Say I have a non-sealed class that does not deal with any unmanaged resources. I need to make a single async call during its disposing stage to do some clean up. There are no other managed resources to deal with.
From what I understand, in order to make the async clean up call, I must implement IAsyncDisposable and use the DisposeAsync() and DisposeAsyncCore() methods. But the guidance says that you should also implement the dispose pattern when you implement the async dispose pattern. This is all fine but there's nothing really I need to do in the Dispose().
So my question is, should the Dispose() logic be empty or do I need something to do the async cleanup in a synchronous way? (see comment in code about "What if anything should go here").
public class MyClass : IDisposable, IAsyncDisposable
{
private bool disposed;
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
public async ValueTask DisposeAsync()
{
await DisposeAsyncCore().ConfigureAwait(false);
Dispose(false);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
if (!disposed)
{
if (disposing)
{
// What if anything should go here?
}
disposed = true;
}
}
protected virtual async ValueTask DisposeAsyncCore()
{
// Make async cleanup call here e.g. Database.CleanupAsync();
}
}
Example to those who still hesitate to implement both:
internal class Program
{
static void Main(string[] args)
{
foreach (var a in new B()){}
//IAsyncDisposable is not called - you leaking resources.
//No deadlocks in UI, no warning in compilation, nothing.
//So it is better to be on safe side and implement both
//because you never know how one will manage lifetime of your class.
}
public class B : IEnumerable, IAsyncEnumerable<object>
{
public IEnumerator GetEnumerator() => new A();
public IAsyncEnumerator<object> GetAsyncEnumerator(CancellationToken ct) => new A();
}
public class A : IAsyncEnumerator<object>, IEnumerator
{
public ValueTask DisposeAsync()
{
Console.WriteLine("Async Disposed");
return ValueTask.CompletedTask;
}
public bool MoveNext() => false;
public void Reset(){}
public ValueTask<bool> MoveNextAsync() => ValueTask.FromResult(false);
public object Current => null;
}
}
Conclusion
You can freely add support for async version only, but beware: some wraps, like foreach or older versions of DI containers (Ninject, StructureMap, etc), code generators like RestSharp, or proxy generators like Castle.Proxy might not support IAsyncDisposable. Failing to cast object to IDisposable will present hard to catch bugs in your app. Whereas if you do implement it, the worst thing that could happen is deadlock in finally block (if you do it through sync-over-async).
In general, it is better to support both operations if you plan to make it public API or you don't have control over your class lifetime (like in DI containers or other widely known wrappers).
How to
There is full Microsoft example on how to implement both of them in inheritable class (non-sealed, like in your example) - https://learn.microsoft.com/en-us/dotnet/standard/garbage-collection/implementing-disposeasync#implement-both-dispose-and-async-dispose-patterns
class ExampleConjunctiveDisposableusing : IDisposable, IAsyncDisposable
{
IDisposable? _disposableResource = new MemoryStream();
IAsyncDisposable? _asyncDisposableResource = new MemoryStream();
public void Dispose()
{
Dispose(disposing: true);
GC.SuppressFinalize(this);
}
public async ValueTask DisposeAsync()
{
await DisposeAsyncCore().ConfigureAwait(false);
Dispose(disposing: false);
#pragma warning disable CA1816 // Dispose methods should call SuppressFinalize
GC.SuppressFinalize(this);
#pragma warning restore CA1816 // Dispose methods should call SuppressFinalize
}
protected virtual void Dispose(bool disposing)
{
if (disposing)
{
_disposableResource?.Dispose();
(_asyncDisposableResource as IDisposable)?.Dispose();
_disposableResource = null;
_asyncDisposableResource = null;
}
}
protected virtual async ValueTask DisposeAsyncCore()
{
if (_asyncDisposableResource is not null)
{
await _asyncDisposableResource.DisposeAsync().ConfigureAwait(false);
}
if (_disposableResource is IAsyncDisposable disposable)
{
await disposable.DisposeAsync().ConfigureAwait(false);
}
else
{
_disposableResource?.Dispose();
}
_asyncDisposableResource = null;
_disposableResource = null;
}
}
Both implementations of dispose feature is from the callers point of view. Your class would then offer both mechanisms to dispose off any managed and unmanaged resources and the caller application decides what to choose. This also ensures that any consumer which is unable to make use of asynchronous patterns are not lost.
You do not really need to implement synchronous dispose if you are sure about or want to force asynchronous consumption of your class.
