New to async await integration in C# 5. I'm working with some basic Task based methods to explore async await and the TPL. In this example below I'm calling a web service with a timeout of 5 seconds. If the timeout expires it should throw an exception so I can return false from the method. However, the timeout never occurs, or maybe it does but the Task never returns.
public static Task<bool> IsConnectedAsync()
{
return Task.Run(() =>
{
try
{
using (WSAppService.AppService svc = new NCSoftware.Common.WSAppService.AppService(GetServiceUrl(WebService.app)){Timeout = 5000})
{
return svc.PingB();
}
}
catch (Exception ex)
{
Logger.LogException(ex.Message, ex, "IsConnectedAsync");
}
return false;
});
}
If you could please help with how to properly handle this so that if the timeout occurs or even better, an exception occurs, the Task does return.
In general, you shouldn't use Task.Run if you're wrapping async services. Since this is a service reference, you should be able to expose an async method (returning Task) directly from the service, in which case you could use:
public async static Task<bool> IsConnectedAsync()
{
try
{
using (WSAppService.AppService svc = new NCSoftware.Common.WSAppService.AppService(GetServiceUrl(WebService.app)){Timeout = 5000})
{
return await svc.PingBAsync();
}
}
catch (Exception ex)
{
Logger.LogException(ex.Message, ex, "IsConnectedAsync");
}
return false;
}
If you must wrap via Task.Run (again, this is not suggested, as it's turning synchronous code into async via the thread pool, which is typically better handled by the user at the top level), you could do:
public async static Task<bool> IsConnectedAsync()
{
try
{
return await Task.Run(() =>
{
using (WSAppService.AppService svc = new NCSoftware.Common.WSAppService.AppService(GetServiceUrl(WebService.app)){Timeout = 5000})
{
return svc.PingB();
}
}
}
catch (Exception ex)
{
Logger.LogException(ex.Message, ex, "IsConnectedAsync");
return false;
}
}
Related
I don't think this question is a duplicate of "Proper way to deal with exceptions in DisposeAsync".
Let's say my class that implements IAsynsDisposable because it has a long-running background task, and DisposeAsync terminates that task. A familiar pattern might be the Completion property, e.g. ChannelReader<T>.Completion (despite ChannelReader doesn't implement IAsynsDisposable).
Is it considered a good practice to propagate the Completion task's exceptions outside DisposeAsync?
Here is a complete example that can be copied/pasted into a dotnet new console project. Note await this.Completion inside DisposeAsync:
try
{
await using var service = new BackgroundService(TimeSpan.FromSeconds(2));
await Task.Delay(TimeSpan.FromSeconds(3));
}
catch (Exception ex)
{
Console.WriteLine(ex);
Console.ReadLine();
}
class BackgroundService: IAsyncDisposable
{
public Task Completion { get; }
private CancellationTokenSource _diposalCts = new();
public BackgroundService(TimeSpan timeSpan)
{
this.Completion = Run(timeSpan);
}
public async ValueTask DisposeAsync()
{
_diposalCts.Cancel();
try
{
await this.Completion;
}
finally
{
_diposalCts.Dispose();
}
}
private async Task Run(TimeSpan timeSpan)
{
try
{
await Task.Delay(timeSpan, _diposalCts.Token);
throw new InvalidOperationException("Boo!");
}
catch (OperationCanceledException)
{
}
}
}
Alternatively, I can observe service.Completion explicitly in the client code (and ignore its exceptions inside DiposeAsync to avoid them being potentially thrown twice), like below:
try
{
await using var service = new BackgroundService(TimeSpan.FromSeconds(2));
await Task.Delay(TimeSpan.FromSeconds(3));
await service.Completion;
}
catch (Exception ex)
{
Console.WriteLine(ex);
Console.ReadLine();
}
class BackgroundService: IAsyncDisposable
{
public Task Completion { get; }
private CancellationTokenSource _diposalCts = new();
public BackgroundService(TimeSpan timeSpan)
{
this.Completion = Run(timeSpan);
}
public async ValueTask DisposeAsync()
{
_diposalCts.Cancel();
try
{
await this.Completion;
}
catch
{
// the client should observe this.Completion
}
finally
{
_diposalCts.Dispose();
}
}
private async Task Run(TimeSpan timeSpan)
{
try
{
await Task.Delay(timeSpan, _diposalCts.Token);
throw new InvalidOperationException("Boo!");
}
catch (OperationCanceledException)
{
}
}
}
Is there a concensus about which option is better?
For now, I've settled on a reusable helper class LongRunningAsyncDisposable (here's a gist, warning: barely tested yet), which allows:
to start a background task;
stop this task (via a cancellation token) by calling IAsyncDisposable.DisposeAsync at any time, in a thread-safe, concurrency-friendly way;
configure whether DisposeAsync should re-throw the task's exceptions (DisposeAsync will await the task's completion either way, before doing a cleanup);
observe the task's status, result and exceptions at any time via LongRunningAsyncDisposable.Completion property.
