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+# Capability APIs
+
+**PM** [Immo Landwerth](https://github.com/terrajobst)
+
+Thoughts I shared on Twitter:
+
+* This morning it happened. I had an epiphany under the shower to make
+  platform-checks much better via a generalized approach for capability APIs.
+  Basically, instead of doing actual platform checks, the analyzer would push you
+  call capability APIs.
+* The nice thing is that this also works for features that aren't OS specific,
+  but more of a combination of runtime and environment, such as whether you can
+  JIT.
+* Imagine in web assembly where we discover certain features wouldn't work. It's
+  hard to annotate an API with "works everywhere but here" in a fashion that
+  doesn't break consumers when the environment changes.
+
+## Scenarios and User Experience
+
+## Requirements
+
+### Goals
+
+### Non-Goals
+
+## Design
+
+### Open Questions
+
+#### How can we reduce the noise?
+
+Basically, we should make sure that people building applications don't have to
+perform capability checks if the answer is statically known. For example,
+registry will work on Windows, so folks building a WinForms/WPF app shouldn't
+have to suppress them. One option is add typical capabilities to the assembly,
+but that's messy. Another option is tag capabilities with platform annotations
+that basically say "on this platform, you don't have to ask for capability
+checks". Then it's only a matter of flowing the platform information from the
+build into the analyzer.
+
+#### Should this analyzer be-opt in?
+
+This might be useful for .NET Standard library developers and much less useful
+for people building .NET Core apps. We might also want different defaults.
+
+#### Should we provide an assert-style attribute?
+
+This would behave the same was tagging the assemblies, modules, types, or
+members with the `[RequiredXxxCapability]` but without requiring callers to
+check it. It's basically a suppression for a specific capability.
+
+#### Should different capabilities have different diagnostic ids?
+
+This probably doesn't work in Roslyn as the diagnostic set needs to be known
+statically.
+
+### Attributes
+
+```C#
+namespace System.Runtime.InteropServices
+{
+    public abstract class CapabilityAttribute : Attribute
+    {
+        public bool Assert { get; set; }
+    }
+
+    [AttributeUsage(AttributeTargets.Class, AllowMultiple = true, Inherited = true)]
+    public sealed class CapabilityCheckAttribute : Attribute
+    {
+        public CapabilityCheckAttribute(Type type, string propertyName)
+        {
+            Type = type;
+            PropertyName = propertyName;
+        }
+
+        public Type Type { get; }
+        public string PropertyName { get; }
+    }
+}
+```
+
+### Example: Registry
+
+```C#
+//[RequiresRegistryCapability]
+static string GetLoggingPath()
+{
+    if (Registry.IsSupported)
+    {
+        var key = new RegistryKey();
+        var value = key.GetValue("foo");
+
+        /*
+        using (var key = Registry.CurrentUser.OpenSubKey(@"SoftwareFabrikamAssetManagement"))
+        {
+            if (key?.GetValue("LoggingDirectoryPath") is string configuredPath)
+                return configuredPath;
+        }
+        */s
+    }
+
+    var appDataPath = Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData);
+    return Path.Combine(appDataPath, "Fabrikam", "AssetManagement", "Logging");
+}
+```
+
+```C#
+namespace Microsoft.Win32
+{
+    [CapabilityCheck(typeof(Registry), nameof(Registry.IsSupported))]
+    [AlwaysAvailabeOn(OSPlatform.Windows)]
+    [AlwaysAvailabeOn(OSPlatform.Linux)]
+    [AlwaysAvailabeOn(OSPlatform.FreeBSD)]
+    [AlwaysUnsupportedOn(WebAssembly)]
+    public sealed class RequiresRegistryCapabilityAttribute : CapabilityAttribute
+    {
+    }
+
+    [RequiresRegistryCapability]
+    public partial class Registry
+    {
+        public static bool IsSupported { get; }
+
+        public static readonly RegistryKey CurrentUser;
+    }
+
+    [RequiresRegistryCapability]
+    public partial class RegistryKey : IDisposable
+    {
+        public RegistryKey OpenSubKey(string path) => throw null;
+        public object GetValue(string name) => throw null;
+        public void Dispose() => throw null;
+    }
+}
+```
+
+### Example: ACLs
+
+```C#
+void Main()
+{
+    if (File.Exits(path))
+    {
+        using (var x = File.OpenRead(path))
+        {
+            if (ObjectSecurity.IsSupported)
+            {
+                var acl = x.GetAccessControl();
+                acl.GiveMePermissions();
+            }
+        }
+    }
+}
+```
+
+```C#
+namespace System.Security.AccessControl
+{
+    [CapabilityCheck(typeof(ObjectSecurity), nameof(ObjectSecurity.IsSupported))]
+    public sealed class RequiresAccessControlCapabilityAttribute : CapabilityAttribute
+    {
+    }
+
+    [RequiresAccessControlCapability]
+    public partial class ObjectSecurity
+    {
+        public static bool IsSupported { get; }
+    }
+
+    [RequiresAccessControlCapability]
+    public partial class FileSecurity
+    {
+    }
+
+    [RequiresAccessControlCapability]
+    public partial class MutexSecurity
+    {
+    }
+}
+
+namespace System.IO
+{
+    public partial class File
+    {
+        [RequiresAccessControlCapability]
+        public static FileSecurity GetAccessControl(string path);
+    }
+
+    public partial class FileInfo
+    {
+        [RequiresAccessControlCapability]
+        public FileSecurity GetAccessControl();
+    }
+}
+
+namespace System.Threading
+{
+    public partial class Mutex
+    {
+        [RequiresAccessControlCapability]
+        public Mutex(bool initiallyOwned, string name, out bool createdNew, MutexSecurity mutexSecurity) => throw null;
+    }
+}
+```
+
+### Example: JIT
+
+```C#
+namespace System.Reflection.Emit
+{
+    [CapabilityCheck(typeof(RuntimeFeature), nameof(RuntimeFeature.IsDynamicCodeSupported))]
+    [CapabilityCheck(typeof(RuntimeFeature), nameof(RuntimeFeature.IsDynamicCodeCompiled))]
+    public partial class RequiresDynamicCodeCapability : CapabilityAttribute
+    {
+    }
+
+    [RequiresDynamicCodeCapability]
+    public partial class DynamicMethod : MethodInfo
+    {
+    }
+
+    [RequiresDynamicCodeCapability]
+    public partial class AssemblyBuilder : Assembly
+    {
+    }
+
+    // Would also mark
+    // - Other *Builder types
+    // - AppDomain.DefineDynamicAssembly
+    // - ILGenerator
+}
+```
+
+## Q & A