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Update Dispose pattern examples to use thread-safe implementation #49370

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Copilot Oct 20, 2025
329f244
Update Dispose pattern examples to use thread-safe implementation
Copilot Oct 20, 2025
20778b1
Revert Disposable.cs and Disposable.vb to non-thread-safe pattern
Copilot Oct 20, 2025
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4 changes: 2 additions & 2 deletions docs/standard/garbage-collection/implementing-dispose.md
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Expand Up @@ -91,7 +91,7 @@ All non-sealed classes (or Visual Basic classes not modified as `NotInheritable`

### Base class with managed resources

Here's a general example of implementing the dispose pattern for a base class that only owns managed resources.
Here's a general example of implementing the dispose pattern for a base class that only owns managed resources. The example uses <xref:System.Threading.Interlocked.CompareExchange%2A?displayProperty=nameWithType> to ensure thread-safe disposal.

:::code language="csharp" source="../../../samples/snippets/csharp/VS_Snippets_CLR_System/system.idisposable/cs/base1.cs":::
:::code language="vb" source="../../../samples/snippets/visualbasic/VS_Snippets_CLR_System/system.idisposable/vb/base1.vb":::
Expand Down Expand Up @@ -153,7 +153,7 @@ The following code demonstrates how to handle unmanaged resources by implementin
> The behavior of the `DisposableBaseWithSafeHandle` class is equivalent to the behavior of the [`DisposableBaseWithFinalizer` class in a previous example](#base-class-with-unmanaged-resources), however the approach demonstrated here is safer:
>
> - There is no need to implement a finalizer, because <xref:System.Runtime.InteropServices.SafeHandle> will take care of finalization.
> - There is no need for synchronization to guarantee thread safety. Even though there is a race condition in the `Dispose` implementation of `DisposableBaseWithSafeHandle`, <xref:System.Runtime.InteropServices.SafeHandle> guarantees that <xref:System.Runtime.InteropServices.SafeHandle.ReleaseHandle%2A?displayProperty=nameWithType> will be called only once.
> - <xref:System.Runtime.InteropServices.SafeHandle> guarantees that <xref:System.Runtime.InteropServices.SafeHandle.ReleaseHandle%2A?displayProperty=nameWithType> will be called only once, even in multi-threaded scenarios.
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The updated bullet point removes important context about why synchronization is unnecessary when using SafeHandle. The original text explained that despite a race condition in the Dispose implementation, SafeHandle provides its own thread-safety guarantees. Consider revising to: '- The Dispose implementation uses xref:System.Threading.Interlocked.CompareExchange%2A?displayProperty=nameWithType to ensure thread-safe disposal. Additionally, xref:System.Runtime.InteropServices.SafeHandle guarantees that xref:System.Runtime.InteropServices.SafeHandle.ReleaseHandle%2A?displayProperty=nameWithType will be called only once, even in multi-threaded scenarios.' This maintains the educational value by explaining both the explicit thread-safety in the Dispose pattern and the built-in SafeHandle protections.

Suggested change
> - <xref:System.Runtime.InteropServices.SafeHandle> guarantees that <xref:System.Runtime.InteropServices.SafeHandle.ReleaseHandle%2A?displayProperty=nameWithType> will be called only once, even in multi-threaded scenarios.
> - The `Dispose` implementation uses <xref:System.Threading.Interlocked.CompareExchange%2A?displayProperty=nameWithType> to ensure thread-safe disposal. Additionally, <xref:System.Runtime.InteropServices.SafeHandle> guarantees that <xref:System.Runtime.InteropServices.SafeHandle.ReleaseHandle%2A?displayProperty=nameWithType> will be called only once, even in multi-threaded scenarios.

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I agree with this rewording. What do you think @gewarren ?


### Built-in safe handles in .NET

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13 changes: 8 additions & 5 deletions samples/snippets/csharp/VS_Snippets_CLR_System/system.idisposable/cs/base1.cs
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@@ -1,10 +1,13 @@
using System;
using System.IO;
using System.Threading;

public class DisposableBase : IDisposable
{
// Detect redundant Dispose() calls.
private bool _isDisposed;
// Detect redundant Dispose() calls in a thread-safe manner.
// _isDisposed == 0 means Dispose(bool) has not been called yet.
// _isDisposed == 1 means Dispose(bool) has been already called.
private int _isDisposed;