We start using CancellationToken in out app a lot, so we have to change exception handling correspondingly:
class Program
{
static async Task Main(string[] args)
{
CancellationTokenSource cts = new CancellationTokenSource(100);
await DoJob(cts.Token);
Console.WriteLine("Successfully finished");
}
private static async Task DoJob(CancellationToken ct)
{
try
{
await Task.Delay(1000, ct);
}
catch (Exception e) when(!(e is OperationCanceledException))
{
Console.WriteLine("Do cleanup in case of error.");
}
}
}
The idea behind this code is that if someone use catch(Exception e) (please do not blame me for this) and forgot to exclude CancellationToken, an error handling is executed, for example, there is a log that operation failed. But it is not true, id doesn't fail, it just has been canceled. And cancellation should be handled differently then failure.
It seems to me like a big boilerplate to write practically in every general catch
catch (Exception e) when(!(e is OperationCanceledException))
Is there some more robust solution with less boilerplate?
You could create a method that accepts a Func<Task> and catches the exception(s), e.g.:
class Program
{
static async Task Main(string[] args)
{
CancellationTokenSource cts = new CancellationTokenSource(100);
await GeneralDoJobAndCatchException(() => DoJob(cts.Token));
Console.WriteLine("Successfully finished");
}
private static async Task GeneralDoJobAndCatchException(Func<Task> func)
{
try
{
await func();
}
catch (OperationCanceledException) { }
catch (Exception e)
{
Console.WriteLine("Do error handling");
}
}
private static async Task DoJob(CancellationToken ct)
{
await Task.Delay(1000, ct);
}
}
We're having the exact same problem. Mainly there is a while-loop that checks for the CancellationToken but you've to catch this exception.
We created the following extension method:
public static async Task<TaskStatus> HideCancellationException(this Task task)
{
try
{
await task;
return task.Status;
}
catch (OperationCanceledException)
{
return TaskStatus.Canceled;
}
}
Having this extension method allows to change this code:
while (!cancellationToken.IsCancellationRequested)
{
// do stuff here...
try
{
await Task.Delay(..., cancellationToken);
}
catch (OperationCanceledException)
{
// expected
}
}
to something like that:
while (!cancellationToken.IsCancellationRequested)
{
// Do stuff here.
await Task.Delay(..., cancellationToken).HideCancellationException();
}
Keep in mind that there is explicitly no overload for Task<T> because the return value in case of cancellation is default. You can't distinguish between default as normal task result and default as result of cancellation. In that case it's better to catch the exception.
You could get rid of the try-catch block altogether by awaiting indirectly with Task.WhenAny, and then querying the status of the completed task:
private static async Task DoJob(CancellationToken ct)
{
var completedTask = await Task.WhenAny(Task.Delay(1000, ct));
if (completedTask.IsFaulted)
{
Console.WriteLine("Error: " + completedTask.Exception.InnerException);
}
else if (completedTask.IsCanceled)
{
// Do nothing
}
else // Success
{
// Do nothing
}
}
I am trying to create the async task for the already existing code in my project for the database connection using EF 6.
I have created async task and it works fine, you can refer Using Async Task section for the code. But I want to rewrite The existing code of the project section as async task to maintain the consistency in the project.
Using Async Task: The below code works fine.
public static async Task<Student> InsertAsync(Student student)
{
try
{
using(PracticeContext context = new PracticeContext())
{
Repository<Student> repository = new Repository<Student>(context);
return await repository.InsertAsync(student);
}
}
catch (Exception ex)
{
Console.WriteLine(ex);
return null;
}
}
The existing code of the project:
I want to implement the below code as async task for both the Insert() and Execute() methods. So, that the data will be added to the database without holding the UI for the long time and make the project more user friendly.
Please suggest or provide the async implementation of the below codes.
public static Student Insert(Student student)
{
try
{
return Execute<Student, Student>((repository) => {
return repository.Insert(student);
});
}
catch (Exception ex)
{
Console.WriteLine(ex);
return null;
}
}
private static TResult Execute<TResult, T>(Func<Repository<T>, TResult> func) where T: class
{
using(PracticeContext context = new PracticeContext())
{
try
{
return func(new Repository<T>(context));
}
catch(Exception ex)
{
Console.WriteLine(ex);
throw new Exception("Error Occured.");
}
}
}
I would be glad to add more stuffs if required to make the questions more accurate and understandable.
