[ObjCRuntime] Implement Runtime.UseAutoreleasePoolInThreadPool for .NET.

The getter just returns the runtime appcontext value, while the setter throws
a PlatformNotSupportedException pointing at our documentation about how to
change the value at build time.

src/ObjCRuntime/Runtime.cs

@@ -281,8 +281,7 @@ unsafe static void Initialize (InitializationOptions* options)
 #endif
  InitializePlatform (options);
 
-#if !XAMMAC_SYSTEM_MONO && !NET_TODO
- // NET_TODO: https:/dotnet/runtime/issues/32543
+#if !XAMMAC_SYSTEM_MONO && !NET
  UseAutoreleasePoolInThreadPool = true;
 #endif
  IsARM64CallingConvention = GetIsARM64CallingConvention (); // Can only be done after Runtime.Arch is set (i.e. InitializePlatform has been called).
@@ -301,8 +300,19 @@ unsafe static void Initialize (InitializationOptions* options)
 #endif
  }
 
-#if !XAMMAC_SYSTEM_MONO && !NET_TODO
- // NET_TODO: https:/dotnet/runtime/issues/32543
+#if !XAMMAC_SYSTEM_MONO
+#if NET
+ [EditorBrowsable (EditorBrowsableState.Never)]
+ [Obsolete ("Use the 'AutoreleasePoolSupport' MSBuild property instead: https://docs.microsoft.com/en-us/dotnet/core/run-time-config/threading#autoreleasepool-for-managed-threads")]
+ public static bool UseAutoreleasePoolInThreadPool {
+ get {
+ return AppContext.TryGetSwitch ("System.Threading.Thread.EnableAutoreleasePool", out var enabled) && enabled;
+ }
+ set {
+ throw new PlatformNotSupportedException ("Use the 'AutoreleasePoolSupport' MSBuild property instead: https://docs.microsoft.com/en-us/dotnet/core/run-time-config/threading#autoreleasepool-for-managed-threads");
+ }
+ }
+#else
  static bool has_autoreleasepool_in_thread_pool;
  public static bool UseAutoreleasePoolInThreadPool {
  get {
@@ -319,6 +329,7 @@ static bool ThreadPoolDispatcher (Func<bool> callback)
  using (var pool = new NSAutoreleasePool ())
  return callback ();
  }
+#endif // NET
 #endif
 
 #if MONOMAC