Imported Upstream version 6.4.0.137

Former-commit-id: 943baa9f16a098c33e129777827f3a9d20da00d6

mcs/class/System.Private.CoreLib/System.Runtime.CompilerServices/DependentHandle.cs

@@ -0,0 +1,69 @@
+namespace System.Runtime.CompilerServices
+{
+	struct Ephemeron
+	{
+		public object key;
+		public object value;
+	}
+
+	//
+	// Instead of dependent handles, mono uses arrays of Ephemeron objects.
+	//
+	struct DependentHandle
+	{
+		Ephemeron[] data;
+
+		public DependentHandle (object primary, object secondary)
+		{
+			data = new Ephemeron [1];
+			data [0].key = primary;
+			data [0].value = secondary;
+			GC.register_ephemeron_array (data);
+		}
+
+		public bool IsAllocated => data != null;
+
+		// Getting the secondary object is more expensive than getting the first so
+		// we provide a separate primary-only accessor for those times we only want the
+		// primary.
+		public object GetPrimary ()
+		{
+			if (!IsAllocated)
+				throw new NotSupportedException ();
+			if (data [0].key == GC.EPHEMERON_TOMBSTONE)
+				return null;
+			return data [0].key;
+		}
+
+		public object GetPrimaryAndSecondary (out object secondary)
+		{
+			if (!IsAllocated)
+				throw new NotSupportedException ();
+			if (data [0].key == GC.EPHEMERON_TOMBSTONE) {
+				secondary = null;
+				return null;
+			}
+			secondary = data [0].value;
+			return data [0].key;
+		}
+
+		public void SetPrimary (object primary)
+		{
+			if (!IsAllocated)
+				throw new NotSupportedException ();
+			data [0].key = primary;
+		}
+
+		public void SetSecondary (object secondary)
+		{
+			if (!IsAllocated)
+				throw new NotSupportedException ();
+			data [0].value = secondary;
+		}
+
+		public void Free ()
+		{
+			data = null;
+		}
+	}
+}