Goal:
I am confused by the behavior I am seeing with exceptions in my .Net Core library. The goal of this question is to understand why it is doing what I am seeing.
Executive Summary
I thought that when an async method is called, the code in it is executed synchronously until it hits the first await. If that is the case, then, if an exception is thrown during that "synchronous code", why is it not propagated up to the calling method? (As a normal synchronous method would do.)
Example Code:
Given the following code in a .Net Core Console Application:
static void Main(string[] args)
{
Console.WriteLine("Hello World!");
try
{
NonAwaitedMethod();
}
catch (Exception e)
{
Console.WriteLine("Exception Caught");
}
Console.ReadKey();
}
public static async Task NonAwaitedMethod()
{
Task startupDone = new Task(() => { });
var runTask = DoStuff(() =>
{
startupDone.Start();
});
var didStartup = startupDone.Wait(1000);
if (!didStartup)
{
throw new ApplicationException("Fail One");
}
await runTask;
}
public static async Task DoStuff(Action action)
{
// Simulate starting up blocking
var blocking = 100000;
await Task.Delay(500 + blocking);
action();
// Do the rest of the stuff...
await Task.Delay(3000);
}
}
Scenarios:
When run as is, this code will throw an exception, but, unless you have a break point on it, you will not know it. The Visual Studio Debugger nor the Console will give any indication that there was an issue (aside from a one line note in the Output screen).
Swap the return type of NonAwaitedMethod from Task to void. This will cause the Visual Studio Debugger to now break on the exception. It will also be printed out in the console. But notably, the exception is NOT caught in the catch statement found in Main.
Leave the return type of NonAwaitedMethod as void, but take off the async. Also change the last line from await runTask; to runTask.Wait(); (This essentially removes any async stuff.) When run, the exception is caught in the catch statement in the Main method.
So, to summarize:
| Scenario | Caught By Debugger | Caught by Catch |
|------------|--------------------|-----------------|
| async Task | No | No |
| async void | Yes | No |
| void | N/A | Yes |
Questions:
I thought that because the exception was thrown before an await was done, that it would execute synchronously up to, and through the throwing of the exception.
Hence my question of: Why does neither scenario 1 or 2 get caught by the catch statement?
Also, why does swapping from Task to void return type cause the exception to get caught by the Debugger? (Even though I am not using that return type.)
exception was thrown before an await was done, that it would execute synchronously
Thought this is fairly true, but it doesn't mean you could catch the exception.
Because your code has async keyword, which turns the method into an async state machine i.e. encapsulated / wrapped by a special type. Any exception thrown from async state machine will get caught and re-thrown when the task is awaited (except for those async void ones) or they go unobserved, which can be caught in TaskScheduler.UnobservedTaskException event.
If you remove async keyword from the NonAwaitedMethod method, you can catch the exception.
A good way to observe this behavior is using this:
try
{
NonAwaitedMethod();
// You will still see this message in your console despite exception
// being thrown from the above method synchronously, because the method
// has been encapsulated into an async state machine by compiler.
Console.WriteLine("Method Called");
}
catch (Exception e)
{
Console.WriteLine("Exception Caught");
}
So your code is compiled similarly to this:
try
{
var stateMachine = new AsyncStateMachine(() =>
{
try
{
NonAwaitedMethod();
}
catch (Exception ex)
{
stateMachine.Exception = ex;
}
});
// This does not throw exception
stateMachine.Run();
}
catch (Exception e)
{
Console.WriteLine("Exception Caught");
}
why does swapping from Task to void return type cause the exception to get caught
If the method returns a Task, the exception is caught by the task.
If the method is void, then the exception gets re-thrown from an arbitrary thread pool thread. Any unhandled exception thrown from thread pool thread will cause the app to crash, so chances are the debugger (or maybe the JIT debugger) is watching this sort of exceptions.
If you want to fire and forget but properly handle the exception, you could use ContinueWith to create a continuation for the task:
NonAwaitedMethod()
.ContinueWith(task => task.Exception, TaskContinuationOptions.OnlyOnFaulted);
Note you have to visit task.Exception property to make the exception observed, otherwise, task scheduler still will receive UnobservedTaskException event.
