How to Unwrap an AggregateException Thrown by AWS Lambda
If you’ve written a .NET Lambda function which calls async code and returns either
Task<T>, you’ve probably seen an
AggregateException which wraps the real exception.
This example comes from the handler below which calls an asnyc method that always throws a
AggregateException can be unwrapped, though. When AWS added .NET Core 2.1 support to Lambda, they also added an undocumented feature toggle. The only real mention of it is by the AWS .NET team on Reddit.
UNWRAP_AGGREGATE_EXCEPTIONS environment variable is set to
true, Lambda will call
GetAwaiter().GetResult() instead of using the
Note that this only works if your method returns
This causes the first exception thrown to be returned instead of an
To avoid compatibility issues, this is opt-in via the environment variable. However, in the Reddit comment, the team says they plan to make it the default behaviour in future versions. I think they’ll keep the toggle, though, because it can be useful.
The reason you get an
AggregateException is that a single
Task can represent multiple parallel tasks, each of which can throw an exception. You’ll most often see this when using
This convoluted example runs three tasks in parallel, two of which throw an exception.
UNWRAP_AGGREGATE_EXCEPTIONS environment variable gets us back to just the
FormatException which was thrown first. In the future, you’ll need to toggle unwrapping exceptions off when you want to see every exception.