.NET Standard vs .NET Core
问：
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?
答：
.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): .NET Standard versions are Interfaces, while frameworks are implementations of those interfaces.
Note: in the original analogy 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.
In code terms:
.NET Standard = interface
.NET Core, .NET Framework, etc = classes