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Implement a Dispose method

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The Dispose method is primarily implemented to release unmanaged resources. When working with instance members that are IDisposable implementations, it's common to cascade Dispose calls. There are other reasons for implementing Dispose, for example, to free memory that was allocated, remove an item that was added to a collection, or signal the release of a lock that was acquired.

The .NET garbage collector doesn't allocate or release unmanaged memory. The pattern for disposing an object, referred to as the dispose pattern, imposes order on the lifetime of an object. The dispose pattern is used for objects that implement the IDisposable interface. This pattern is common when interacting with file and pipe handles, registry handles, wait handles, or pointers to blocks of unmanaged memory, because the garbage collector is unable to reclaim unmanaged objects.

To help ensure that resources are always cleaned up appropriately, a Dispose method should be idempotent, such that it's callable multiple times without throwing an exception. Furthermore, subsequent invocations of Dispose should do nothing.

The code example provided for the GC.KeepAlive method shows how garbage collection can cause a finalizer to run while an unmanaged reference to the object or its members is still in use. It may make sense to utilize GC.KeepAlive to make the object ineligible for garbage collection from the start of the current routine to the point where this method is called.

Tip

With regard to dependency injection, when registering services in an IServiceCollection, the service lifetime is managed implicitly on your behalf. The IServiceProvider and corresponding IHost orchestrate resource cleanup. Specifically, implementations of IDisposable and IAsyncDisposable are properly disposed at the end of their specified lifetime.

For more information, see Dependency injection in .NET.

Cascade dispose calls

If your class owns an instance of another type that implements IDisposable, the containing class itself should also implement IDisposable. Typically a class that instantiates an IDisposable implementation and stores it as an instance member (or property) is also responsible for its cleanup. This helps ensure that the referenced disposable types are given the opportunity to deterministically perform cleanup through the Dispose method. In the following example, the class is sealed (or NotInheritable in Visual Basic).

C# 
using System;

public sealed class Foo : IDisposable
{
    private readonly IDisposable _bar;

    public Foo()
    {
        _bar = new Bar();
    }

    public void Dispose() => _bar.Dispose();
}

Tip

  • If your class has an IDisposable field or property but doesn't own it, then the class doesn't need to implement IDisposable. Typically a class creating and storing the IDisposable child object also becomes the owner, but in some cases the ownership can be transferred to another IDisposable type.
  • There are cases when you might want to perform null-checking in a finalizer (which includes the Dispose(false) method invoked by a finalizer). One of the primary reasons is if you're unsure whether the instance got fully initialized (for example, an exception might be thrown in a constructor).

Dispose() and Dispose(bool)

The IDisposable interface requires the implementation of a single parameterless method, Dispose. Also, any non-sealed class should have an Dispose(bool) overload method.

Method signatures are:

  • public non-virtual (NotOverridable in Visual Basic) (IDisposable.Dispose implementation).
  • protected virtual (Overridable in Visual Basic) Dispose(bool).

The Dispose() method

Because the public, non-virtual (NotOverridable in Visual Basic), parameterless Dispose method is called when it's no longer needed (by a consumer of the type), its purpose is to free unmanaged resources, perform general cleanup, and to indicate that the finalizer, if one is present, doesn't have to run. Freeing the actual memory associated with a managed object is always the domain of the garbage collector. Because of this, it has a standard implementation:

C# 
public void Dispose()
{
    // Dispose of unmanaged resources.
    Dispose(true);
    // Suppress finalization.
    GC.SuppressFinalize(this);
}

The Dispose method performs all object cleanup, so the garbage collector no longer needs to call the objects' Object.Finalize override. Therefore, the call to the SuppressFinalize method prevents the garbage collector from running the finalizer. If the type has no finalizer, the call to GC.SuppressFinalize has no effect. The actual cleanup is performed by the Dispose(bool) method overload.

