreCLR package is compatible with  NET Core  but not with  NET Standard   What is the difference between Class Library   NET Standard  and Class Library   NET Core    Compatibility  Libraries that target  NET Standard will run on any  NET Standard compliant runtime  such as  NET Core   NET Framework  Mono Xamarin  On the other hand  libraries that target  NET Core can only run on the  NET Core runtime  API Surface Area   NET Standard libraries come with everything in NETStandard Library  whereas  NET Core libraries come with everything in Microsoft NETCore App  The latter includes approximately 20 additional libraries  some of which we can add manually to our  NET Standard library  such as System Threading Thread  and some of which are not compatible with the  NET Standard  such as Microsoft NETCore CoreCLR   Also   NET Core libraries specify a runtime and come with an application model  That s important  for instance  to make unit test class libraries runnable   Why do both exist   Ignoring libraries for a moment  the reason that  NET Standard exists is for portability  it defines a set of APIs that  NET platforms agree to implement  Any platform that implements a  NET Standard is compatible with libraries that target that  NET Standard  One of those compatible platforms is  NET Core  Coming back to libraries  the  NET Standard library templates exist to run on multiple runtimes  at the expense of API surface area   Conversely  the  NET Core library templates exist to access more API surface area  at the expense of compatibility  and to specify a platform against which to build an executable  Here is an interactive matrix that shows which  NET Standard supports which  NET implementation s  and how much API surface area is available

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I hope this will help to understand the relationship between  NET Standard API surface and other  NET platforms  Each interface represents a target framework and methods represents groups of APIs available on that target framework  namespace Analogy           NET Standard      interface INetStandard10               void Primitives            void Reflection            void Tasks            void Xml            void Collections            void Linq               interface INetStandard11   INetStandard10               void ConcurrentCollections            void LinqParallel            void Compression            void HttpClient               interface INetStandard12   INetStandard11               void ThreadingTimer               interface INetStandard13   INetStandard12                  NET Standard 1 3 specific APIs               And so on               NET Framework      interface INetFramework45   INetStandard11               void FileSystem            void Console            void ThreadPool            void Crypto            void WebSockets            void Process            void Drawing            void SystemWeb            void WPF            void WindowsForms            void WCF               interface INetFramework451   INetFramework45  INetStandard12                   NET Framework 4 5 1 specific APIs            interface INetFramework452   INetFramework451  INetStandard12                   NET Framework 4 5 2 specific APIs            interface INetFramework46   INetFramework452  INetStandard13                   NET Framework 4 6 specific APIs            interface INetFramework461   INetFramework46  INetStandard14                   NET Framework 4 6 1 specific APIs            interface INetFramework462   INetFramework461  INetStandard15                   NET Framework 4 6 2 specific APIs                NET Core     interface INetCoreApp10   INetStandard15                  TODO   NET Core 1 0 specific APIs              Windows Universal Platform     interface IWindowsUniversalPlatform   INetStandard13               void GPS            void Xaml                  Xamarin     interface IXamarinIOS   INetStandard15               void AppleAPIs               interface IXamarinAndroid   INetStandard15               void GoogleAPIs                 Future platform      interface ISomeFuturePlatform   INetStandard13                  A future platform chooses to implement a specific  NET Standard version             All libraries that target that version are instantly compatible with this new            platform           Source

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Another way of explaining the difference could be with real world examples  as most of us mere mortals will use existing tools and frameworks  Xamarin  Unity  etc   to do the job  So  with  NET Framework you have all the  NET tools to work with  but you can only target Windows applications  UWP  Windows Forms  ASP NET  etc    Since  NET Framework is closed source there isn t much to do about it  With  NET Core you have fewer tools  but you can target the main desktop platforms  Windows  Linux  and Mac   This is specially useful in ASP NET Core applications  since you can now host ASP NET on Linux  cheaper hosting prices   Now  since  NET Core was open sourced  it s technically possible to develop libraries for other platforms  But since there aren t frameworks that support it  I don t think that s a good idea  With  NET Standard you have even fewer tools  but you can target all most platforms  You can target mobile thanks to Xamarin  and you can even target game consoles thanks to Mono Unity  It s also possible to target web clients with the UNO platform and Blazor  although both are kind of experimental right now   In a real-world application you may need to use all of them  For example  I developed a point of sale application that had the following architecture  Shared both server and slient   A  NET Standard library that handles the models of my application  A  NET Standard library that handles the validation of data sent by the clients   Since it s a  NET Standard library  it can be used in any other project  client and server   Also a nice advantage of having the validation on a  NET standard library since I can be sure the same validation is applied on the server and the client  Server is mandatory  while client is optional and useful to reduce traffic  Server side  Web API    A  NET Standard  could be  NET Core as well  library that handles all the database connections   A  NET Core project that handles the Rest API and makes use of the database library    As this is developed in  NET Core  I can host the application on a Linux server  Client side  MVVM with WPF   Xamarin Forms Android iOS    A  NET Standard library that handles the client API connection   A  NET Standard library that handles the ViewModels logic  It is used in all the views   A  NET Framework WPF application that handles the WPF views for a windows application  WPF applications can be  NET core now  although they only work on Windows currently  AvaloniaUI is a good alternative for making desktop GUI applications for other desktop platforms   A  NET Standard library that handles Xamarin forms views   A Xamarin Android and Xamarin iOS project    So you can see that there s a big advantage here on the client side of the application  since I can reuse both  NET Standard libraries  client API and ViewModels  and just make views with no logic for the WPF  Xamarin and iOS applications

[.net] What is the difference between .NET Core and .NET Standard Class Library project types?

.NET Framework

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.NET Core

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