I ran into this problem at work and wanted to capture the findings for future me.

ETW uses the ActivityId field on the TEB to track an activity. This gets logged with the ETW events.

When adding async Tasks to the mix, Tasks have an abstraction on the execution context that is per Task. Tasks may run on multiple different threads and so it’s possible that a Task gets hooked up to a thread that already has an activityId set. This messes up logging.

AsyncLocal provides a way to “flow” an ambient value across multiple Tasks as part of a logical operation. This can be used to track activityId on Tasks, but the trick is getting it pushed down onto the native thread.

AsyncLocal has a constructor argument that is a callback that is executed when the value changes. Values are always refreshed from empty when a Task is assigned to a thread. This provides a place where the activityId can be flowed down to the executing thread so that ETW logging works.

See ActivityTracker.cs on ReferenceSource to see how this works in .NET Framework.