src/Lucene.Net/Support/IdentityWeakReference.cs

using System;
using System.Runtime.CompilerServices;

namespace Lucene.Net.Support
{
    /*
     * Licensed to the Apache Software Foundation (ASF) under one or more
     * contributor license agreements.  See the NOTICE file distributed with
     * this work for additional information regarding copyright ownership.
     * The ASF licenses this file to You under the Apache License, Version 2.0
     * (the "License"); you may not use this file except in compliance with
     * the License.  You may obtain a copy of the License at
     *
     *     http://www.apache.org/licenses/LICENSE-2.0
     *
     * Unless required by applicable law or agreed to in writing, software
     * distributed under the License is distributed on an "AS IS" BASIS,
     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     * See the License for the specific language governing permissions and
     * limitations under the License.
     */

    internal class IdentityWeakReference<T> : WeakReference
        where T : class
    {
        private readonly int hash;
        private static readonly object NULL = new object();

        public IdentityWeakReference(T target)
            : base(target ?? NULL)
        {
            hash = RuntimeHelpers.GetHashCode(target);
        }

        public override int GetHashCode()
        {
            return hash;
        }

        public override bool Equals(object o)
        {
            if (ReferenceEquals(this, o))
            {
                return true;
            }
            if (o is IdentityWeakReference<T> iwr && ReferenceEquals(this.Target, iwr.Target))
            {
                return true;
            }
            return false;
        }

        public new T Target
        {   // note: if this.NULL is the target, it will not cast to T, so the "as" will return null as we would expect.
            get => base.Target as T;
            set => base.Target = value;
        }
    }
}