C#:如何将开关与if语句组合？

有什么不对:

if(starttime<=endtime)
{
    switch (periodtype)
    {
        case 0: nextRunTime = nextRunTime.AddHours(period); break;
        case 1: nextRunTime = nextRunTime.AddMinutes(period); break;
        case 2: nextRunTime = nextRunTime.AddSeconds(period); break;
    }
}