Service.cs

interface IAuditor
{
    void RecordThatSomethingWasDone();
}

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