// Instantiate a disposable object owned by this class.
private Stream? _managedResource = new MemoryStream();
Expand All @@ -19,10 +22,10 @@ public void Dispose()
// Protected implementation of Dispose pattern.
protected virtual void Dispose(bool disposing)
{
if (!_isDisposed)
// In case _isDisposed is 0, atomically set it to 1.
// Enter the branch only if the original value is 0.
if (Interlocked.CompareExchange(ref _isDisposed, 1, 0) == 0)
{
_isDisposed = true;

if (disposing)
{
// Dispose managed state.
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13 changes: 8 additions & 5 deletions samples/snippets/csharp/VS_Snippets_CLR_System/system.idisposable/cs/safe.cs
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@@ -1,6 +1,7 @@
using System;
using System.IO;
using System.Runtime.InteropServices;
using System.Threading;
using Microsoft.Win32.SafeHandles;

// Wraps the IntPtr allocated by Marshal.AllocHGlobal() into a SafeHandle.
Expand All @@ -27,8 +28,10 @@ public static LocalAllocHandle Allocate(int numberOfBytes)

public class DisposableBaseWithSafeHandle : IDisposable
{
// Detect redundant Dispose() calls.
private bool _isDisposed;
// Detect redundant Dispose() calls in a thread-safe manner.
// _isDisposed == 0 means Dispose(bool) has not been called yet.
// _isDisposed == 1 means Dispose(bool) has been already called.
private int _isDisposed;

// Managed disposable objects owned by this class
private LocalAllocHandle? _safeHandle = LocalAllocHandle.Allocate(10);
Expand All @@ -44,10 +47,10 @@ public void Dispose()
// Protected implementation of Dispose pattern.
protected virtual void Dispose(bool disposing)
{
if (!_isDisposed)
// In case _isDisposed is 0, atomically set it to 1.
// Enter the branch only if the original value is 0.
if (Interlocked.CompareExchange(ref _isDisposed, 1, 0) == 0)
{
_isDisposed = true;

if (disposing)
{
// Dispose managed state.
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13 changes: 8 additions & 5 deletions samples/snippets/visualbasic/VS_Snippets_CLR_System/system.idisposable/vb/base1.vb
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Original file line number Diff line number Diff line change
@@ -1,10 +1,13 @@
Imports System.IO
Imports System.Threading

Public Class DisposableBase
Implements IDisposable

' Detect redundant Dispose() calls.
Private _isDisposed As Boolean
' Detect redundant Dispose() calls in a thread-safe manner.
' _isDisposed = 0 means Dispose(bool) has not been called yet.
' _isDisposed = 1 means Dispose(bool) has been already called.
Private _isDisposed As Integer

' Instantiate a disposable object owned by this class.
Private _managedResource As Stream = New MemoryStream()
Expand All @@ -17,9 +20,9 @@ Public Class DisposableBase

' Protected implementation of Dispose pattern.
Protected Overridable Sub Dispose(disposing As Boolean)
If Not _isDisposed Then
_isDisposed = True

' In case _isDisposed is 0, atomically set it to 1.
' Enter the branch only if the original value is 0.
If Interlocked.CompareExchange(_isDisposed, 1, 0) = 0 Then
If disposing Then
' Dispose managed state.
_managedResource?.Dispose()
Expand Down
13 changes: 8 additions & 5 deletions samples/snippets/visualbasic/VS_Snippets_CLR_System/system.idisposable/vb/safe.vb
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
Imports System
Imports System.IO
Imports System.Runtime.InteropServices
Imports System.Threading
Imports Microsoft.Win32.SafeHandles

' Wraps the IntPtr allocated by Marshal.AllocHGlobal() into a SafeHandle.
Expand Down Expand Up @@ -29,8 +30,10 @@ End Class
Public Class DisposableBaseWithSafeHandle
Implements IDisposable

' Detect redundant Dispose() calls.
Private _isDisposed As Boolean
' Detect redundant Dispose() calls in a thread-safe manner.
' _isDisposed = 0 means Dispose(bool) has not been called yet.
' _isDisposed = 1 means Dispose(bool) has been already called.
Private _isDisposed As Integer

' Managed disposable objects owned by this class
Private _safeHandle As LocalAllocHandle = LocalAllocHandle.Allocate(10)
Expand All @@ -44,9 +47,9 @@ Public Class DisposableBaseWithSafeHandle

' Protected implementation of Dispose pattern.
Protected Overridable Sub Dispose(disposing As Boolean)
If Not _isDisposed Then
_isDisposed = True

' In case _isDisposed is 0, atomically set it to 1.
' Enter the branch only if the original value is 0.
If Interlocked.CompareExchange(_isDisposed, 1, 0) = 0 Then
If disposing Then
' Dispose managed state.
_otherUnmanagedResource?.Dispose()
Expand Down