How to run Application subclass first?

Hello,

I am a new user of Xamarin, and am studying how to use.

http://docs.xamarin.com/guides/cross-platform/application_fundamentals/data/part_5_using_data_in_an_app/

Following the article I want to copy a database file to equipment.

"you can create an Application subclass that is loaded before any activities"

I created a subclass, but When I debug, the subclass was not run into.

How to config to let the Application subclass run first?

Thanks / Berry Wang