Sometimes we want to execute code whenever we hit Play button in Unity Editor. The reason for this usually is that we want to enforce some workflow automatically such as getting specific environment information (e.g. git hash, date) and fill it in a prefab or something. We could do these things by creating a Custom Window with a button somewhere, but it feels too manual.

Fortunately, Unity has offered a simple way to do this, that is InitializeOnLoadAttribute.

http://blog.andreimarks.com/2012/08/16/unity-how-to-run-specific-code-only-when-building-out-a-project/ shows an usecase of InitializeOnLoadAttribute: to generate a tracking tag of the build with the build date.

Beware of a potential problem here

An awesome review of other ways to execute code on startup are listed here http://www.codingjargames.com/blog/2015/08/04/unity-and-initialization-order/