c# 8.0

static string RepairCar(Car brokenCar)
{
    return brokenCar switch
    {
        Ferari p => $"Ferari details: {p.Model}",
        BMW s => $"BMW details: {s.Model} {s.Age} ({s.CurrentSpeed})",
        _ => $"{brokenCar.Model} {brokenCar.Age}" //// Default
    };
}

c# 7.0

static string RepairCar(Car brokenCar)
{
    string result = string.empty;

    switch (brokenCar)
    {
        case Ferrari p: { result = $"Ferari details: {p.Model}"; }
        case BMW s: { result = $"BMW details: {s.Model} {s.Age} ({s.CurrentSpeed})"; }
        case _: { result = $"{brokenCar.Model} {brokenCar.Age}"; }
    }

    return result;
}