Effective Date Field

Looking into a solution to turn the effective_date field to a null when a lifecycle state is occurring. I'm assuming I need put a server method on a lifecycle transition, but I am having trouble writing the proper code in C#. The issue is when the lifecycle states are changing, but there is no actual change to the item. When the item changes it's not an issue because the effective_date field automatically becomes null when the item reverts to Preliminary state.