So depending on your vision of class usage, you can choose how to dispose objects. If you choose to keep both mechanisms, you can dispose all resources both ways.
As you have said, the class is non-sealed. For sealed classes, it's enough to implement I(Async)Disposable interface. The Disposable pattern exists because the derived class may want to add cleanup logic that can be either sync or async. You can't know. That's why you need to implement the whole pattern for sync and async cases.
For your question. Never block async call in sync Dispose method. It's a caller's responsibility to use your class correctly. If he decides to call Dispose instead of DisposeAsync and clear only sync resources it's his decision/mistake. If this async call in DisposeAsync is absolutely necessary for proper cleanup and it is controlled by you, consider adding sync equivalent to be used in Dispose method.
Related
Microsoft provides a refence implementation for classes that need to implement both IDisposable and IAsyncDisposable, say because they have members of both of these.
https://learn.microsoft.com/en-us/dotnet/standard/garbage-collection/implementing-disposeasync
I include the full implementation below. These lines in Dispose(disposing) are the part I don't understand.
(_asyncDisposableResource as IDisposable)?.Dispose();
...
_asyncDisposableResource = null;
It seems like if I have an instance of CustomDisposable and Dispose() gets called on it before DisposeAsync() then the _asyncDispoableResource field will have Dispose() called on it instead of DisposeAsync (if it has one), and then get set to null unconditionally. Seems like _asyncDispoableResource never gets disposed of properly in this case, even if DisposeAsync() gets called on the CustomDisposable instance later.
Full reference code:
using System;
using System.IO;
using System.Threading.Tasks;
namespace Samples
{
public class CustomDisposable : IDisposable, IAsyncDisposable
{
IDisposable _disposableResource = new MemoryStream();
IAsyncDisposable _asyncDisposableResource = new MemoryStream();
public void Dispose()
{
Dispose(disposing: true);
GC.SuppressFinalize(this);
}
public async ValueTask DisposeAsync()
{
await DisposeAsyncCore();
Dispose(disposing: false);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
if (disposing)
{
_disposableResource?.Dispose();
(_asyncDisposableResource as IDisposable)?.Dispose();
}
_disposableResource = null;
_asyncDisposableResource = null;
}
protected virtual async ValueTask DisposeAsyncCore()
{
if (_asyncDisposableResource is not null)
{
await _asyncDisposableResource.DisposeAsync().ConfigureAwait(false);
}
if (_disposableResource is IAsyncDisposable disposable)
{
await disposable.DisposeAsync().ConfigureAwait(false);
}
else
{
_disposableResource.Dispose();
}
_asyncDisposableResource = null;
_disposableResource = null;
}
}
}
Quoting your reference:
It is typical when implementing the IAsyncDisposable interface that classes will also implement the IDisposable interface. A good implementation pattern of the IAsyncDisposable interface is to be prepared for either synchronous or asynchronous dispose.
This means that it should be sufficient to dispose an object via "classic" Dispose, it should not matter that DisposeAsync is never called. This is why the resource is set to null, so it is clear that it has already been disposed asynchonously. The actual implementation of the disposable reference has to be prepared for this.
I am working with asp.net core signalR. I made a HubClientBase (which is a client for communicating with the signalR hub/server). It uses an HubConnection object to send/receive data in the derived classes.
public abstract class HubClientBase : IDisposable, IAsyncDisposable
{
#region Properties and fields
protected readonly ILogger<HubClientBase> Logger;
protected readonly HubConnection Connection;
#endregion
...
// Start connection
// Some other stuff
}
// Derived:
public class AlarmHubClient : HubClientBase, IAlarmHubClient, IAlarmHub
{
/// <inheritdoc />
public AlarmHubClient(ILogger<HubClientBase> logger, HubConnection connection)
: base(logger, connection)
{
}
protected override void AttachReceiveHandlers(HubConnection connection)
{
connection.On<AlarmBarMessageDto>(nameof(ReceiveCurrentAlarmBarMessage), ReceiveCurrentAlarmBarMessage);
...
}
#region Implementation of IAlarmHubClient
/// <inheritdoc />
public async Task SendCurrentAlarmBarMessage(AlarmBarMessageDto alarmBarMessage)
{
await Connection.SendAsync(nameof(SendCurrentAlarmBarMessage), alarmBarMessage);
}
I run into 2 problems pertaining disposale however.
