How to Promote Properties from an Orchestration

"The best reason to blog something is so that you can Google your own brain." -- Poorly paraphrased Scott Hanselman, yet so very true.

In that spirit, today I'm posting a reminder of a very useful trick for promoting properties out of Orchestrations.  Kudos go to Saravana Kumar and David Hurtado for posting about this long before I did, but I record this information here to make sure I can find it when next I need it.

Normally when you assign a value to a message context property from within an Orchestration, that property is not promoted, even if it was promoted prior to you assigning a value to it.  In order to promote a property within an orchestration we use an "elegant hack" as detailed below:

1. Create a new Correlation Set, and as such likely a new Correlation Type, which contains the property or properties which you wish to promote.
2. When you use a "Send" shape in your orchestration, there is a property called "Initializing Correlation Sets", check off your new Correlation Set from the drop down on this property.
3. Rejoice, you have now promoted those properties.

Now before anyone asks, no this does not create orphaned subscriptions in the Message Box, we are never creating a Receive using this Correlation Set so we're never creating a subscription.  This can be very useful, in particular I've used it just this week on a 2006 R2 project I'm working on to enable batching from an Orchestration. That is a long story, but the short version is we're creating a new message in the Orchestration which we then intend to immediately queue for batching, so we need to promote these properties without ever having passed through a pipeline with this message.