Math.Round()并不是严格意义上的是四舍五入函数。它默认的执行的是"银行家舍入"算法，即四舍六入五取偶。概括为：四舍六入五考虑、五后非零就进一，五后皆零看奇偶，五前为偶应舍去、五前为奇要进一。
当为5时，取离着最近的偶数。见下图：
测试代码如下：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace MathRoundTest
{
class Program
{
static void Main(string[] args)
{
Console.WriteLine(Math.Round(1234.5).ToString()); //输出：1234
Console.WriteLine(Math.Round(1234.50).ToString()); //输出：1234
Console.WriteLine(Math.Round(1235.5).ToString()); //输出：1236
Console.WriteLine(Math.Round(1235.50).ToString()); //输出：1236
Console.WriteLine("---------------------------------------------");
Console.WriteLine(Math.Round(-1234.5).ToString()); //输出：-1234
Console.WriteLine(Math.Round(-1234.50).ToString()); //输出：-1234
Console.WriteLine(Math.Round(-1235.5).ToString()); //输出：-1236
Console.WriteLine(Math.Round(-1235.5).ToString()); //输出：-1236
Console.WriteLine("---------------------------------------------");
Console.WriteLine(Math.Round(123.45, 1).ToString()); //输出：123.4
Console.WriteLine(Math.Round(123.55, 1).ToString()); //输出：123.6
Console.WriteLine("---------------------------------------------");
Console.WriteLine(Math.Round(-123.45, 1).ToString()); //输出：-123.4
Console.WriteLine(Math.Round(-123.55, 1).ToString()); //输出：-123.6
Console.WriteLine("---------------------------------------------");
Console.ReadLine();
}
}
}
如果要实现我们所需要的四舍五入，需要使用使用四舍五入策略参数：MidpointRounding.AwayFromZero （当为5时，取远离0的数值）。
测试代码如下：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace MathRoundTest
{
class Program
{
static void Main(string[] args)
{
Console.WriteLine(Math.Round(1234.5,MidpointRounding.AwayFromZero).ToString()); //输出：1235
Console.WriteLine(Math.Round(1234.50, MidpointRounding.AwayFromZero).ToString()); //输出：1235
Console.WriteLine(Math.Round(1235.5, MidpointRounding.AwayFromZero).ToString()); //输出：1236
Console.WriteLine(Math.Round(1235.50, MidpointRounding.AwayFromZero).ToString()); //输出：1236
Console.WriteLine("---------------------------------------------");
Console.WriteLine(Math.Round(-1234.5, MidpointRounding.AwayFromZero).ToString()); //输出：-1235
Console.WriteLine(Math.Round(-1234.50, MidpointRounding.AwayFromZero).ToString()); //输出：-1235
Console.WriteLine(Math.Round(-1235.5, MidpointRounding.AwayFromZero).ToString()); //输出：-1236
Console.WriteLine(Math.Round(-1235.5, MidpointRounding.AwayFromZero).ToString()); //输出：-1236
Console.WriteLine("---------------------------------------------");
Console.WriteLine(Math.Round(123.45, 1, MidpointRounding.AwayFromZero).ToString()); //输出：123.5
Console.WriteLine(Math.Round(123.55, 1, MidpointRounding.AwayFromZero).ToString()); //输出：123.6
Console.WriteLine("---------------------------------------------");
Console.WriteLine(Math.Round(-123.45, 1, MidpointRounding.AwayFromZero).ToString()); //输出：-123.5
Console.WriteLine(Math.Round(-123.55, 1, MidpointRounding.AwayFromZero).ToString()); //输出：-123.6
Console.WriteLine("---------------------------------------------");
Console.ReadLine();
}
}
}