TPL – exception handling and UnobservedTaskException issue

1. Introduction

Working with Tasks is a modern way of writing asynchronous code in an easy and flexible manner. It is quite straightforward to start using them, so usually developers do not investigate thoroughly the topic. Unfortunately this often leads to unpleasant surprises – especially when it comes to exception handling. Having this in mind let’s take a look how to handle exceptions in Task and what can happen if we do it wrong.

2. Exception handling

Because of the fact that Tasks are run asynchronously we can’t just use a standard try catch block surrounding Task

In that case catch block is unreachable (I will describe later what happens with that exception), of course we could use try catch inside Task.Factory.StartNew lambda expression but this is not always the case. Usually we want to handle exception in the caller of the Task, not inside the Task itself. There are couple of ways of achieving that – depending on the situation you can use one of the following ways. Exception are propagated to caller once you start waiting for the result of Task. So basically you can use try catch block if you wait for a Task to finish – either by accessing Result property or using Wait method

However accessing Result or calling Wait method are blocking calls, so this might not be a perfect solution for every scenario. Fortunately we can also leverage ContinueWith method which gives you an ability to specify what should be done once Task finishes processing the operation. What is more important you can configure this method to only be called if exception occurs. Nonetheless there is one crucial thing you have to remember – in order to make exception handled or observed you have to access Exception property of Task

or call Handle method from AggregateException object

Otherwise exception will not be handled which may cause a lot of troubles described in section 3 of this post. Lastly if you use .NET 4.5 you can also just use async await and try catch block

3. Handling UnobservedTaskException

If it happens that exception thrown by Task is not handled, once the Task is garbage-collected, finalizer thread will throw UnobservedTaskException.

The implication of this depends on framework we are running our application on. If you run your app on .NET 4.5 or later version, with default escalation policy, TaskScheduler.UnobservedTaskException event will be raised and basically that is all. No more further problems, so basically you may not even realize that there is something like UnobservedTaskException. However if we add this entry

to app.configour process will be killed once UnobservedTaskExceptionis is thrown.
The problem arises in .NET 4.0 (if you run your app on .NET 4.0 not only compile it against this framework version), in this version of framework default escalation policy is very strict and will kill our process once the UnobservedTaskException is thrown by finalizer thread. Furthermore you cannot change this policy using app.config. Fortunately there is a one last chance to handle UnobservedExceptions and prevent our process from being killed. All you have to do is to wire up TaskScheduler.UnobservedTaskException static event handler and then set exception to observed state

There is one more thing to clarify. You can compile your library against .NET 4.0 but if you have .NET 4.5 installed on your machine, your application is launched and run on .NET 4.5. This means that escalation policy from .NET 4.5 will be used – so by default process will not be killed. In order to simulate pure .NET 4.0 behavior while having .NET 4.5 installed you have to add

in your app.config as it was mentioned before. Otherwise you would have to uninstall .NET 4.5 in order to, for example reproduce production issue. Source code for this post can be found here

TPL – exception handling and UnobservedTaskException issue

Easy way to suppress code analysis warnings

Everyone who has ever been using a code analysis knows that sometimes it is hard to follow the Microsoft coding guidelines and it is necessary to suppress some warnings. For me it always was a painful operation because I didn’t know any automated way of doing that. So basically I had to find the warning in the MSDN, copy category, code and other necessary stuff and then manually add appropriate attribute in code. Fortunately a friend of mine yesterday showed me a simple trick to generate this attribute with couple of mouse clicks. All you have to do is to right-click on warning message and choose Suppress Message context menu …
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There are two options in there. The first option – In Source, will add attribute directly in the code

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Easy way to suppress code analysis warnings