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, , mode);
}
[System.Security.SecuritySafeCritical] // auto-generated
public static Decimal Round(Decimal d, int decimals, MidpointRounding mode) {
if ((decimals < ) || (decimals > ))
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 <= ) {
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 > );
// Round away from zero at the mid point
if (lastRemainder >= (lastDivisor >> )) {
InternalAddUInt32RawUnchecked(ref d, );
}
// 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。