I use the client in a .NET 4.6.1 project. The dll gets loaded into a WPF application. This means, that .Dispose() is called by the UI thread. The hubconnection only has a .DisposeAsync implementation, so i need to call that from the .Dispose method. We follow a hierachical dispose pattern, so i need to be sure the dispose is blocking until it is done. I however cannot Wait() in the dispose, since that would block the ui thread (a deadlock occurs, where the task will be scheduled to run on the main ui thread, but the thread cannot do any work because its waiting)
This means, I have to use Task.Run - which is far from an ideal solution...
#region IDisposable
protected virtual void Dispose(bool disposing)
{
if (disposing)
{
if (Connection != null)
{
// Force the async method on the thread pool with Task.Run.
// WARN A library / base should 99.9% of the time never use Task.Run, since the creation of threads should be a responsibility of the application level.
Task.Run(async () => await Connection.DisposeAsync())
.ConfigureAwait(true).GetAwaiter().GetResult();
}
}
}
/// <inheritdoc />
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
#endregion
What would be a better way to go about Async disposing a member in the sync dispose?
I tried using IAsyncDisposable aswell, so I can maybe work some magic upstream and use that instead of Dispose(). I came up with the following, but i found ample documentation about how to correctly use IAsyncDispose. especially since this is a baseclass that will be derived from:
#region Implementation of IAsyncDisposable
/// <inheritdoc />
public ValueTask DisposeAsync()
{
try
{
return new ValueTask(Connection.DisposeAsync());
}
catch (Exception exc)
{
return new ValueTask(Task.FromException(exc));
}
}
#endregion
Is this a good way of async dispoing a member in the async dispose? Any recommendations for derived classes?
I've been familiarizing myself with some of the things (that are planned to be) added in C# 8 & .NET Core 3.0, and am unsure on the correct way to implement IAsyncDisposable (at time of writing, this link has literally no guidance).
In particular, it is unclear to me what to do in the case when an instance isn't explicitly disposed - that is, it isn't wrapped in an async using(...) and .DisposeAsync() isn't explicitly called.
My first thought was to do that same thing I'd do when implementing IDisposable:
My DisposeAsync() implementation calls a DisposeAsync(bool disposing) with disposing: true
Implement a finalizer (with ~MyType()) that calls DisposeAsync(disposing: false)
DisposeAsync(bool disposing) actually frees and/or disposes everything, and suppresses finalizing if disposing == true.
My concerns are that there's nothing to await the results of DisposeAsync(bool) in the finalizer, and explicitly waiting in a finalizer seems really really dangerous.
Of course "just leak" also seems less than ideal.
To make this concrete, here's a (simplified) example class that does have a finalizer:
internal sealed class TestAsyncReader: IAsyncDisposable
{
private bool IsDisposed => Inner == null;
private TextReader Inner;
internal TestAsyncReader(TextReader inner)
{
Inner = inner;
}
// the question is, should this exist?
~TestAsyncReader()
{
DisposeAsync(disposing: false);
}
private ValueTask DisposeAsync(bool disposing)
{
// double dispose is legal, but try and do nothing anyway
if (IsDisposed)
{
return default;
}
// should a finalizer even exist?
if (disposing)
{
GC.SuppressFinalize(this);
}
// in real code some resources explicitly implement IAsyncDisposable,
// but for illustration purposes this code has an interface test
if (Inner is IAsyncDisposable supportsAsync)
{
var ret = supportsAsync.DisposeAsync();
Inner = null;
return ret;
}
// dispose synchronously, which is uninteresting
Inner.Dispose();
Inner = null;
return default;
}
public ValueTask DisposeAsync()
=> DisposeAsync(disposing: true);
}
So, is there any guidance around proper handling of leaked IAsyncDisposable instances?
Basing on examples of how it's implemented inside .NET Core classes (like here) and some recommendations from there, I'd say that when you need to implement IAsyncDisposable, the good practice would be to implement both IAsyncDisposable and IDisposable. In this case IAsyncDisposable will be only responsible for explicit scenarios when asyncronus disposal is needed, while IDisposable is supposed to be implemented as usual according to disposable pattern practices and it's going to serve all fallback scenarios including those when things come to finalization. Thus you don't need to have anything like DisposeAsync(bool disposing) - the asynchronous disposal cannot and shouldn't happen in a finalizer. The only bad news that you'll have to support both paths for resources reclaiming (synchronous and asynchronous).
Microsoft released their own guidance regarding this problem.
