In your documentation, (Application Developper, Tracking), you say that you have access to number of secondaries but there is no method of GetSecondaries in G4ParticleChange. I find one in G4VParticleChange but I don't understand how to use it. Could you help me? Thanks.
ParticleChange is NOT meant to the user's use. Once G4SteppingManager collects secondaries made by the particular ParticleChange, it contains no secondaries anymore. To get information about the current step, I advise to make your own SteppingVerbose class, or to import some of its code into your SteppingAction (or sensitive datector). Refer to the exampleN03 for the use of G4SteppingVerbose.