Service4.cs

class Service
{
    class NullAuditor : IAuditor
    {
        void RecordThatSomethingWasDone()
        {
            // does nothing
        }
    }
    
    private readonly IAuditor _nullAuditor = new NullAuditor();
    
    public void DoSomething()
    {
        DoSomething(_nullAuditor);
    }

    public void DoSomething(IAuditor auditor)
    {
        // do something…
        auditor.RecordThatSomethingWasDone();
    }
 }