As in the accepted answer you should implement both interfaces
If you implement the IAsyncDisposable interface but not the IDisposable interface, your app can potentially leak resources. If a class implements IAsyncDisposable, but not IDisposable, and a consumer only calls Dispose, your implementation would never call DisposeAsync. This would result in a resource leak.
But also you may need to implement 2 dispose patterns:
using System;
using System.IO;
using System.Threading.Tasks;
class ExampleConjunctiveDisposableusing : IDisposable, IAsyncDisposable
{
IDisposable? _disposableResource = new MemoryStream();
IAsyncDisposable? _asyncDisposableResource = new MemoryStream();
public void Dispose()
{
Dispose(disposing: true);
GC.SuppressFinalize(this);
}
public async ValueTask DisposeAsync()
{
await DisposeAsyncCore().ConfigureAwait(false);
Dispose(disposing: false);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
if (disposing)
{
_disposableResource?.Dispose();
(_asyncDisposableResource as IDisposable)?.Dispose();
_disposableResource = null;
_asyncDisposableResource = null;
}
}
protected virtual async ValueTask DisposeAsyncCore()
{
if (_asyncDisposableResource is not null)
{
await _asyncDisposableResource.DisposeAsync();
}
if (_disposableResource is IAsyncDisposable disposable)
{
await disposable.DisposeAsync();
}
else
{
_disposableResource?.Dispose();
}
_asyncDisposableResource = null;
_disposableResource = null;
}
}
Forgive me in advance if this question is a little too open-ended, but I've seen similar language discussion posts here so I figured I'd take the plunge.
Anyway, I have read several MSDN help pages and various other blogs on the subject of properly implementing IDisposable classes. I feel like I understand things pretty well, but I have to wonder if there's a flaw in the suggested class structure:
public class DisposableBase : IDisposable
{
private bool mDisposed;
~DisposableBase()
{
Dispose(false);
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
if (!mDisposed)
{
if (disposing)
{
// Dispose managed resources
mManagedObject.Dispose();
}
// Dispose unmanaged resources
CloseHandle(mUnmanagedHandle);
mUnmanagedHandle = IntPtr.Zero;
mDisposed = true;
}
}
}
Anytime the above is supposed to serve as a base class, you rely on the implementer of the subclass to properly override the Dispose(bool) method where necessary. In short, derived classes must ensure they invoke the base Dispose(bool) method from within their overridden version. If not, the base class' unmanaged resources may never get freed, defeating the primary purpose of the IDisposable interface.
We all know the benefits of virtual methods, but it seems like in this case their design falls short. In fact, I think this particular shortcoming of virtual methods manifests itself frequently when trying to design visual components and similar base/derived class structures.
Consider the following change, using a protected event rather than a protected virtual method:
public class DisposeEventArgs : EventArgs
{
public bool Disposing { get; protected set; }
public DisposeEventArgs(bool disposing)
{
Disposing = disposing;
}
}
public class DisposableBase : IDisposable
{
private bool mDisposed;
protected event EventHandler<DisposeEventArgs> Disposing;
~DisposableBase()
{
Dispose(false);
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
// This method is now private rather than protected virtual
private void Dispose(bool disposing)
{
if (!mDisposed)
{
// Allow subclasses to react to disposing event
AtDisposing(new DisposeEventArgs(disposing));
if (disposing)
{
// Dispose managed resources
mManagedObject.Dispose();
}
// Dispose unmanaged resources
CloseHandle(mUnmanagedHandle);
mUnmanagedHandle = IntPtr.Zero;
mDisposed = true;
}
}
private void AtDisposing(DisposeEventArgs args)
{
try
{
EventHandler<DisposeEventArgs> handler = Disposing;
if (handler != null) handler(this, args);
}
catch
{
}
}
}
With this design, the base class' Dispose(bool) method will always be called, regardless of whether subclasses subscribe to the Disposing event or not. The biggest flaw that I can see with this revised setup is that there is no predetermined order for when event listeners are called. This could be problematic if there are multiple levels of inheritance, e.g. SubclassA's listener might be triggered before its child SubclassB's listener. Is this flaw serious enough to invalidate my revised design?
This design dilemma makes me wish there were some sort of modifier for methods that was similar to virtual but which would ensure that the base class' method was always called, even if a subclass overrode that function. If there's a better way to achieve this, I would greatly appreciate your suggestions.
