Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/mscorlib/system/weakreferenceoft.cs

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+// ==++==
+// 
+//   Copyright (c) Microsoft Corporation.  All rights reserved.
+// 
+// ==--==
+/*============================================================
+**
+** Class: WeakReference<T>
+**
+** Purpose: A wrapper for establishing a WeakReference to a generic type.
+**
+===========================================================*/
+namespace System
+{
+    using System;
+    using System.Runtime.Serialization;
+    using System.Security;
+    using System.Runtime;
+    using System.Runtime.CompilerServices;
+    using System.Runtime.Versioning;
+    using System.Diagnostics.Contracts;
+
+    [Serializable]
+    // This class is sealed to mitigate security issues caused by Object::MemberwiseClone.
+    public sealed class WeakReference<T> : ISerializable where T : class
+    {
+        // If you fix bugs here, please fix them in WeakReference at the same time.
+
+        // This field is not a regular GC handle. It can have a special values that are used to prevent ----s between setting the target and finalization.
+        internal IntPtr m_handle;
+
+        // Creates a new WeakReference that keeps track of target.
+        // Assumes a Short Weak Reference (ie TrackResurrection is false.)
+        //
+        public WeakReference(T target)
+            : this(target, false)
+        {
+        }
+
+        //Creates a new WeakReference that keeps track of target.
+        //
+        public WeakReference(T target, bool trackResurrection)
+        {
+            Create(target, trackResurrection);
+        }
+
+        internal WeakReference(SerializationInfo info, StreamingContext context)
+        {
+            if (info == null) {
+                throw new ArgumentNullException("info");
+            }
+            Contract.EndContractBlock();
+
+            T target = (T)info.GetValue("TrackedObject", typeof(T));
+            bool trackResurrection = info.GetBoolean("TrackResurrection");
+
+            Create(target, trackResurrection);
+        }
+
+        //
+        // We are exposing TryGetTarget instead of a simple getter to avoid a common problem where people write incorrect code like:
+        //
+        //      WeakReference ref = ...;
+        //      if (ref.Target != null)
+        //          DoSomething(ref.Target)
+        //
+        [MethodImplAttribute(MethodImplOptions.AggressiveInlining)]
+        public bool TryGetTarget(out T target)
+        {
+            // Call the worker method that has more performant but less user friendly signature.
+            T o = this.Target;
+            target = o;
+            return o != null;
+        }
+
+        public void SetTarget(T target)
+        {
+            this.Target = target;
+        }
+
+        // This is property for better debugging experience (VS debugger shows values of properties when you hover over the variables)
+        private extern T Target
+        {
+            [ResourceExposure(ResourceScope.None)]
+            [MethodImplAttribute(MethodImplOptions.InternalCall)]
+            [SecuritySafeCritical]
+            get;
+            [ResourceExposure(ResourceScope.None)]
+            [MethodImplAttribute(MethodImplOptions.InternalCall)]
+            [SecuritySafeCritical]
+            set;
+        }
+
+        // Free all system resources associated with this reference.
+        //
+        // Note: The WeakReference<T> finalizer is not usually run, but
+        // treated specially in gc.cpp's ScanForFinalization
+        // This is needed for subclasses deriving from WeakReference<T>, however.
+        // Additionally, there may be some cases during shutdown when we run this finalizer.
+        [ResourceExposure(ResourceScope.None)]
+        [MethodImplAttribute(MethodImplOptions.InternalCall)]
+        [SecuritySafeCritical]
+        extern ~WeakReference();
+
+        [SecurityCritical]
+        public void GetObjectData(SerializationInfo info, StreamingContext context)
+        {
+            if (info == null) {
+                throw new ArgumentNullException("info");
+            }
+            Contract.EndContractBlock();
+
+            info.AddValue("TrackedObject", this.Target, typeof(T));
+            info.AddValue("TrackResurrection", IsTrackResurrection());
+        }
+
+        [ResourceExposure(ResourceScope.None)]
+        [MethodImplAttribute(MethodImplOptions.InternalCall)]
+        [SecuritySafeCritical]
+        private extern void Create(T target, bool trackResurrection);
+
+        [ResourceExposure(ResourceScope.None)]
+        [MethodImplAttribute(MethodImplOptions.InternalCall)]
+        [SecuritySafeCritical]
+        private extern bool IsTrackResurrection();
+    }
+}