Or if the exception needs to be caught and processed in Main, the correct way to do that is using async Main methods.
if an exception is thrown during that "synchronous code", why is it not propagated up to the calling method? (As a normal synchronous method would do.)
Good question. And in fact, the early preview versions of async/await did have that behavior. But the language team decided that behavior was just too confusing.
It's easy enough to understand when you have code like this:
if (test)
throw new Exception();
await Task.Delay(TaskSpan.FromSeconds(5));
But what about code like this:
await Task.Delay(1);
if (test)
throw new Exception();
await Task.Delay(TaskSpan.FromSeconds(5));
Remember that await acts synchronously if its awaitable is already completed. So has 1 millisecond gone by by the time the task returned from Task.Delay is awaited? Or for a more realistic example, what happens when HttpClient returns a locally cached response (synchronously)? More generally, the direct throwing of exceptions during the synchronous part of the method tends to result in code that changes its semantics based on race conditions.
So, the decision was made to unilaterally change the way all async methods work so that all exceptions thrown are placed on the returned task. As a nice side effect, this brings their semantics in line with enumerator blocks; if you have a method that uses yield return, any exceptions will not be seen until the enumerator is realized, not when the method is called.
Regarding your scenarios:
Yes, the exception is ignored. Because the code in Main is doing "fire and forget" by ignoring the task. And "fire and forget" means "I don't care about exceptions". If you do care about exceptions, then don't use "fire and forget"; instead, await the task at some point. The task is how async methods report their completion to their callers, and doing an await is how calling code retrieves the results of the task (and observe exceptions).
Yes, async void is an odd quirk (and should be avoided in general). It was put in the language to support asynchronous event handlers, so it has semantics that are similar to event handlers. Specifically, any exceptions that escape the async void method are raised on the top-level context that was current at the beginning of the method. This is how exceptions also work for UI event handlers. In the case of a console application, exceptions are raised on a thread pool thread. Normal async methods return a "handle" that represents the asynchronous operation and can hold exceptions. Exceptions from async void methods cannot be caught, since there is no "handle" for those methods.
Well, of course. In this case the method is synchronous, and exceptions travel up the stack just like normal.
On a side note, never, ever use the Task constructor. If you want to run code on the thread pool, use Task.Run. If you want to have an asynchronous delegate type, use Func<Task>.
The async keyword indicates that the compiler should transform the method to an async state machine, which is not configurable regarding the handling of the exceptions. If you want the sync-part-exceptions of the NonAwaitedMethod method to be thrown immediately, there is no other option than removing the async keyword from the method. You can have the best of both worlds by moving the async part into an async local function:
public static Task NonAwaitedMethod()
{
Task startupDone = new Task(() => { });
var runTask = DoStuff(() =>
{
startupDone.Start();
});
var didStartup = startupDone.Wait(1000);
if (!didStartup)
{
throw new ApplicationException("Fail One");
}
return ImlpAsync(); async Task ImlpAsync()
{
await runTask;
};
}
Instead of using a named function, you could also use an anonymous one:
return ((Func<Task>)(async () =>
{
await runTask;
}))();
Related
I've argued with my colleague about handling exceptions in Task.Run blocks.
For example, I have this code:
private static async Task MainAsync()
{
try
{
await Task.Run(() => throw new Exception());
}
catch (Exception)
{
Console.WriteLine("oops");
}
}
If I run this code (in debug mode), I'll get the message from vs2019 about the unhandled exception. But if I press the continue button, the app will work correctly and the exception will be handled.
Is it the correct way to catch exceptions from Task.Run?
Generally, exception in tasks (objects of class Task) are placed on tasks, i.e. if some code in a task throws exception inside that task - it is stored in Exception property in Task class.
await operator automatically "unpacks" exception and throws. So your way is totally correct, but I think stack trace might change - so you need to keep that in mind.
What will happen (and why) if the following if statement is satisfied, and Bar() throws an exception?
async Task Foo()
{
Task<object> myTask = Bar();
if (condition)
{
return;
}
else
{
await myTask;
// ....
return;
}
}
Will the exception be caught? By who?
No, the exception won't be caught. You need to specifically add a continuation to the Task (note that when you await a task you're adding a continuation to it).
If Bar throws an exception, it will be thrown right at the point where you call it.
