If you’ve written a .NET Lambda function which calls async code and returns either `Task` or `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 `FormatException`.

![](https://hackernoon.com/hn-images/1*JoypzysM4eC2Qg0iZ74pNA.png)

The `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](https://www.reddit.com/r/aws/comments/98witj/we_are_the_aws_net_team_ask_the_experts/e98xinf/).

When the `UNWRAP_AGGREGATE_EXCEPTIONS` environment variable is set to `1` or `true`, Lambda will call `GetAwaiter().GetResult()` instead of using the`Result` property.

> Note that this only works if your method returns `Task` or `Task<T>`.

![](https://hackernoon.com/hn-images/1*4ou5ZjjOBs5sZk46SZ_v0A.png)

This causes the **first exception thrown** to be returned instead of an `AggregateException`.

![](https://hackernoon.com/hn-images/1*s71J796Elu0iw8yIPV4eBA.png)

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 `Task.WaitAll` or `Task.WhenAll`.

![](https://hackernoon.com/hn-images/1*Z-3m-8iOJiwwT53fxXZt5w.png)

This convoluted example runs three tasks in parallel, two of which throw an exception.

![](https://hackernoon.com/hn-images/1*z3Bv3Rel9Ar-X0xKzPfS_A.png)

Setting the `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.

For more like this, please follow me on [Medium](https://medium.com/@zaccharles) and [Twitter](https://twitter.com/zaccharles).

Twitter

Too Long; Didn't Read

Get hands-on learning from ML experts on Coursera 

How to Unwrap an AggregateException Thrown by AWS Lambda

Too Long; Didn't Read

Companies Mentioned

@zaccharles

RELATED STORIES