#if NET20 || NET30 || NET35 || NET40 || !NET_4_6

using System.Runtime.InteropServices;
using System.Runtime.Serialization;
//using System.Security.Permissions;

namespace System
{
    [Serializable]
    public sealed class WeakReference<T> : ISerializable
       where T : class
    {
        private readonly bool _trackResurrection;
        private GCHandle _handle;

        public WeakReference(T target)
            : this(target, false)
        {
            //Empty
        }

        public WeakReference(T target, bool trackResurrection)
        {
            _trackResurrection = trackResurrection;
            SetTarget(target);
        }

        internal WeakReference(SerializationInfo info, StreamingContext context)
        {
            if (info == null)
            {
                throw new ArgumentNullException("info");
            }
            var value = (T)info.GetValue("TrackedObject", typeof(T));
            _trackResurrection = info.GetBoolean("TrackResurrection");
            SetTarget(value);
        }

        //[SecurityPermission(SecurityAction.LinkDemand, Flags = SecurityPermissionFlag.SerializationFormatter)]
        public void GetObjectData(SerializationInfo info, StreamingContext context)
        {
            if (info == null)
            {
                throw new ArgumentNullException("info");
            }
            T value;
            TryGetTarget(out value);
            info.AddValue("TrackedObject", value, typeof(T));
            info.AddValue("TrackResurrection", _trackResurrection);
        }

        //[SecurityPermission(SecurityAction.Demand, UnmanagedCode = true)]
        public void SetTarget(T value)
        {
            var oldHandle = _handle;
            _handle = GetNewHandle(value, _trackResurrection);
            if (oldHandle.IsAllocated)
            {
                oldHandle.Free();
                try
                {
                    oldHandle.Free();
                }
                catch (InvalidOperationException exception)
                {
                    // The handle was freed or never initialized.
                    // Nothing to do.
                    GC.KeepAlive(exception);
                }
            }
            _handle = GetNewHandle(value, _trackResurrection);
        }

        //[SecurityPermission(SecurityAction.Demand, UnmanagedCode = true)]
        public bool TryGetTarget(out T target)
        {
            target = default(T);
            if (!_handle.IsAllocated)
            {
                return false;
            }
            try
            {
                var obj = _handle.Target;
                if (obj == null)
                {
                    return false;
                }
                target = obj as T;
                return true;
            }
            catch (InvalidOperationException)
            {
                return false;
            }
        }

        //[SecurityPermission(SecurityAction.Demand, UnmanagedCode = true)]
        private GCHandle GetNewHandle(T value, bool trackResurrection)
        {
            return GCHandle.Alloc(value, trackResurrection ? GCHandleType.WeakTrackResurrection : GCHandleType.Weak);
        }
    }
}

#endif