//******************************************************************************************************//  NullableWeakReference.cs - Gbtc////  Copyright © 2017, Grid Protection Alliance.  All Rights Reserved.////  Licensed to the Grid Protection Alliance (GPA) under one or more contributor license agreements. See//  the NOTICE file distributed with this work for additional information regarding copyright ownership.//  The GPA licenses this file to you under the MIT License (MIT), 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.opensource.org/licenses/MIT////  Unless agreed to in writing, the subject software distributed under the License is distributed on an//  "AS-IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. Refer to the//  License for the specific language governing permissions and limitations.////  Code Modification History://  ----------------------------------------------------------------------------------------------------//  02/14/2017 - Steven E. Chisholm//       Generated original version of source code.////****************************************************************************************************** using System; namespace GSF{    /// <summary>    /// A <see cref="WeakReference"/> implementation that can have the <see cref="Target"/> object set to null.    /// Natively, setting <see cref="WeakReference.Target"/> to null will throw an <see cref="InvalidOperationException"/>,    /// </summary>    [Serializable]    public class NullableWeakReference : WeakReference    {        private bool m_cleared;                 /// <summary>        /// Creates a <see cref="NullableWeakReference"/>        /// </summary>        /// <param name="target">the object to maintain the weak reference to. Cannot be null.</param>        public NullableWeakReference(object target)            : base(target)        {            if (target == null)                throw new ArgumentNullException(nameof(target));        }         /// <summary>        /// Gets an indication whether the object referenced by the current <see cref="T:NullableWeakReference" /> object has been cleared or garbage collected.        /// </summary>        /// <returns>        /// true if the object referenced by the current <see cref="T:NullableWeakReference" /> object has not been garbage collected or cleared         /// and is still accessible; otherwise, false.        /// </returns>        /// <filterpriority>2</filterpriority>        public override bool IsAlive => !m_cleared && base.IsAlive;         /// <summary>        /// Gets the object (the target) referenced by the current <see cref="T:NullableWeakReference" /> object.        /// Set will only accept null.        /// </summary>        /// <returns>        /// null if the object referenced by the current <see cref="T:NullableWeakReference" /> object has been garbage collected or cleared;        /// otherwise, a reference to the object referenced by the current <see cref="T:NullableWeakReference" /> object.        /// </returns>        /// <exception cref="T:System.InvalidOperationException">If setting this property to anything other than null</exception>        /// <filterpriority>2</filterpriority>        public override object Target        {            get            {                if (m_cleared)                    return null;                return base.Target;            }            set            {                if (value == null)                {                    Clear();                }                else                {                    throw new InvalidOperationException("This target must be set in the constructor.");                }            }        }         /// <summary>        /// Sets <see cref="Target"/> to null so subsequent calls to <see cref="Target"/> returns null        /// </summary>        public virtual void Clear()        {            m_cleared = true;        }    } }