decimal a = 0.45M;
           Console.WriteLine(Math.Round(a,1,MidpointRounding.AwayFromZero));
           Console.WriteLine(Math.Round(a,1,MidpointRounding.ToEven));
           Console.WriteLine(Math.Round(a,1));
           string dateStr = DateTime.Now.ToString("MMM dd", CultureInfo.CreateSpecificCulture("en-US"));
            Console.ReadLine();

结果是 ：

0.5

0.4

0.4

Math.Round(a,1,MidpointRounding.AwayFromZero) 为常用的 四舍五入

MidpointRounding.AwayFromZero

当一个数字是其他两个数字的中间值时，会将其舍入为两个值中绝对值较小的值 应该是 绝对值较大的值