【转】C#中Math.Round()实现中国式四舍五入

原文地址：https://www.cnblogs.com/xiaofengfeng/p/4729973.html

C#中的Math.Round()并不是使用的"四舍五入"法。其实在VB、VBScript、C#、J#、T-SQL中Round函数都是采用Banker's rounding（银行家算法），即：四舍六入五取偶。事实上这也是IEEE的规范，因此所有符合IEEE标准的语言都应该采用这样的算法。

.NET 2.0 开始，Math.Round 方法提供了一个枚举选项 MidpointRounding.AwayFromZero 可以用来实现传统意义上的"四舍五入"。即： Math.Round(4.5, MidpointRounding.AwayFromZero) = 5。

Round(Decimal)
Round(Double)
Round(Decimal, Int32)
Round(Decimal, MidpointRounding)
Round(Double, Int32)
Round(Double, MidpointRounding)
Round(Decimal, Int32, MidpointRounding)
Round(Double, Int32, MidpointRounding)

 如：

Math.Round(0.4) //result:0

Math.Round(0.6) //result:1

Math.Round(0.5) //result:0

Math.Round(1.5) //result:2

Math.Round(2.5) //result:2

Math.Round(3.5) //result:4

Math.Round(4.5) //result:4

使用MidpointRounding.AwayFromZero重载后对比：   

Math.Round(0.4, MidpointRounding.AwayFromZero); // result:0

Math.Round(0.6, MidpointRounding.AwayFromZero); // result:1

Math.Round(0.5, MidpointRounding.AwayFromZero); // result:1

Math.Round(1.5, MidpointRounding.AwayFromZero); // result:2

Math.Round(2.5, MidpointRounding.AwayFromZero); // result:3

Math.Round(3.5, MidpointRounding.AwayFromZero); // result:4

Math.Round(4.5, MidpointRounding.AwayFromZero); // result:5

 但是悲剧的是，如果用这个计算小数的话，就不灵了！！！

必须用第七个重载方法，

decimal Round(decimal d, int decimals, MidpointRounding mode)

这样计算出来的小数才是真正的中国式四舍五入！！

?Math.Round(526.925, 2)
526.92

?Math.Round(526.925, 2,MidpointRounding.AwayFromZero)
526.92

?Math.Round((decimal)526.925, 2)
526.92

?Math.Round((decimal)526.925, 2,MidpointRounding.AwayFromZero)
526.93

 

扩展

只入不舍

 /// <summary>
        /// 按小数位四舍五入
        /// </summary>
        /// <param name="value"></param>
        /// <param name="decimals"></param>
        /// <returns></returns>
        public static double ToRound(this double value, int decimals)
        {
            var c = Math.Pow(10, decimals);
            var s = Math.Truncate(value * c);
            if (s < value * c)
            {
                s += 1;
            }
            return Math.Round(s / 100, decimals);
        }