You're using an event here when really you want to use an inheritance mechanism like virtual. For scenarios like this where I want to ensure my implementation is always called but want to allow for base class customization I use the following pattern
private void Dispose(bool disposing)
if (mDisposed) {
return;
}
if (disposing) {
mManagedObject.Dispose();
}
// Dispose unmanaged resources
CloseHandle(mUnmanagedHandle);
mUnmanagedHandle = IntPtr.Zero;
mDisposed = true;
DisposeCore(disposing);
}
protected virtual void DisposeCore(bool disposing) {
// Do nothing by default
}
With this pattern I've ensured my base class Dispose implementation will always be called. Derived classes can't stop me by simply forgetting to call a base method. They can still opt into the dispose pattern by overriding DisposeCore but they can't break the base class contract.
The derived class can simply re-implement IDisposable and thus prevent your dispose method from being called, so you can't ensure that either.
Personally I wouldn't use either pattern. I prefer building on SafeHandle and similar mechanisms, instead of implementing finalizers myself.
Consider making it apparent that Dispose is not being called so someone will catch it. Of course Debug.WriteLine will only be called when the code is compiled with DEBUG compiler directive defined.
public class DisposableBase : IDisposable
{
private bool mDisposed;
~DisposableBase()
{
if (!mDisposed)
System.Diagnostics.Debug.WriteLine ("Object not disposed: " + this + "(" + GetHashCode() + ")";
Dispose(false);
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
You can break it down:
A destructor (finalizer) is only needed for unmanaged resources.
Using a Safehandle can turn an unmanged resource into a managed resource.
Ergo: You won't need a destructor. That halves the Dispose pattern.
The reference design uses a virtual void Dispose(bool) to cater for the Base/Derived class problem. This puts the burden on the derived class to call base.Dispose(disposing), the core of your question. I use 2 approaches:
1) Prevent it. With a sealed base-class you won't have to worry.
sealed class Foo:IDisposable
{
void Dispose() { _member.Dispose(); }
}
2) Check it. Like #j-agent's answer but conditional. When performance could be an issue then you don't want the finalizers in Production code:
class Foo:IDisposable
{
void Dispose() { Dispose(true); }
[Conditional("TEST")] // or "DEBUG"
~Foo { throw new InvalidOperation("somebody forgot to Dispose") }
}
The destructor is going to be called no matter if any subclass overrides Dispose() (can be via override or new) but your destructor is going to be called ( ~DisposableBase() ) so i bet putting your logic for cleanup there can be a good starting point.
Here is an intersting article about destructors: http://www.c-sharpcorner.com/UploadFile/chandrahundigam/UnderstandingDestructors11192005021208AM/UnderstandingDestructors.aspx
I am using Quartz.NET in an application. What is the proper way to dispose of Quartz.NET.
Right now I am just doing
if (_quartzScheduler != null)
{
_quartzScheduler = null;
}
Is that enough or should I implement a dispose or something in the jobType class?
Seth
scheduler.Shutdown(waitForJobsToComplete: true);
Of course, if you're not on C# 4.0 yet, named parameters don't work:
scheduler.Shutdown(true);
This is not a complete example but might get you on the right path. I would implement something like this:
class customSchedulerClass : IDisposable
{
private Component component = new Component();
private bool disposed = false;
public void scheduleSomeStuff()
{
//This is where you would implement the Quartz.net stuff
}
public void Dispose()
{
Dispose(true);
GC.SupressFinalize(this);
}
private void Dispose(bool disposing)
{
if(!this=disposed)
{
if(disposing)
{
component.dispose;
}
}
disposed = true;
}
}
Then with this you can do cool stuff like using statements:
public static void Main()
{
using (customSchedulerClass myScheduler = new customSchedulerClass())
{
c.scheduleSomeStuff();
}
console.WriteLine("Now that you're out of the using statement the resources have been disposed");
}
So basically by implementing you code while inheriting the functionality of IDisposable you can then us the using statement and when you're done it will cleanly dispose your resources and keep things nice and clean. (Disclaimer, again this is not a complete example, just to get you in the right direction).
The docs don't say anything about IScheduler implementing IDisposable. If you have custom job types that grab and hold resources (file locks, database connections), you can implement IDispoable and override Dispose() on your object to release resources.
Generally we don't need to set an object to null in order to dispose it off.
If an object contains unmanaged resources then it should implement IDisposable (and be called by all its clients).
You can refere this similar post.