Change Environment.OSVersion to return actual OS version on macOS

[eerhardt]

Similar to #33643, but for macOS (or OSX). We are now returning an understandable version for Windows 10 (ex. 10.0.18363.0), we should start returning understandable versions for macOS.

Today if I run:

Console.WriteLine(System.Environment.OSVersion.Version.ToString());

On my macOS Mojave computer it says 18.7.0.0. But the "About This Mac" window says 10.14.6.

We should change OSVersion.Version to return 10.14.6 instead.

[spouliot]

I don' know (or recall) why mono returns the Darwin kernel version number (and did not know this was the same for dotnet).

It is not very user/developer friendly. However the runtime code is identical for macOS, iOS, tvOS and watchOS. If we break this to return the macOS version (like 10.15.4.0) then a different implementation will be needed for iOS, tvOS and watchOS runtimes

[wfurt]

My guess we tried to match Apple's terminoplogy:

Shining:FunctionalTests furt$ sysctl -a| grep -e release -e version
kern.osrelease: 19.3.0
kern.version: Darwin Kernel Version 19.3.0: Thu Jan  9 20:58:23 PST 2020; root:xnu-6153.81.5~1/RELEASE_X86_64
kern.posix1version: 200112
kern.osversion: 19D76
kern.osproductversion: 10.15.3
kern.iossupportversion: 13.0

In this case, osversion seems completely bogus so we picked up osrelease.
Do you know @spouliot what we return for various *OS flavors? It would be also nice to return user visible string like 13.3.1 and not underlying kernel.

[spouliot]

@wfurt 19.4.0.0 (the kernel version) comes out of my iOS phone - which is what I expected since it's the same (Darwin-based) code from mono.

The SDK themselves do offer other API to get and compare the "marketing" versions.

[eerhardt]

With the proposal #33331, it is going to be super inconsistent if Environment.OSVersion.Version is returning 19.4.0.0, but RuntimeInformation is returning false for IsOSPlatformOrLater(OSPlatform.iOS, 13, 5).

[spouliot]

I think it's better (and a good time) to fix this inconsistency :)

But TBH this situation has existed since forever and rarely comes up in issues (I can't recall any for iOS/macOS SDK). I think the existing API (similar to the proposed ones) makes it easy to check for features/API that (almost) no one ever used Environment.OSVersion on the mobile SDK.

[eerhardt]

Note that this issue isn't just about mobile. In the dotnet/sdk we have to work around this issue in order to return the actual OS version when running dotnet --info. It would be super confusing to users to report that they are running on OS Version 19.4.0.0.

See

https://github.com/dotnet/sdk/blob/ac704f6190eb2a10c768f188ac57fc139b7e0aac/src/Cli/Microsoft.DotNet.Cli.Utils/RuntimeEnvironment.cs#L68-L86