Unify MinimumOSPlatformAttribute and UnsupportedOSPlatformAttribute

[terrajobst]

The original proposal for marking-platform-specific APIs only considered cases where an API was platform-specific, such as the Windows registry. When trying to design a solution for marking APIs as unsupported by Blazor, it became obvious that we need a way to exclude some platforms without treating the API as platform-specific. Hence, a proposal for platform-exclusion was born dotnet/designs#143.

During the design, it emerged that it would be desirable to align these two proposals.

API proposal

• Rename MinimumOSPlatformAttribute to SupportedOSPlatformAttribute
• Rename RemovedInOSPlatformAttribute to UnsupportedOSPlatformAttribute

namespace System.Runtime.Versioning
{
    [AttributeUsage(AttributeTargets.Assembly |
                    AttributeTargets.Class |
                    AttributeTargets.Constructor |
                    AttributeTargets.Enum |
                    AttributeTargets.Event |
                    AttributeTargets.Field |
                    AttributeTargets.Method |
                    AttributeTargets.Module |
                    AttributeTargets.Property |
                    AttributeTargets.Struct,
                    AllowMultiple = true, Inherited = false)]
    public sealed class SupportedOSPlatformAttribute : OSPlatformAttribute
    {
        public SupportedOSPlatformAttribute(string platformName);
    }
  
    [AttributeUsage(AttributeTargets.Assembly |
                    AttributeTargets.Class |
                    AttributeTargets.Constructor |
                    AttributeTargets.Enum |
                    AttributeTargets.Event |
                    AttributeTargets.Field |
                    AttributeTargets.Method |
                    AttributeTargets.Module |
                    AttributeTargets.Property |
                    AttributeTargets.Struct,
                    AllowMultiple = true, Inherited = false)]
    public sealed class UnsupportedOSPlatformAttribute : OSPlatformAttribute
    {
        public UnsupportedOSPlatformAttribute(string platformName);
    }
}

The semantics of these new attributes are as follows:

• An API that doesn't have any of these attributes is considered supported by all platforms.
• If either [SupportedOSPlatform] or [UnsupportedOSPlatform] attributes are present, we group all attributes by OS platform identifier:
  • Allow list. If the lowest version for each OS platform is a [SupportedOSPlatform] attribute, the API is considered to only be supported by the listed platforms and unsupported by all other platforms.
  • Deny list. If the lowest version for each OS platform is a [UnsupportedOSPlatform] attribute, then the API is considered to only be unsupported by the listed platforms and supported by all other platforms.
  • Inconsistent list. If for some platforms the lowest version attribute is [SupportedOSPlatform] while for others it is [UnsupportedOSPlatform], the analyzer will produce a warning on the API definition because the API is attributed inconsistently.
• Both attributes can be instantiated without version numbers. This means the version number is assumed to be 0.0. This simplifies guard clauses, see examples below for more details.
• [ObsoletedInOSPlatform] continuous to require a version number.
• [ObsoletedInOSPlatform] by itself doesn't imply support. However, it doesn't make sense to apply [ObsoletedInOSPlatform] unless that platform is supported.

API Usage

Unsuported platform

In .NET 5:

[UnsupportedOSPlatform("windows")]
public void DoesNotWorkOnWindows();

In .NET 6 we change the code to support the API, but only on Windows 10:

[UnsupportedOSPlatform("windows")]
[SupportedOSPlatform("windows10.0.1903")]
public void DoesNotWorkOnWindows();

Platform-specific API

[SupportedOSPlatform("ios12.0")]
[ObsoletedInOSPlatform("ios13.0")]
[UnsupportedOSPlatform("ios14.0")]
[SupportedOSPlatform("ipados13.0")]
public void OnlyWorksOniOS();

Related

• dotnet/designs#143 Spec for platform exclusion
• dotnet/designs#142 Spec for Windows-specific APIs
• dotnet/designs#144 Spec for Blazor-specific APIs

[jeffhandley]

As I commented on in the spec PR, I wonder if we should support params string[] platformNames on these attributes. That would make it easier to annotate the iOS family of platforms.

[SupportedOSPlatform("ios12.0", "ipados13.0")]
[ObsoletedInOSPlatform("ios13.0")]
[UnsupportedOSPlatform("ios14.0")]
public void OnlyWorksOniOS();

[eerhardt]

Overall, this proposal looks good - it will allow us to model unsupported APIs in Blazor WASM.

namespace System.Runtime.Versioning

RuntimeInformation.IsOSPlatform() and the OSPlatform struct are in the System.Runtime.InteropServices namespace. But these attributes are in a different namespace. Why the difference? They are all about "OS Platforms". The rest of the types in System.Runtime.Versioning seem to be primarily about versioning. But these attributes are primarily about which Operating Systems an API is supported/unsupported in. Versioning seems to be a secondary concern.

It feels like the types OSPlatform and OSPlatformAttribute should be in the same namespace.