I wanted to have a Task implementation that whenever it is get disposed, the task associated is canceled. So I have the following naive implementation in mind
public class AutoCancelTask : Task, IDisposable
{
public static AutoCancelTask Create(Action<CancellationToken> a)
{
return new AutoCancelTask(a, new CancellationTokenSource());
}
private CancellationTokenSource cts;
internal AutoCancelTask(Action<CancellationToken> a, CancellationTokenSource cts)
:base((() => a(cts.Token)))
{ this.cts = cts; }
void IDisposable.Dispose(){
cts.Cancel();
Dispose();
}
}
To simplify I only consider the simplest Task constructor. Adding other constructors would not be hard anyway.
My concern is how the IDisposable interface is being implemented. Since Task also implements IDisposable, would the above code cause any problems in certain scenarios? More importantly, would the above code do the right thing?
Related
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.
I have a memory leak caused by the code below, probably due to undetached events, and I don't know how to properly unsubscribe from them.
I have the interface IEventsHandler, declared as singleton, which exposes some events. This interface is used inside a BackgroundService, where a subscriber is attached to the event. When an AlertEvent occurs, a scope is created to use a service which stores the event inside the database.
Here is the code:
public class MyWorker : BackgroundService
{
private readonly IEventsHandler _eventsHandler;
private readonly IServiceProvider _serviceProvider;
public MyWorker(IEventsHandler eventsHandler,
IServiceProvider serviceProvider)
{
_eventsHandler = eventsHandler;
_serviceProvider = serviceProvider;
_eventsHandler.AlertOccurred += AlertEventOccurred;
}
private async void AlertEventOccurred(object sender, AlertEvent e)
{
using var scope = _serviceProvider.CreateScope();
var service = scope.ServiceProvider.GetRequiredService<AlertService>();
await service.SaveAlert(e);
}
}
public interface IEventsHandler
{
event EventHandler<AlertEvent> AlertOccurred;
Task AddAlertEventAsync(AlertEvent ev);
}
public class EventsHandler : IEventsHandler
{
public event EventHandler<AlertEvent> AlertOccurred;
public async Task AddAlertEventAsync(AlertEvent ev)
{
await Task.Run(() => AlertOccurred?.Invoke(this, ev));
}
}
PS any advice on bad practices used in the example code is welcome :-)
Thanks
BackgroundService implements IHostedService interface which has a couple of methods named StartAsync and StopAsync.
It's better to use these methods instead of class constructor to subscribe to your events and then unsubscribe from them.
This way
public override Task StartAsync(CancellationToken cancellationToken)
{
_eventsHandler.AlertOccurred += AlertEventOccurred;
return Task.CompletedTask;
}
public override Task StopAsync(CancellationToken cancellationToken)
{
_eventsHandler.AlertOccurred -= AlertEventOccurred;
return Task.CompletedTask;
}
If this is actually the cause of your memory leak, the fix is pretty simple. Just create a destructor and unsubscribe from the event.
A destructor is the opposite of a constructor in that it gets called right before the object gets garbage collected. It's denoted by a tilde (~) preceding the same syntax as a constructor (minus the parameters). Yours would look something like this:
~MyWorker(){
_eventsHandler.AlertOccurred -= AlertEventOccurred;
}
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?
We have a generic Job class which have an abstract HeavyTask method like this:
abstract class Job {
private Task m_task;
protected abstract void HeavyTask();
public void StartJob(){
m_task = Task.Run(() => HeavyTask());
}
public async Task WaitJob(){
await m_task;
}
}
And the derived class override the HeavyTask function and also make it async:
class JobFoo : Job {
protected override async void HeavyTask()
{
await Task.Delay(1000);
Debug.WriteLine("JobFoo is done");
}
}
Then when we are using this method, it seems that the HeavyTask() is not awaited:
Job job = new JobFoo();
job.StartJob();
await job.WaitJob();
Debug.WriteLine("All Done");
Output:
All Done
JobFoo is Done
If we don't have async for the override HeavyTask, then it is working as expected. But I cannot guarantee those whose override the Job won't make the HeavyTask async. I want to understand why it is not awaited successfully and is there a way to make sure it will awaited? If you can, could you also explain whether it is a good practice to override a non-async function as async as shown above?
It's not awaited because there's no awaitable (i.e. Task) to await. That method has a void return type. And you should avoid using async void outside of event handlers.
If you want to enable a derived class to use async have the method return a Task to begin with:
protected abstract Task HeavyTaskAsync();
And if you then need to have a synchronous override return a Task synchronously:
override Task HeavyTaskAsync()
{
// do stuff;
return Task.CompletedTask;
}
I don't think this line is awaitable:
m_task = Task.Run(() => HeavyTask());
What is it spouse to wait for? No value is returned.
how about
Task.Run(() => HeavyTask()).Wait();
This question already has answers here:
Error: "Cannot use 'async' on methods without bodies". How to force async child overrides?
(2 answers)
Closed 1 year ago.
I'm unable to declare
interface IMyInterface
{
async Task<myObject> MyMethod(Object myObj);
}
The compiler tells me:
The modifier async isn't valid for this item
The async modifier can only be used for methods that have a body
Is this something that should be implemented, or does the nature of async & await prohibit this from ever occurring?
Whether a method is implemented using async/await or not is an
implementation detail. How the method should behave is a contract
detail, which should be specified in the normal way.
Note that if you make the method return a Task or a Task<T>, it's more
obvious that it's meant to be asynchronous, and will probably be hard
to implement without being asynchronous.
From https://stackoverflow.com/a/6274601/4384
Whether or not your implementation is async, has no relevance to your interface. In other words, the interface cannot specify that a given method must be implemented in an asynchronous way.
Just take async out of your interface and it will compile; however, there is no way to enforce asynchronous implementation just by specifying an interface.
If you have an interface with two implementations (one that is truly async and the other that is synchronous) this is what it would look like for each implementation - with both returning a Task<bool>.
public interface IUserManager
{
Task<bool> IsUserInRole(string roleName);
}
public class UserManager1 : IUserManager
{
public async Task<bool> IsUserInRole(string roleName)
{
return await _userManager.IsInRoleAsync(_profile.Id, roleName);
}
}
public class UserManager2 : IUserManager
{
public Task<bool> IsUserInRole(string roleName)
{
return Task.FromResult(Roles.IsUserInRole(roleName));
}
}
If it is a void method you need to return Task.CompletedTask; from the non async method
(I think .NET 4.5 and later)
See also : Return Task<bool> instantly
The async modifier is an 'implementation detail', it affects how a method can do stuff asynchronously, not if it does so.
So async has no business being inside an interface.
interface IService { Task DoSomethingAsync(); }
class A : IService { public async Task DoSomethingAsync() { ... } }
class B : IService { public Task DoSomethingAsync() { ... } }
Classes A and B are both perfectly valid. Both methods are awaitable. A consumer of the interface doesn't know or care if they use async.
B.DoSomethingAsync() can be and probably will be asynchronous.