The Dispose(bool) method overload

In the overload, the disposing parameter is a Boolean that indicates whether the method call comes from a Dispose method (its value is true) or from a finalizer (its value is false).

C# 
protected virtual void Dispose(bool disposing)
{
    if (_disposed)
    {
        return;
    }

    if (disposing)
    {
        // Dispose managed state (managed objects).
        // ...
    }

    // Free unmanaged resources.
    // ...

    _disposed = true;
}

Important

The disposing parameter should be false when called from a finalizer, and true when called from the IDisposable.Dispose method. In other words, it is true when deterministically called and false when nondeterministically called.

The body of the method consists of three blocks of code:

  • A block for conditional return if object is already disposed.

  • A conditional block that frees managed resources. This block executes if the value of disposing is true. The managed resources that it frees can include:

    • Managed objects that implement IDisposable. The conditional block can be used to call their Dispose implementation (cascade dispose). If you have used a derived class of System.Runtime.InteropServices.SafeHandle to wrap your unmanaged resource, you should call the SafeHandle.Dispose() implementation here.
    • Managed objects that consume large amounts of memory or consume scarce resources. Assign large managed object references to null to make them more likely to be unreachable. This releases them faster than if they were reclaimed nondeterministically.

  • A block that frees unmanaged resources. This block executes regardless of the value of the disposing parameter.

If the method call comes from a finalizer, only the code that frees unmanaged resources should execute. The implementer is responsible for ensuring that the false path doesn't interact with managed objects that may have been disposed. This is important because the order in which the garbage collector disposes managed objects during finalization is nondeterministic.

Implement the dispose pattern

All non-sealed classes (or Visual Basic classes not modified as NotInheritable) should be considered a potential base class, because they could be inherited. If you implement the dispose pattern for any potential base class, you must add the following methods to your class:

  • A Dispose implementation that calls the Dispose(bool) method.
  • A Dispose(bool) method that performs the actual cleanup.
  • If your class deals with unmanaged resources, either provide an override to the Object.Finalize method or wrap the unmanaged resource in a SafeHandle.

Important

A finalizer (a Object.Finalize override) is only required if you directly reference unmanaged resources. This is a highly advanced scenario that can be typically avoided:

  • If your class references only managed objects, it's still possible for the class to implement the dispose pattern. There's no need to implement a finalizer.
  • If you need to deal with unmanaged resources, we strongly recommend wrapping the unmanaged IntPtr handle into a SafeHandle. The SafeHandle provides a finalizer so you don't have to write one yourself. For more information, see the Safe handles paragraph.

Base class with managed resources

Here's a general example of implementing the dispose pattern for a base class that only owns managed resources.

C# 
using System;
using System.IO;

public class DisposableBase : IDisposable
{
    // Detect redundant Dispose() calls.
    private bool _isDisposed;

    // Instantiate a disposable object owned by this class.
    private Stream? _managedResource = new MemoryStream();

    // Public implementation of Dispose pattern callable by consumers.
    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    // Protected implementation of Dispose pattern.
    protected virtual void Dispose(bool disposing)
    {
        if (!_isDisposed)
        {
            _isDisposed = true;

            if (disposing)
            {
                // Dispose managed state.
                _managedResource?.Dispose();
                _managedResource = null;
            }
        }
    }
}

Note

The previous example uses a dummy MemoryStream object to illustrate the pattern. Any IDisposable could be used instead.

Base class with unmanaged resources

Here's an example for implementing the dispose pattern for a base class that overrides Object.Finalize in order to clean up unmanaged resources it owns. The example also demonstrates a way to implement Dispose(bool) in a thread-safe manner. Synchronization might be critical when dealing with unmanaged resources in a multi-threaded application. As mentioned earlier, this is an advanced scenario.

