Imported Upstream version 6.4.0.137

Former-commit-id: 943baa9f16a098c33e129777827f3a9d20da00d6

mcs/class/System.Private.CoreLib/System/WeakReference.cs

@@ -0,0 +1,44 @@
+using System.Runtime.InteropServices;
+
+namespace System
+{
+	partial class WeakReference
+	{
+		bool trackResurrection;
+		GCHandle handle;
+
+		public virtual bool IsAlive {
+			get {
+				return Target != null;
+			}
+		}
+
+		public virtual object Target {
+			get {
+				if (!handle.IsAllocated)
+					return null;
+				return handle.Target;
+			}
+			set {
+				handle.Target = value;
+			}
+		}
+
+		~WeakReference ()
+		{
+			handle.Free ();
+		}
+
+		void Create (object target, bool trackResurrection)
+		{
+			if (trackResurrection) {
+				this.trackResurrection = true;
+				handle = GCHandle.Alloc (target, GCHandleType.WeakTrackResurrection);
+			} else {
+				handle = GCHandle.Alloc (target, GCHandleType.Weak);
+			}
+		}
+
+		bool IsTrackResurrection () => trackResurrection;
+	}
+}