However, if the Task that Bar returns wraps an exception, what happens depends on your version of .NET runtime - for .NET 4.0, it will bring down your entire process, because it eventually causes the exception to be thrown on a finalizer thread (or a thread-pool thread). For .NET 4.5+, the exception will be silently disposed of.
In any case, you don't want either. You should always explicitly handle any asynchronous exceptions that can be propagated in the asynchronous task. If you don't want to await the task in some branch of your code (say, you're pre-loading data you think you'll need, but don't), at least bind a continuation on the task to handle any possible exceptions gracefully.
I know this question has been asked several times, but, I'm looking slightly at a different variant.
public async Task<string> SomeAsyncMethod(string url)
{
// do some URL validation
if (!valid)
{
throw new Exception("some error");
}
// do async stuff now
return await GetFromUrl(url)
}
// now in caller
public async Task<string> SomeOtherAsyncMethod(string input)
{
var task = SomeAsyncMethod(input);
// there is potential chance that a validation fails and
//exception is thrown even before entering async parts of the called function
// do some independent stuff here
try
{
await task;
}
catch(Exception e)
{
// log error
}
// is the following code correct way to handle exceptions?
if (!task.IsFaulted)
{
return task.Result;
}
// log error from task.Exception
return null;
}
In the above code it may so happen that validation fails and exception is thrown even before the control enters async part of the method. Do we need to wrap the first call also around a try..catch block? My experiment showed that this is not useful. Instead, the task status is set to Faulted. So, I believe it is correct to check Task status and return data accordingly. Can C# pros comment on this?
As you have already stated, when you have an async method that throws an exception calling the method will never throw, instead the returned tasks will simply be faulted. This is true even if an exception is thrown before the first await. If that's your desired functionality, then you already have it, and there is no need to change anything.
Do we need to wrap the first call also around a try..catch block?
You may want to do so, as a defensive coding measure. "Precondition" exceptions in async methods suffer from the same problems as they do with enumerator blocks. In the async case, the precondition exceptions are used to fault the task, not raised directly. This is how I do precondition exceptions.
However, there is an alternative. It is possible for an implementation to "eagerly" do the precondition checks and only use faulted tasks to represent asynchronous exceptions. I.e.:
public Task<string> SomeMethodAsync(string url)
{
// do some URL validation
if (!valid)
{
throw new Exception("some error");
}
// do async stuff now
return SomeMethodImplAsync(url);
}
private async Task<string> SomeMethodImplAsync(string url)
{
return await GetFromUrl(url)
}
I don't do this myself, but this kind of approach does have its supporters. Most notably, Jon Skeet.
With that in mind, unless the documentation explicitly specifies that precondition exceptions will be placed on the returned task, you probably should include the call to SomeMethdAsync within a try block.
When I try to raise transient exception manually, it is always handled as AggregateException. Since it is handled as AggregateException , it is not handled as transient error in my retry policy and not retried for the predefined retry count.
Transient errors are shown here .
Therefore I have tried CommunicationException and ServerErrorException but it is handled as an AggregateException.
When I look for AggregateException, it says "Represents one or more errors that occur during application execution." Yeah, it is so helpful!!!
Here is the example code of my case:
I have a retry policy which uses ServiceBusTransientErrorDetectionStrategy
public void TestManually()
{
var retryPolicy = new RetryPolicy<ServiceBusTransientErrorDetectionStrategy>(RetryStrategy.DefaultFixed);
retryPolicy.Retrying += (obj, eventArgs) =>
{
Trace.TraceError("Hey!! I'm Retrying, CurrentRetryCount = {0} , Exception = {1}", eventArgs.CurrentRetryCount, eventArgs.LastException.Message);
};
retryPolicy.ExecuteAsync(() =>
MyTestFunction().ContinueWith(t =>
{
if (t.Exception != null)
{
// A non-transient exception occurred or retry limit has been reached
Trace.TraceError("This was not a transient exxception... It was: " + t.Exception.GetType().ToString());
}
}));
}
public Task MyTestFunction()
{
Task task = Task.Factory.StartNew(() => RaiseTransientErrorManually());
return task;
}
public void RaiseTransientErrorManually()
{
//throw new CommunicationException();
throw new ServerErrorException();
}
Let's say I call my function like this:
TestManually();
I'm very confused why the manually thrown exception (which is defined as Transient Error) is handled as AggregateException ? What I'm missing there?