The first thing you do is change the lowest-level call to use the asynchronous version and use async in its enclosing method/lambda. Then let the async grow from there. So the first step would be:
return Execute<Student, Student>(async (repository) => {
return await repository.Insert(student);
});
This code then requires Execute to allow asynchronous delegates. At this step, we probably want other code to continue using the synchronous Execute, so we can copy/paste it and make the new overload take an asynchronous delegate:
private static async Task<TResult> Execute<TResult, T>(Func<Repository<T>, Task<TResult>> func) where T: class
{
using(PracticeContext context = new PracticeContext())
{
try
{
return await func(new Repository<T>(context));
}
catch(Exception ex)
{
Console.WriteLine(ex);
throw new Exception("Error Occured.");
}
}
}
Now that the asynchronous Execute returns a Task<T>, we then need to await it back in the calling method:
return await Execute<Student, Student>(async (repository) => {
return await repository.Insert(student);
});
which then makes the calling method asynchronous as well:
public static async Task<Student> InsertAsync(Student student)
{
try
{
return await Execute<Student, Student>(async (repository) => {
return await repository.Insert(student);
});
}
catch (Exception ex)
{
Console.WriteLine(ex);
return null;
}
}
I have the following method
public async Task<bool> Connect()
{
lock (_connectingLock)
{
if (_connecting)
throw new IOException("Already connecting");
_connecting = true;
}
try {
await tcpClient.ConnectAsync(...);
}
catch (SocketException e)
{
return false;
}
finally
{
lock (_connectingLock)
{
_connecting = false;
}
}
}
Now, I would expect consecutive calls to Connect() to throw an IOException, but it doesn't happen!
What could be the cause?
Calls to Connect() can't throw exceptions directly. Async methods do not throw exceptions. Instead, they will return tasks which, when awaited, will throw IOException. (i.e. the tasks are faulted.)
If that's not what you want, you should separate the calls:
public Task<bool> Connect()
{
// Eager validation of state...
lock (_connectingLock)
{
if (_connecting)
throw new IOException("Already connecting");
_connecting = true;
}
return ConnectImpl();
}
private async Task<bool> ConnectImpl()
{
try {
await tcpClient.ConnectAsync(...);
}
catch (SocketException e)
{
return false;
}
finally
{
lock (_connectingLock)
{
_connecting = false;
}
}
}
It's not clear whether that's appropriate in this case though. It's generally fine to throw things like ArgumentException eagerly, but if the error doesn't represent a bug in the calling code itself, I think returning a faulted task instead is fine.
This example "fails":
static async void Main(string[] args)
{
try
{
await TaskEx.Run(() => { throw new Exception("failure"); });
}
catch (Exception)
{
throw new Exception("success");
}
}
That is, the exception with the text "failure" bubbles up.
Then I tried this workaround:
static async void Main(string[] args)
{
try
{
await SafeRun(() => { throw new Exception("failure"); });
}
catch (Exception)
{
throw new Exception("success");
}
}
static async Task SafeRun(Action action)
{
var ex = default(Exception);
await TaskEx.Run(() =>
{
try
{
action();
}
catch (Exception _)
{
ex = _;
}
});
if (ex != default(Exception))
throw ex;
}
That didn't help either.
I suppose my Async CTP refresh installation could be hosed.
Should this code work as I expect ("success" bubbles up, not "failure"), or is this not "supposed" to work that way. And if not, how would you work around it?
The behavior you are seeing is likely an edge case bug or may even be correct, if unintuitive. Normally when you invoke an async method synchronously, it wraps a task around to execute and since there is no one waiting on the task to finish, the exception never makes it to the main thread. If you were to call Main directly it would succeed, but then your runtime would see an exception of "success" on another thread.
Since main is the entrypoint of your application, it is invoked synchronously and likely as the entrypoint doesn't trigger the Task wrapping behavior, so that await isn't run properly and the TaskEx.Run throws on its own thread, which shows up in the runtime as an exception being thrown on another thread.
If you were to run main as an async method, i.e. returning a Task (since an async that returns void can only really be called via await) and blocking on it from your synchronous main context, you would get the appropriate behavior as the below test illustrates:
static async Task Main() {
try {
await TaskEx.Run(() => { throw new Exception("failure"); });
} catch(Exception) {
throw new Exception("success");
}
}
static async Task Main2() {
await Main();
}
[Test]
public void CallViaAwait() {
var t = Main2();
try {
t.Wait();
Assert.Fail("didn't throw");
} catch(AggregateException e) {
Assert.AreEqual("success",e.InnerException.Message);
}
}
[Test]
public void CallDirectly() {
var t = Main();
try {
t.Wait();
Assert.Fail("didn't throw");
} catch(AggregateException e) {
Assert.AreEqual("success", e.InnerException.Message);
}
}
I.e. the Task faults with an AggregateException which contains the success exception as it's inner exception.