Can .NET 4.5 Applications Define a Global Exception Handler for Async Task Exceptions?

Exploration on global asynchronous task exception handling mechanism

With the introduction of asynchronous programming in .NET 4.5, a key question has surfaced: Can an application define a global exception handler to catch exceptions thrown in asynchronous Task methods?

Exception propagation mechanism in asynchronous tasks

Exceptions that occur in asynchronous Task methods must be observed by the code that handles task completion. If the task is not observed, the exception will lie dormant.

For example code snippet:

<code>string x = await DoSomethingAsync();</code>

If DoSomethingAsync throws an exception, it will only be observed when handling the await expression, such as placing the code in a try-catch block. Failure to observe an exception results in the exception being unhandled and possibly ignored.

AppDomain and unobserved task exceptions

.NET 4.5 provides two events to handle unobserved exceptions:

• AppDomain.CurrentDomain.UnhandledException
• TaskScheduler.UnobservedTaskException

The UnhandledException event is typically used to handle unobserved exceptions in synchronous code. However, exceptions for asynchronous Tasks are typically handled by the UnobservedTaskException event when the task attempts garbage collection.

Enable UnhandledException for asynchronous tasks

By default, .NET 4.5 does not fire the UnhandledException event for unobserved asynchronous task exceptions. This behavior can be changed by setting the ThrowUnobservedTaskExceptions property to true in the application configuration file.

Asynchronous void method

Exception handling behavior is different for asynchronous void methods. Since there is no task reference to observe, the exception is immediately thrown in the current synchronization context. In WPF, this can cause a series of events to be fired, including Dispatcher.UnhandledException, and can cause the application to crash. Since there is no task, TaskScheduler.UnobservedTaskException is not triggered in this case.

Summary

While it is possible to define a global exception handler for synchronous unobserved exceptions, the behavior of exceptions thrown in asynchronous Task methods is more nuanced. It is important to understand how these exceptions propagate and handle them accordingly to prevent them from going unobserved and causing unexpected application behavior.

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