C# 
using System;
using System.IO;
using System.Runtime.InteropServices;
using System.Threading;

public class DisposableBaseWithFinalizer : IDisposable
{
    // 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();

    // A pointer to 10 bytes allocated on the unmanaged heap.
    private IntPtr _unmanagedResource = Marshal.AllocHGlobal(10);

    ~DisposableBaseWithFinalizer() => Dispose(false);

    // Public implementation of Dispose pattern callable by consumers.
    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    // Protected implementation of Dispose pattern.
    protected virtual void Dispose(bool disposing)
    {
        // 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)
        {
            if (disposing)
            {
                _managedResource?.Dispose();
                _managedResource = null;
            }

            Marshal.FreeHGlobal(_unmanagedResource);
        }
    }
}

Note

  • The previous example uses AllocHGlobal to allocate 10 bytes on the unmanaged heap in the constructor and free the buffer in Dispose(bool) by calling FreeHGlobal. This is a dummy allocation for illustrational purpose.
  • Again, it's recommended to avoid implementing a finalizer. See Implement the dispose pattern using a custom safe handle for an equivalent of the previous example that delegates non-deterministic finalization and synchronization to SafeHandle.

Tip

In C#, you implement a finalization by providing a finalizer, not by overriding Object.Finalize. In Visual Basic, you create a finalizer with Protected Overrides Sub Finalize().

Implement the dispose pattern for a derived class

A class derived from a class that implements the IDisposable interface shouldn't implement IDisposable, because the base class implementation of IDisposable.Dispose is inherited by its derived classes. Instead, to clean up a derived class, you provide the following:

  • A protected override void Dispose(bool) method that overrides the base class method and performs the actual cleanup of the derived class. This method must also call the base.Dispose(bool) (MyBase.Dispose(bool) in Visual Basic) method passing it the disposing status (bool disposing parameter) as an argument.
  • Either a class derived from SafeHandle that wraps your unmanaged resource (recommended), or an override to the Object.Finalize method. The SafeHandle class provides a finalizer that frees you from having to code one. If you do provide a finalizer, it must call the Dispose(bool) overload with false argument.

Here's an example of the general pattern for implementing the dispose pattern for a derived class that uses a safe handle:

C# 
using System.IO;

public class DisposableDerived : DisposableBase
{
    // To detect redundant calls
    private bool _isDisposed;

    // Instantiate a disposable object owned by this class.
    private Stream? _managedResource = new MemoryStream();

    // Protected implementation of Dispose pattern.
    protected override void Dispose(bool disposing)
    {
        if (!_isDisposed)
        {
            _isDisposed = true;

            if (disposing)
            {
                _managedResource?.Dispose();
                _managedResource = null;
            }
        }

        // Call base class implementation.
        base.Dispose(disposing);
    }
}

Note

The previous example uses a SafeFileHandle object to illustrate the pattern; any object derived from SafeHandle could be used instead. Note that the example does not properly instantiate its SafeFileHandle object.

Here's the general pattern for implementing the dispose pattern for a derived class that overrides Object.Finalize:

C# 
using System.Threading;

public class DisposableDerivedWithFinalizer : DisposableBaseWithFinalizer
{
    // 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;

    ~DisposableDerivedWithFinalizer() => Dispose(false);

    // Protected implementation of Dispose pattern.
    protected override void Dispose(bool disposing)
    {
        // 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)
        {
            if (disposing)
            {
                // TODO: dispose managed state (managed objects).
            }

            // TODO: free unmanaged resources (unmanaged objects) and override a finalizer below.
            // TODO: set large fields to null.
        }

        // Call the base class implementation.
        base.Dispose(disposing);
    }
}

Safe handles

Writing code for an object's finalizer is a complex task that can cause problems if not done correctly. Therefore, we recommend that you construct System.Runtime.InteropServices.SafeHandle objects instead of implementing a finalizer.

