Clever use of interfaces

gistfile1.cs

        private void setInitialUsedSpace()
        {
            _sizeCalculator.Calculate().ToList()
                .ForEach(calculation => DecreaseUsedSpaceForVolume(calculation.Key).With(calculation.Value));
            _volumes.ForEach(x => logInitialSize(x));
        }

        private void handle(UploadFinishedMessage message)
        {
            var task = message.Task;
            IncreaseUsedSpaceForVolume(task.StorageVolumeID).With(task.FileSize);
        }

        private void handle(RemoveFinishedMessage message)
        {
            var task = message.Task;
            DecreaseUsedSpaceForVolume(task.StorageVolumeID).With(task.FileSize);
        }