Thanks.
Exceptions within asynchronous code are a tricky subject for two reasons.
The manner in which exceptions are handled (e.g. by catch blocks) is not always intuitive, and may seem inconsistent.
The manner in which libraries document the behavior for exceptions thrown by asynchronous methods is not always obvious.
I'll address each of these items below.
Important Note: This answer uses the term asynchronous method to refer to any method with a return type of Task or Task<T>. Languages with built-in support for asynchronous programming have their own related terminology which may differ in meaning.
Exceptions Thrown by Asynchronous Methods
Asynchronous methods are capable of throwing exceptions before creating a Task or during the asynchronous execution of the task itself. While projects are not always consistent in the way exceptions are documented for asynchronous code, I like to include the following note with my projects to make things clear for my users.
Note: only assume the following quote is true for a library which explicitly states it. The statement is specifically meant to address the second problem area described above.
The documentation for asynchronous methods does not distinguish between these two cases, allowing for any of the specified exceptions to be thrown in either manner.
Exceptions Prior to Task Creation
Exceptions thrown prior to the creation of the Task object representing the asynchronous operation must be caught directly by the calling code. For example, if the code throws an ArgumentNullException in this manner, the calling code would need to contain an exception handler for ArgumentNullException or ArgumentException to handle the exception.
Example code which throws a direct exception:
public Task SomeOperationAsync()
{
throw new ArgumentException("Directly thrown.");
}
Example code which handles a directly-thrown exception:
try
{
Task myTask = SomeOperationAsync();
}
catch (ArgumentException ex)
{
// ex was thrown directly by SomeOperationAsync. This cannot occur if
// SomeOperationAsync is an async function (§10.15 - C# Language Specification
// Version 5.0).
}
Exceptions During Task Execution
Exceptions thrown during the asynchronous execution of the task are wrapped in an AggregateException object and returned by the Exception property. Exceptions thrown in this manner must be handled either by a task continuation that checks the Exception property, or by calling Wait or checking the Result property within an exception handling block that includes a handler for AggregateException.
In libraries that I create, I provide an additional guarantee for users which reads as follows:
Note: only assume the following quote is true for a library which explicitly states it.
This library additionally ensures that exceptions thrown by asynchronous operations are not wrapped in multiple layers of AggregateException. In other words, an ArgumentException thrown during the asynchronous execution of a task will result in the Exception property returning an AggregateException, and that exception will not contain any nested instances of AggregateException in the InnerExceptions collection. In most cases, the AggregateException wraps exactly one inner exception, which is the original ArgumentException. This guarantee simplifies the use of the API is languages that support async/await, since those operators automatically unwrap the first layer of AggregateException.
Example methods which each throw an exception during task execution:
public Task SomeOperationAsync()
{
return Task.StartNew(
() =>
{
throw new ArgumentException("Directly thrown.");
});
}
public async Task SomeOtherOperationAsync()
{
throw new ArgumentException("async functions never throw exceptions directly.");
}
Example code which handles an exception during task execution:
try
{
Task myTask = SomeOperationAsync();
myTask.Wait();
}
catch (AggregateException wrapperEx)
{
ArgumentException ex = wrapperEx.InnerException as ArgumentException;
if (ex == null)
throw;
// ex was thrown during the asynchronous portion of SomeOperationAsync. This is
// always the case if SomeOperationAsync is an async function (§10.15 - C#
// Language Specification Version 5.0).
}
Consistent Exception Handling
Applications implementing specialized handling for exception which occur during asynchronous calls have multiple options available for consistent handling. The simplest solution, when available, involves using async/await. These operators automatically unwrap the first exception instance in the InnerExceptions collection of an AggregateException, resulting in behavior that appears to calling code as though the exception was directly thrown by the invoked method. The second method involves treating the original call as a continuation of another task, ensuring that all exceptions are presented as an AggregateException to the exception handling code. The following code shows the application of this strategy to an existing asynchronous call. Note that the CompletedTask class and Then() extension method are part of the separate Rackspace Threading Library (open-source, Apache 2.0).
