I want to send a message using an asynchronous call but disposable resources are used in the process. I'm not sure how to properly handle this.
using(IDisposable disposeMe = new DisposableThing())
{
MethodAsync(disposeMe);
}
Any ideas how to do this. The async method does not have any callback.
EDIT:
Based on the suggestion the accepted answer by #Servy. The implementation was changed to:
public async Task Method()
{
using(IDisposable disposeMe = new DisposableThing())
{
await MethodAsync(disposeMe);
}
}
The async method does not have any callback
Well that's a problem. A very big problem. If at all possible you should fix that; unless there is a very good reason not to, all asynchronous methods should provide some means for the caller to know when they have completed, whether it's a callback, returning a Task, firing an event, etc. It's particularly troubling for an asynchronous method that is dependent on a disposable resource to do this.
If you really do have no way of being notified when the operation completes, then there is no way for you to call Dispose after the method completes. The only means you have of ensuring you don't dispose of the resource while the async operation still needs it is to leak the resource and not dispose of it at all. If the disposable resource has a finalizer it's possible it will be cleaned up eventually, but not all disposable resources do so.
If you can modify the asynchronous method somehow then you simply need to call Dispose in the callback, or in a continuation of the task, or in a handler of the relevant event. You won't be able to use a using unless you can await the asynchronous operation (because await is just awesome like that).
Related
I want to send a message using an asynchronous call but disposable resources are used in the process. I'm not sure how to properly handle this.
using(IDisposable disposeMe = new DisposableThing())
{
MethodAsync(disposeMe);
}
Any ideas how to do this. The async method does not have any callback.
EDIT:
Based on the suggestion the accepted answer by #Servy. The implementation was changed to:
public async Task Method()
{
using(IDisposable disposeMe = new DisposableThing())
{
await MethodAsync(disposeMe);
}
}
The async method does not have any callback
Well that's a problem. A very big problem. If at all possible you should fix that; unless there is a very good reason not to, all asynchronous methods should provide some means for the caller to know when they have completed, whether it's a callback, returning a Task, firing an event, etc. It's particularly troubling for an asynchronous method that is dependent on a disposable resource to do this.
If you really do have no way of being notified when the operation completes, then there is no way for you to call Dispose after the method completes. The only means you have of ensuring you don't dispose of the resource while the async operation still needs it is to leak the resource and not dispose of it at all. If the disposable resource has a finalizer it's possible it will be cleaned up eventually, but not all disposable resources do so.
If you can modify the asynchronous method somehow then you simply need to call Dispose in the callback, or in a continuation of the task, or in a handler of the relevant event. You won't be able to use a using unless you can await the asynchronous operation (because await is just awesome like that).
I needed to implement a simple transaction. For that case my transaction class implements the IDisposable interface. That way I can use my transaction class within a using statement and if any error happens within that scope, everything get's rolled back when the transaction is disposed.
using (var transaction = new Transaction())
{
// do some stuff
}
The "do some stuff" also includes some client/server connections etc.
Now I roll back everything if "do some stuff" raised any error.
public async void Dispose(){ // roll back everything on error }
That probably includes to cleanup some resources on a server where I need to call asynchronous operations. That leads to my question: can I safely just use the async keyword for my Dispose() implementation to enable the await keyword within its implementation? Or could this lead probably to race conditions due to synchronisation context issues or something like that?
How about calling Task.Run(...). It queues the specified work to run on the ThreadPool.
public void Dispose()
{
if (_thereWasAnError)
{
Task.Run(async () => await RollbackAsync()).Wait();
}
}
You can only use await when the method returns Task, Task<T> or another implementation of an awaitable.
IDisposable does not define a Task Dispose() method. You can always write your own, but it will not be called when you exit the using block.
Depending on the locking mechanism in your resource, you may risk race conditions with an async void (avoid those!).
As Gusdor already mentioned in his answer, you cannot simply add the async keyword to the Dispose() method of the IDisposable interface.
