CarsCache.cs

class CarsCache
{
    private AsyncExpiringLazy<List<Car>> Cars { get; set; }

    public CarService CarService { get; set; }

    public CarsCache()
    {
        Cars = new AsyncExpiringLazy<List<Car>>(async metadata =>
        {
            return await Task.Run(() =>
            {
                return new ExpirationMetadata<List<Car>>
                {
                    Result = CarService.GetAllCars(),
                    ValidUntil = DateTimeOffset.UtcNow.AddHours(12)
                };
            });
        });
    }

    public Task<List<Car>> GetAllCars()
    {
        return Cars.Value();
    }
}

// Par la suite, l'appel se fait ainsi
var cache = new CarsCache();
var cars = await cache.GetAllCars();