A System.Runtime.InteropServices.SafeHandle is an abstract managed type that wraps an System.IntPtr that identifies an unmanaged resource. On Windows it might identify a handle, and on Unix, a file descriptor. The SafeHandle provides all of the logic necessary to ensure that this resource is released once and only once, either when the SafeHandle is disposed of or when all references to the SafeHandle have been dropped and the SafeHandle instance is finalized.

The System.Runtime.InteropServices.SafeHandle is an abstract base class. Derived classes provide specific instances for different kinds of handle. These derived classes validate what values for the System.IntPtr are considered invalid and how to actually free the handle. For example, SafeFileHandle derives from SafeHandle to wrap IntPtrs that identify open file handles and descriptors, and overrides its SafeHandle.ReleaseHandle() method to close it (via the close function on Unix or CloseHandle function on Windows). Most APIs in .NET libraries that create an unmanaged resource wrap it in a SafeHandle and return that SafeHandle to you as needed, rather than handing back the raw pointer. In situations where you interact with an unmanaged component and get an IntPtr for an unmanaged resource, you can create your own SafeHandle type to wrap it. As a result, few non-SafeHandle types need to implement finalizers. Most disposable pattern implementations only end up wrapping other managed resources, some of which might be SafeHandle objects.

Implement the dispose pattern using a custom safe handle

The following code demonstrates how to handle unmanaged resources by implementing a SafeHandle.

C# 
using System;
using System.IO;
using System.Runtime.InteropServices;
using Microsoft.Win32.SafeHandles;

// Wraps the IntPtr allocated by Marshal.AllocHGlobal() into a SafeHandle.
class LocalAllocHandle : SafeHandleZeroOrMinusOneIsInvalid
{
    private LocalAllocHandle() : base(ownsHandle: true) { }

    // No need to implement a finalizer - SafeHandle's finalizer will call ReleaseHandle for you.
    protected override bool ReleaseHandle()
    {
        Marshal.FreeHGlobal(handle);
        return true;
    }

    // Allocate bytes with Marshal.AllocHGlobal() and wrap the result into a SafeHandle.
    public static LocalAllocHandle Allocate(int numberOfBytes)
    {
        IntPtr nativeHandle = Marshal.AllocHGlobal(numberOfBytes);
        LocalAllocHandle safeHandle = new LocalAllocHandle();
        safeHandle.SetHandle(nativeHandle);
        return safeHandle;
    }
}

public class DisposableBaseWithSafeHandle : IDisposable
{
    // Detect redundant Dispose() calls.
    private bool _isDisposed;

    // Managed disposable objects owned by this class
    private LocalAllocHandle? _safeHandle = LocalAllocHandle.Allocate(10);
    private Stream? _otherUnmanagedResource = new MemoryStream();

    // Public implementation of Dispose pattern callable by consumers.
    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    // Protected implementation of Dispose pattern.
    protected virtual void Dispose(bool disposing)
    {
        if (!_isDisposed)
        {
            _isDisposed = true;

            if (disposing)
            {
                // Dispose managed state.
                _otherUnmanagedResource?.Dispose();
                _safeHandle?.Dispose();
                _otherUnmanagedResource = null;
                _safeHandle = null;
            }
        }
    }
}

Note

The behavior of the DisposableBaseWithSafeHandle class is equivalent to the behavior of the DisposableBaseWithFinalizer class in a previous example, however the approach demonstrated here is safer:

  • There is no need to implement a finalizer, because 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, SafeHandle guarantees that SafeHandle.ReleaseHandle will be called only once.

Built-in safe handles in .NET

The following derived classes in the Microsoft.Win32.SafeHandles namespace provide safe handles.

Class Resources it holds
SafeFileHandle
SafeMemoryMappedFileHandle
SafePipeHandle		 Files, memory mapped files, and pipes
SafeMemoryMappedViewHandle Memory views
SafeNCryptKeyHandle
SafeNCryptProviderHandle
SafeNCryptSecretHandle		 Cryptography constructs
SafeRegistryHandle Registry keys
SafeWaitHandle Wait handles

See also