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
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).
What prevents me from constantly using the async/wait pattern in my .net code is that once you create an async method the async keyword tends to spread thru my code forcing me to make all methods async. Is there a pattern to stop this efficiently?
What prevents me from constantly using the async/wait pattern in my .net code is that once you create an async method the async keyword tends to spread thru my code forcing me to make all methods async. Is there a pattern to stop this efficiently?
Let me ask you this, then: why are you using async/await? You need to take a step back and decide if you want the benefits of asynchronous code. If you do want those benefits, then your code must be asynchronous. As soon as you block a thread, you lose all the benefits of async. So really, why use asynchronous code at all, if you're just going to block a thread anyway?
That said, there are some scenarios where a partially-async stack makes sense. For example, if your code base is temporarily in a transition state. In this case, you can use one of the hacks described in my article on brownfield async:
Blocking directly (may cause deadlocks).
Blocking on a thread pool thread (executes code on a different thread and different context, may cause unexpected parallelism).
Blocking on the current thread with a thread pool context (executes code in a different context, may cause unexpected parallelism).
Blocking on a thread with a single-threaded context (executes code on a different thread and different context).
Blocking on a nested message loop (may cause unexpected reentrancy).
All of these are hacks, and all have different drawbacks. There is no hack that works in every scenario.
As commented earlier you should go async all the way...
In some cases we use a litle helper class to cut off the async pattern. This class alows you to wait for the result without ending up with a deadlock by replacing the SynchronizationContext.
public class NoSynchronizationContextScope : IDisposable
{
private readonly SynchronizationContext synchronizationContext;
public NoSynchronizationContextScope()
{
synchronizationContext = SynchronizationContext.Current;
SynchronizationContext.SetSynchronizationContext(null);
}
public void Dispose() => SynchronizationContext.SetSynchronizationContext(synchronizationContext);
}
void fn()
{
using (new NoSynchronizationContextScope())
{
fnAsync().Wait();
// or
// var result = fnAsync().Result();
// in case you have to wait for a result
}
}
In the very particular case of a method that you want to start running but not wait for the result of (i.e. triggering an event handler) you can use async void. Just make sure you catch all the errors at that point:
public async void DoSomething() {
try {
var o = await DoThing1();
await DoThing2(o);
} catch (Exception e) {
// Log the error somewhere (to a file, database, etc.)
}
}
You can use async method and invoke not async method in task so outer method will behave like async and inner method is still sync.
public async Task OuterFuncAsync()
{
await Task.Run(() => InnerFunc());
}
This question already has answers here:
How Do I Call an Async Method from a Non-Async Method? [duplicate]
(2 answers)
How to call asynchronous method from synchronous method in C#?
(17 answers)
Closed 6 years ago.
Consider this code:
public async Task DoStuffAsync() {
await WhateverAsync(); // Stuff that might take a while
}
// Elsewhere in my project...
public async Task MyMethodAsync() {
await DoStuffAsync();
}
public void MyMethod() {
DoStuffAsync(); // <-- I want this to block until Whatever() is complete.
}
I want to provide two methods with the same functionality: an async method which runs asynchronously, and a non-async method that blocks, and either of these two methods can be called depending on whether the code I'm calling from is also async.
I don't want to repeat loads of code everywhere and create two versions of each method, wasting time and effort and reducing maintanability.
I'm guessing I can do something like one of the following:
// Call it just like any other method...
// the return type would be Task though which isn't right.
DoStuffAsync();
// Wrap it in a Task.Run(...) call, this seems ok, but
// but is doesn't block, and Task.Run(...) itself returns an
// awaitable Task... back to square one.
Task.Run(() => DoStuffAsync());
// Looks like it might work, but what if I want to return a value?
DoStuffAsync().Wait();
What is the correct way to do this?
UPDATE
Thanks for your answers. The general advice seems to be just provide an Async method and let the consumer decide, rather than creating a wrapper method.
However, let me try to explain my real-world problem...
I have a UnitOfWork class that wraps the the SaveChanges() and SaveChangesAsync() methods of an EF6 DbContext. I also have a List<MailMessage> of emails that need to be sent only when the SaveChanges() succeeds. So my code looks like this:
private readonly IDbContext _ctx;
private readonly IEmailService _email;
private readonly List<MailMessage> _emailQueue;
// ....other stuff
public void Save() {
try {
_ctx.SaveChanges();
foreach (var email in _emailQueue) _email.SendAsync(email).Wait();
} catch {
throw;
}
}
public async Task SaveAsync() {
try {
await _ctx.SaveChangesAsync();
foreach (var email in _emailQueue) await _email.SendAsync(email);
} catch {
throw;
}
}
So you can see that because an EF6 DbContext provides an async and non-async version, I kinda have to as well... Based on the answers so far, I've added .Wait() to my SendAsync(email) method, which asynchronously sends out an email via SMTP.
