在日常开发中经常遇到四舍五入的情况比如 Math.Round(1.25, 1),首先我们要知道这里的Round 其实是银行家算法,具体可以参考Round() 四舍五入 js银行家算法 那么C#是如何实现的了,我们来看看decimal的round实现如下:
[System.Security.SecuritySafeCritical] // auto-generated public static Decimal Round(Decimal d, int decimals) { FCallRound (ref d, decimals); return d; } public static Decimal Round(Decimal d, MidpointRounding mode) { return Round(d, 0, mode); } [System.Security.SecuritySafeCritical] // auto-generated public static Decimal Round(Decimal d, int decimals, MidpointRounding mode) { if ((decimals < 0) || (decimals > 28)) throw new ArgumentOutOfRangeException("decimals", Environment.GetResourceString("ArgumentOutOfRange_DecimalRound")); if (mode < MidpointRounding.ToEven || mode > MidpointRounding.AwayFromZero) { throw new ArgumentException(Environment.GetResourceString("Argument_InvalidEnumValue", mode, "MidpointRounding"), "mode"); } Contract.EndContractBlock(); if (mode == MidpointRounding.ToEven) { FCallRound (ref d, decimals); } else { InternalRoundFromZero(ref d, decimals); } return d; } [System.Security.SecurityCritical] // auto-generated [ResourceExposure(ResourceScope.None)] [MethodImplAttribute(MethodImplOptions.InternalCall)] private static extern void FCallRound(ref Decimal d, int decimals); // Does an in-place round the specified number of digits, rounding mid-point values // away from zero private static void InternalRoundFromZero(ref Decimal d, int decimalCount) { Int32 scale = (d.flags & ScaleMask) >> ScaleShift; Int32 scaleDifference = scale - decimalCount; if (scaleDifference <= 0) { return; } // Divide the value by 10^scaleDifference UInt32 lastRemainder; UInt32 lastDivisor; do { Int32 diffChunk = (scaleDifference > MaxInt32Scale) ? MaxInt32Scale : scaleDifference; lastDivisor = Powers10[diffChunk]; lastRemainder = InternalDivRemUInt32(ref d, lastDivisor); scaleDifference -= diffChunk; } while (scaleDifference > 0); // Round away from zero at the mid point if (lastRemainder >= (lastDivisor >> 1)) { InternalAddUInt32RawUnchecked(ref d, 1); } // the scale becomes the desired decimal count d.flags = ((decimalCount << ScaleShift) & ScaleMask) | (d.flags & SignMask); }
所以我们如果想要四舍五入 那么方法必须穿入MidpointRounding 参数,调用托管代码;不传这是银行家算法调用非托管代码,从性能的角度讲应该竟可能调用托管代码。所以除非有要求,尽力调用托管代码吧。