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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
using System.Diagnostics;
namespace System.Threading
{
/// <summary>
/// Provides callbacks to objects whose lifetime is managed by <see cref="DeferredDisposableLifetime{T}"/>.
/// </summary>
internal interface IDeferredDisposable
{
/// <summary>
/// Called when the object's refcount reaches zero.
/// </summary>
/// <param name="disposed">
/// Indicates whether the object has been disposed.
/// </param>
/// <remarks>
/// If the refount reaches zero before the object is disposed, this method will be called with
/// <paramref name="disposed"/> set to false. If the object is then disposed, this method will be
/// called again, with <paramref name="disposed"/> set to true. If the refcount reaches zero
/// after the object has already been disposed, this will be called a single time, with
/// <paramref name="disposed"/> set to true.
/// </remarks>
void OnFinalRelease(bool disposed);
}
/// <summary>
/// Manages the lifetime of an object which implements IDisposable, but which must defer the actual
/// cleanup of state until all existing uses of the object are complete.
/// </summary>
/// <typeparam name="T">The type of object whose lifetime will be managed.</typeparam>
/// <remarks>
/// This type maintains a reference count, and tracks whether the object has been disposed. When
/// Callbacks are made to <see cref="IDeferredDisposable.OnFinalRelease(bool)"/> when the refcount
/// reaches zero. Objects that need to defer cleanup until they have been disposed *and* they have
/// no more references can do so in <see cref="IDeferredDisposable.OnFinalRelease(bool)"/> when
/// 'disposed' is true.
/// </remarks>
internal struct DeferredDisposableLifetime<T> where T : class, IDeferredDisposable
{
//
// _count is positive until Dispose is called, after which it's (-1 - refcount).
//
private int _count;
public bool AddRef(T obj)
{
while (true)
{
int oldCount = Volatile.Read(ref _count);
// Have we been disposed?
if (oldCount < 0)
throw new ObjectDisposedException(typeof(T).ToString());
int newCount = checked(oldCount + 1);
if (Interlocked.CompareExchange(ref _count, newCount, oldCount) == oldCount)
return true;
}
}
public void Release(T obj)
{
while (true)
{
int oldCount = Volatile.Read(ref _count);
if (oldCount > 0)
{
// We haven't been disposed. Decrement _count.
int newCount = oldCount - 1;
if (Interlocked.CompareExchange(ref _count, newCount, oldCount) == oldCount)
{
if (newCount == 0)
obj.OnFinalRelease(disposed: false);
return;
}
}
else
{
Debug.Assert(oldCount != 0 && oldCount != -1);
// We've been disposed. Increment _count.
int newCount = oldCount + 1;
if (Interlocked.CompareExchange(ref _count, newCount, oldCount) == oldCount)
{
if (newCount == -1)
obj.OnFinalRelease(disposed: true);
return;
}
}
}
}
public void Dispose(T obj)
{
while (true)
{
int oldCount = Volatile.Read(ref _count);
if (oldCount < 0)
return; // already disposed
int newCount = -1 - oldCount;
if (Interlocked.CompareExchange(ref _count, newCount, oldCount) == oldCount)
{
if (newCount == -1)
obj.OnFinalRelease(disposed: true);
return;
}
}
}
}
}
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