Code:
/ DotNET / DotNET / 8.0 / untmp / whidbey / REDBITS / ndp / clr / src / BCL / System / WeakReference.cs / 2 / WeakReference.cs
// ==++== // // Copyright (c) Microsoft Corporation. All rights reserved. // // ==--== /*============================================================ ** ** Class: WeakReference ** ** Purpose: A wrapper for establishing a WeakReference to an Object. ** ===========================================================*/ namespace System { using System; using System.Runtime.Remoting; using System.Runtime.Serialization; using System.Security.Permissions; using System.Runtime.InteropServices; using System.Threading; [System.Runtime.InteropServices.ComVisible(true)] [SecurityPermissionAttribute(SecurityAction.InheritanceDemand, Flags=SecurityPermissionFlag.UnmanagedCode)] [Serializable()] public class WeakReference : ISerializable { // Most methods using m_handle should use GC.KeepAlive(this) // to avoid potential handle recycling attacks. The GC special // cases the finalizer for WeakReference & only clears them // when all threads are suspended, but you might still run into // problems if your code is at least partially interruptible. // It's just too much complexity to think about. internal volatile IntPtr m_handle; internal bool m_IsLongReference; // Creates a new WeakReference that keeps track of target. // Assumes a Short Weak Reference (ie TrackResurrection is false.) // public WeakReference(Object target) : this(target, false) { } //Creates a new WeakReference that keeps track of target. // public WeakReference(Object target, bool trackResurrection) { m_IsLongReference=trackResurrection; m_handle = GCHandle.InternalAlloc(target, trackResurrection ? GCHandleType.WeakTrackResurrection : GCHandleType.Weak); } protected WeakReference(SerializationInfo info, StreamingContext context) { if (info==null) { throw new ArgumentNullException("info"); } Object temp = info.GetValue("TrackedObject",typeof(Object)); m_IsLongReference = info.GetBoolean("TrackResurrection"); m_handle = GCHandle.InternalAlloc(temp, m_IsLongReference ? GCHandleType.WeakTrackResurrection : GCHandleType.Weak); } //Determines whether or not this instance of WeakReference still refers to an object //that has not been collected. // public virtual bool IsAlive { get { IntPtr h = m_handle; // In determining whether it is valid to use this object, we need to at least expose this // without throwing an exception. if (IntPtr.Zero == h) return false; bool result = (GCHandle.InternalGet(h)!=null); // We want to ensure that if the target is live, then we will // return it to the user. We need to keep this WeakReference object // live so m_handle doesn't get set to 0 or reused. // Since m_handle is volatile, the following statement will // guarantee the weakref object is live till the following // statement. return (m_handle == IntPtr.Zero) ? false : result; } } //Returns a boolean indicating whether or not we're tracking objects until they're collected (true) //or just until they're finalized (false). // public virtual bool TrackResurrection { get { return m_IsLongReference; } } //Gets the Object stored in the handle if it's accessible. // Or sets it. // public virtual Object Target { get { IntPtr h = m_handle; // Should only happen when used illegally, like using a // WeakReference from a finalizer. if (IntPtr.Zero == h) return null; Object o = GCHandle.InternalGet(h); // We want to ensure that if the target is live, then we will // return it to the user. We need to keep this WeakReference object // live so m_handle doesn't get set to 0 or reused. // Since m_handle is volatile, the following statement will // guarantee the weakref object is live till the following // statement. return (m_handle == IntPtr.Zero) ? null : o; } set { IntPtr h = m_handle; if (h == IntPtr.Zero) throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_HandleIsNotInitialized")); // There is a race w/ finalization where m_handle gets set to // NULL and the WeakReference becomes invalid. Here we have to // do the following in order: // // 1. Get the old object value // 2. Get m_handle // 3. HndInterlockedCompareExchange(m_handle, newValue, oldValue); // // If the interlocked-cmp-exchange fails, then either we lost a race // with another updater, or we lost a race w/ the finalizer. In // either case, we can just let the other guy win. Object oldValue = GCHandle.InternalGet(h); h = m_handle; if (h == IntPtr.Zero) throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_HandleIsNotInitialized")); GCHandle.InternalCompareExchange(h, value, oldValue, false /* isPinned */); // Ensure we don't have any handle recycling attacks in this // method where the finalizer frees the handle. GC.KeepAlive(this); } } // Free all system resources associated with this reference. // // Note: The WeakReference finalizer is not actually run, but // treated specially in gc.cpp's ScanForFinalization // This is needed for subclasses deriving from WeakReference, however. #pragma warning disable 420 ~WeakReference() { IntPtr old_handle = m_handle; if (old_handle != IntPtr.Zero) { if (old_handle == Interlocked.CompareExchange(ref m_handle, IntPtr.Zero, old_handle)) GCHandle.InternalFree(old_handle); } } public virtual void GetObjectData(SerializationInfo info, StreamingContext context) { if (info==null) { throw new ArgumentNullException("info"); } info.AddValue("TrackedObject", Target, typeof(Object)); info.AddValue("TrackResurrection", m_IsLongReference); } } }
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