Service2.cs

class Service
{
    private readonly IAuditor _defaultAuditor = new StandardAuditor();
    
    public void DoSomething()
    {
        DoSomething(_defaultAuditor);
    }
    
    public void DoSomething(IAuditor auditor)
    {
        // do something…
        auditor.RecordThatSomethingWasDone();
    }
 }