So... all good, apart from the deadlock problem... how do I avoid a deadlock?
What is the correct way to do this?
The correct way would be to do what you don't want to do, expose two different methods, one which is completely synchronous and another which is completely asynchronous.
Exposing async over sync and sync over async are poth anti-patterns when dealing with async code.
If the methods aren't aren't naturally async, i would expose a synchronous version only and let the invoker decide if he wants to wrap it with a Task.Run himself.
If the method is naturally async, i would expose the async method only. If you have both sync and async versions, expose them independently.
Async methods go "all the way", and they slowly bubble up and down the call stack as you use them. Embrace that fact, and you'll avoid many pitfalls along the way.
There is a Github project for that : AsyncBridge
Example included in the project homepage :
public async Task<string> AsyncString()
{
await Task.Delay(1000);
return "TestAsync";
}
public void Test()
{
string string1 = "";
string string2 = "";
using (var A = AsyncHelper.Wait)
{
A.Run(AsyncString(), res => string1 = res);
A.Run(AsyncString(), res => string2 = res);
}
}
I used it on several project without issue.
How do I do a blocking wait.
// Looks like it might work, but what if I want to return a value?
DoStuffAsync().Wait();
And it does. Regarding the
what if I want to return a value?
Suppose you also had this async method that returned a Task<string>:
public async Task<string> MyStringGetterAsync() {
return await GetStringAsync();
}
You could to the following to perform a blocking wait and retrieve its result:
var myString = MyStringGetterAsync().Result;
With regards to your intention:
I want to provide two methods with the same functionality: an async
method which runs asynchronously, and a non-async method that blocks,
and either of these two methods can be called depending on whether the
code I'm calling from is also async.
Creating a MyWhatEver() overload that just does MyWhatEverAsync.Wait() is not a recommended pattern, because it may have unintended side effects for clients calling that method, as explained e.g. here. The client calling your synchronous version of the method, might not expect its thread to be blocked as the result of calling your method, and cannot decide for itself if that is acceptable (e.g. it cannot specify ConfigureAwait). As mentioned by #Default in the comments, this could lead to deadlocks.
The same is generally true for the reverse case: i.e. having a MyWhatEverAsync() that only does return Task.Run(() => MyWhatEver()); If a client needs to, they could do this themselves and include e.g. a cancellation token, so they can control the task.
It is better to be explicit in your interface and only provide methods that do what their signature says they do.
For your real world problem (after question edit):
I have a UnitOfWork class that wraps the the SaveChanges() and
SaveChangesAsync() methods of an EF6 DbContext. I also have a
List<MailMessage> of emails that need to be sent only when the
SaveChanges() succeeds.
And (from the comments)
The success of sending an email isn't critical.
Your current UnitOfWork design for the Save and SaveAsync methods is not correct, because it violates the atomicity promises it makes.
In the scenario where "SaveChanges" succeeds, but an exception occurs on sending one of the emails. The client code calling one of the Save variants, will observe that exception, will (and should) assume that saving the changes failed, and could retry applying them.
If a client was wrapping this operation in a transaction context with a two phase commit, the "SaveChanges" would be rolled back, and when a client retries the operation, it will attempt to send the emails again, potentially causing a number of recipients to receive the email multiple times, although the changes they received the email about were not committed successfully (and may never be if sending emails fails repeatedly).
So you will need to change something.
If you really don't care about the success of sending the emails, I suggest you add the following method.
private Task SendEmailsAndIgnoreErrors(IEmailService emailService, List<MailMessage> emailQueue) {
return Task.WhenAll(emailQueue.Select(async (m) => {
try {
await emailService.SendAsync(m).ConfigureAwait(false);
} catch {
// You may want to log the failure though,
// or do something if they all/continuously fail.
}
}));
}
And use it like this:
public async Task SaveAsync() {
await _ctx.SaveChangesAsync();
await SendEmailsAndIgnoreErrors(_email, _emailQueue);
}
I would personally omit the synchronous variant, and let clients decide how to wait on the task, but if you believe you really need it, you could do this:
public void Save() {
_ctx.SaveChanges();
SendEmailsAndIgnoreErrors(_email, _emailQueue).Wait().ConfigureAwait(false);
}
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'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?