// original asynchronous method invocation
Task task1 = SomeOperationAsync();
// method invocation treated as a continuation
Task task2 = CompletedTask.Default.Then(_ => SomeOperationAsync());
Code using the continuation strategy for consistent error handling may benefit from the use of the Catch() methods, which are also part of the Rackspace Threading Library. This extension method behaves in a manner similar to await, automatically unwrapping the first exception instance in the InnerExceptions collection of an AggregateException before invoking the continuation function which handles the exception.
As of Transient Fault Handling v6.0.1304.0 , the following code successfully retries as per the configured detection strategy:
Strategy:
public class SimpleHandlerStartegy : ITransientErrorDetectionStrategy
{
public bool IsTransient(Exception ex)
{
if (ex is WebException)
{
return true;
}
return false;
}
}
Code That Throws WebException:
async Task<int> SomeAsyncWork()
{
await Task.Delay(1000);
throw new WebException("This is fake");
return 1; // Unreachable!!
}
Client Code:
var retryStrategy = new Incremental(3, TimeSpan.FromSeconds(1), TimeSpan.FromSeconds(2));
var retryPolicy = new RetryPolicy<SimpleHandlerStartegy>(retryStrategy);
retryPolicy.Retrying += (sender, retryArgs) =>
{
Console.WriteLine("Retrying {0}, Delay{1}, Last Exception: {2}", retryArgs.CurrentRetryCount, retryArgs.Delay, retryArgs.LastException);
};
// In real world, await this to get the return value
retryPolicy.ExecuteAsync(() => SomeAsyncWorkThatThrows());
As far as I understand, exceptions that are raised within an asynchronous code block are delivered back to the main thread within an aggregate exception. I suppose this is because raising an exception doesn't necessarily cause execution to be returned to the main thread and therefore we could have more than one exception returned.
I have the following code that runs without throwing an exception:
var t = Task.Factory.StartNew(() => LongRunningMethod(cancellationToken, progress), cancellationToken);
t.ContinueWith(Callback, TaskScheduler.FromCurrentSynchronizationContext());
Inside 'LongRunningMethod', I call cancellationToken.ThrowIfCancellationRequested(). The callback will always get called (which is what I want), and the Task that gets passed to the callback correctly has IsCancelled set to true or false.
Using the async/await keywords, I have to modify the above lines to the following:
try
{
await Task.Factory.StartNew(() => LongRunningMethod(cancellationToken, progress), cancellationToken);
textEdit1.Text = "Done";
}
catch (OperationCanceledException)
{
textEdit1.Text = "Cancelled";
}
Why, in this case, does the ThrowIfCancellationRequested() throw an actual exception that I need to catch?
With ContinueWith, you're given the Task that was previously run and you can ask it if it cancelled or not (Task.IsCancelled). With await, you don't have that. The only way to communicate cancel is through the exception.
Now, await is simply uses Tasks, so you can "interject" with a continuation. For example:
await Task.Factory
.StartNew(() => LongRunningMethod(cancellationToken, progress), cancellationToken)
.ContinueWith(t=>Trace.WriteLine("Canceled"), TaskContinuationOptions.OnlyOnCanceled);
You can still use await and then use ContinueWith to handle only the cancellation scenario. so, technically the await is awaiting on the continuation.
async was designed to make asynchronous code easier, and as much as possible like the equivalent synchronous code.
First off, note that there was always an exception being thrown. ThrowIfCancellationRequested will (surprise) throw an exception if cancellation has been requested.
In your existing code, this exception is caught and then placed on the Task (wrapped in an AggregateException). The Task interprets this condition as being "canceled". Then you can just check the boolean flag in your continuation.
But consider the equivalent synchronous code:
try
{
LongRunningMethod(cancellationToken, progress);
}
catch (OperationCanceledException)
{
}
And that looks a lot like the async approach. Even if you use ContinueWith, there is still an exception that is thrown and being caught - logically, you are doing a try/catch. Personally, I prefer the explicit try/catch because:
The intent is clearer, so the code is easier to read and maintain.
The code is more accessible (the vast majority of C# programmers understand try/catch; a relative minority understand ContinueWith).
However, ContinueWith is (slightly) more efficient.