ISystemClock not available in netstandard2.0

Question

I'm still getting to grips with .Net Standard vs .Net Core after many years of development ASP.NET Framework. I have set up a new Web Api "app" which targets netcoreapp3.1 framework along with a middle-tier/Business Logic ClassLib and Data Access ClassLib, both of which target .Net Standard for maximum future compatibility. However, I can't seem to use ISystemClock from the Microsoft.AspNetCore.Authentication namespace in the .Net Standard classlibs!

From reading this SO question, I believe this might be because netstandard2.0 might not implement Microsoft.AspNetCore.App framework. Is this correct?

If it doesn't, should I:

1. Provide my own IMySystemClock interface in my class libraries which the "app" itself can implement a trivial concrete class for?; or

2. Change my class middle and data access tiers to netcoreapp3.1 framework (seems over kill and restrictive to do this)?

3. Something else? Maybe I am missing the point of .Net Standard`?

A service (like ISystemClock) to provide the current (real or mock) time seems quite a fundamental service so I'm unsure why it's not appearing in .net standard framework?

Thanks BloodBaz

Answer 1

If you want to access that functionality from a library, change the target framework of your library to .NET Core 3.1. Libraries can be built in .NET Standard or .NET Core. You can't use .NET Core functionality within a .NET Standard library, but you can do the reverse. Use .NET Standard functionality within a .NET Core library.

Also as a side note, unless it's required for compatibility reasons, I recommend you switch to .NET Standard 2.1. Visual Studio defaults new projects to .NET Standard 2.0, but a ton of new functionality was added in .NET Standard 2.1.

I spent years in the .NET Framework, so I feel the confusion. It took me a while to get used to it. I have a huge project I recently migrated over from .NET Framework to .NET Standard / .NET Core. What I ended up doing was dividing my code up into three sections. A .NET Standard 2.1 library with all the non .NET Core specific common code, a .NET Core 3.1 library with all my .NET Core specific common code (which referenced my .NET Standard library), and my applications as .NET Core 3.1 projects (which referenced my .NET Core 3.1 library).

Make sure you take a read at the answer in this question as well: What is the difference between .NET Core and .NET Standard Class Library project types?