Decorate all APIs with correct SupportedOSPlatform attributes

[marek-safar]

As part of netcore migration, we need to correctly annotate all APIs with .NET 6 to work correctly with existing tooling (primary existing analyzers). The attributes already exist in .NET5 and could be extended if necessary.

Available attributes are listed at https://github.com/dotnet/designs/blob/main/accepted/2020/platform-checks/platform-checks.md#attributes

Required Actions

• Generator: read existing attributes but generate the new ones for dotnet
• Manual bindings: Add new attributes under a #if NET and hide existing ones under #else -> in progress
• Update introspection tests to use the new attributes when doing the checks -> done, but parts disabled until manual bindings updates are completed

---

Time estimate: 2 weeks.

[spouliot]

Beside the two issues above the migration means we lose the helper messages, usually the alternative API to use for obsolete/deprecated attributes. E.g. bindings

[Deprecated (PlatformName.iOS, 7, 0, message : "Use 'UbiquitousItemDownloadingStatusKey' instead.")]
[Deprecated (PlatformName.MacOSX, 10, 9, message : "Use 'UbiquitousItemDownloadingStatusKey' instead.")]
[Field ("NSMetadataUbiquitousItemIsDownloadedKey")]
NSString UbiquitousItemIsDownloadedKey { get; }

becomes

[BindingImpl (BindingImplOptions.GeneratedCode | BindingImplOptions.Optimizable)]
static NSString? _UbiquitousItemIsDownloadedKey;
[Field ("NSMetadataUbiquitousItemIsDownloadedKey",  "Foundation")]
[UnsupportedOSPlatform ("ios7.0")]
[UnsupportedOSPlatform ("macos10.9")]
public static NSString UbiquitousItemIsDownloadedKey {
...
}

[marek-safar]

You could consider using ObsoleteAttribute for that (extended in .net5).

https://github.com/dotnet/designs/blob/main/accepted/2020/better-obsoletion/better-obsoletion.md

[spouliot]

Unlike both [SupportedOSPlatform] and [UnsupportedOSPlatform] the [Obsolete] attribute is not allowed multiple times - but it does allow to use our Message property where it's the most useful.

We can use conditionals to work around that limit. That would look like:

[SupportedOSPlatform ("macos10.12.2")]
[SupportedOSPlatform ("watchos5.0")]
#if __WATCHOS__
[Obsolete ("API obsoleted in 'watchos7.0': Removed in Xcode 12.")]
#endif
[SupportedOSPlatform ("tvos14.0")]
public unsafe partial class MPMediaEntity : NSObject, INSCoding, INSSecureCoding {
...
}

The new Url and DiagnosticId could be used. That would allow developers to ignore the Apple obsoletion warnings (per platform?) independently of other normal obsolete.

Note that we might have case where .NET [Obsolete] and Xamarin [Obsoleted] (d suffix) were both used. That should not be common since they were used to indicate different things.