However, since C# 8.0, a new interface for async disposal is available: IAsyncDisposable
With this you can implement true async disposal.
To see how it works, have a look at this excellent article from Microsoft: https://learn.microsoft.com/en-us/dotnet/standard/garbage-collection/implementing-disposeasync
Consider a piece of code like:
private Task<string> Download()
{
var wc = new WebClient();
Task<string> backgroundDownload = wc.DownloadStringTaskAsync(this.Uri);
// Make sure the WebClient is disposed no matter what.
backgroundDownload.ContinueWith((downloadTask) => { wc.Dispose(); });
return backgroundDownload;
}
Can I be certain that the WebClient.Dispose() call occurs and that any exception produced is rethrown to the caller as if there was no call to ContinueWith?
Can clients observe this ContinueWith? (e.g. will later calls to ContinueWith remove the Dispose call?)
With the code that you have you can be certain that the continuation will be fired regardless of whether or not the code completed successfully, was cancelled, or throws an exception.
The one potential problem with the solution that you have is that other continuations can potentially run, before, during, or after the web client is disposed. If you don't have a problem with other continuations running before this cleanup runs then what you have is fine. If that's a problem then you'll need to return the continuation, not the original task, but you'll also need to propagate the result (and exceptions/cancellation) correctly. The use of async makes all of this way easier:
private async Task<string> Download()
{
using(var wc = new WebClient())
return await wc.DownloadStringTaskAsync(this.Uri);
}
First of all, the continuation will be performed even if a regular exception occurred. However it is less likely to run than a regular finally block in the eventuality of exceptional conditions such as an OutOfMemoryException.
Now I would not try to dispose the webclient. Remember that disposing is an optimization, because native resources will be disposed by the finalizer anyway. The only reason we are disposing is that a finalizer is expensive because it triggers a second GC pass.
But in the order to perform your optimization the system may have to create new threads. Besides you may be prolongating the lifetime of your webclient a lot if the threadpool is filled with long running tasks.
Basically, you have to choose the lesser of two evils and I am not convinced that one less GC run is worth what you're doing. You should consider this decision in the context of your application.
I've noticed the following pattern recently, but I don't entirely grasp the usage of the CompletedSynchronously property:
IAsyncResult channelOpenResult = channel.BeginOpen(new AsyncCallback(OnOpenCompleteChannel), channel);
if (channelOpenResult.CompletedSynchronously)
{
CompleteOpenChannel(channelOpenResult);
}
And then again, in the callback:
void OnOpenCompleteChannel(IAsyncResult result)
{
if (result.CompletedSynchronously)
return;
else
CompleteOpenChannel(result);
}
And somewhere in the code there is of course a function:
void CompleteOpenChannel(IAsyncResult result) ...
Is this a way to handle the asynchronous call differently depending on whether it completes directly or not? But why use it in this case, since the AsyncCallback always will be called (will it?)?
Could someone give an example where the call is made synchronously?
See this blog. A common pattern does async work in a loop, and checking CompletedSynchronously helps avoid the case where you get 'unlucky' and a bunch of async calls happen to complete sync and you risk StackOverflowException. (E.g. if you're reading data over the network, and the data you're reading has already come over the wire and is buffered, your async call may complete synchronously, which means your callback is called on the same thread (with a taller stack), which means you better not schedule another async call in a loop.)
According to this document you can supply the call with a synchronous and ASync callback, and only if the call was not handled synchronously, will it call the ASync methods. I do not think this is really applicable to Silverlight (because all Silverlight calls are ASync to a degree) but is probably used more for custom factories in other .NET applications.
Hope this helps.
I've noticed that in some case, Visual Studio recommends to do this
await using var disposable = new Disposable();
// Do something
Instead of this
using var disposable = new Disposable();
// Do something
What is the difference between using and await using?
How should I decide which one to use?
Classic sync using
Classic using calls the Dispose() method of an object implementing the IDisposable interface.
using var disposable = new Disposable();
// Do Something...
Is equivalent to
IDisposable disposable = new Disposable();
try
{
// Do Something...
}
finally
{
disposable.Dispose();
}
New async await using
The new await using calls and await the DisposeAsync() method of an object implementing the IAsyncDisposable interface.
await using var disposable = new AsyncDisposable();
// Do Something...
Is equivalent to
IAsyncDisposable disposable = new AsyncDisposable();
try
{
// Do Something...
}
finally
{
await disposable.DisposeAsync();
}
The IAsyncDisposable Interface was added in .NET Core 3.0 and .NET Standard 2.1.
In .NET, classes that own unmanaged resources usually implement the IDisposable interface to provide a mechanism for releasing unmanaged resources synchronously. However, in some cases they need to provide an asynchronous mechanism for releasing unmanaged resources in addition to (or instead of) the synchronous one. Providing such a mechanism enables the consumer to perform resource-intensive dispose operations without blocking the main thread of a GUI application for a long time.
The IAsyncDisposable.DisposeAsync method of this interface returns a ValueTask that represents the asynchronous dispose operation. Classes that own unmanaged resources implement this method, and the consumer of these classes calls this method on an object when it is no longer needed.
Justin Lessard's answer explains the difference between using and await using, so I'll focus on which one to use. There are two cases: either the two methods Dispose/DisposeAsync are complementary, or they are doing something different.
If the methods are complementary, which is the common case, you can call either one of them and the result will be the same: the unmanaged resources will be released. There is no reason to call both of them, sequentially. If you do, the second call will be a no-op: the resources are already released, so there will be nothing more to do. Choosing which one to call is easy: if you are in a synchronous context, call the Dispose() (use the using). If you are in an asynchronous context, call the await DisposeAsync() (use the await using)¹.
If the methods are doing something different, you should read the documentation and decide which behavior is more suitable for the scenario at hand. Let's talk for example for the System.Threading.Timer class, that implements both interfaces (IDisposable and IAsyncDisposable). The Dispose method releases the unmanaged resources as expected, but the DisposeAsync is doing something more than that: it also awaits the completion of any callbacks that are currently running. Let's make an experiment to demonstrate this difference:
var stopwatch = Stopwatch.StartNew();
using (new Timer(_ => Thread.Sleep(1000), null, 0, Timeout.Infinite))
{
Thread.Sleep(100);
}
Console.WriteLine($"Duration: {stopwatch.ElapsedMilliseconds:#,0} msec");
We create a timer that fires after 0 msec, so practically immediately, then we wait for 100 msec to be sure that the callback has been invoked (it is invoked on a ThreadPool thread), and then we dispose the timer synchronously. Here is the output of this experiment:
Duration: 102 msec
Now let's switch from using to await using. Here is the output of the second experiment:
Duration: 1,005 msec
The implicit call to DisposeAsync returned a ValueTask that completed only after the completion of the timer's callback.
So in the case of a Timer, choosing between using and await using is not just a choice that depends on the context. You might prefer to use the synchronous using in an async context, because you don't care about the callback (you know that it's not harmful to let it become fire-and-forget). Or you might be in a synchronous context but you might prefer the behavior of await using, because fire-and-forget is unacceptable. In that case you'll have to abandon the convenience of using, and invoke instead the DisposeAsync explicitly in a finally block:
var timer = new Timer(_ => Thread.Sleep(1000), null, 0, Timeout.Infinite);
try { Thread.Sleep(100); }
finally { timer.DisposeAsync().AsTask().Wait(); }
¹ Be aware, especially if you are writing a library, that the await using captures the synchronization context by default. In case this is undesirable, which usually is for library code, you'll have to configure it with ConfigureAwait(false). This has some implications that are discussed here: How do I get the "await using" syntax correct?