如何在C#中将一个数字四舍五入到两位小数？

你可能需要检查Math.Round的舍入机制：

http://msdn.microsoft.com/en-us/library/system.midpointrounding.aspx

除此之外，我建议使用Math.Round(inputNumer, numberOfPlaces)方法而不是*100/100的方法，因为它更简洁。

使用Math.Round(YourNumber, 2)可以指定你想要保留的小数位数。

你可以在这里阅读更多信息。

Math.Floor(123456.646 * 100) / 100 会返回 123456.64

1.5 => 2
2.5 => 2
3.5 => 4
4.5 => 4

让我来澄清一下。实际上，你可以采用五种策略之一来获取你从 Math.Round() 中期望得到的结果：

1. ToEven (默认) 当一个数恰好处于两个其他数的中间时，它会被舍入为最接近的偶数。

2. AwayFromZero (数学舍入) 当一个数恰好处于两个其他数的中间时，它会被舍入为离零最远的数。（也就是四舍五入）

3. ToZero 根据指定的精度向下舍入该数。

4. ToNegativeInfinity (取整) 根据指定的精度向下舍入该数。

5. ToPositiveInfinity (取整) 根据指定的精度向上舍入该数。

以下是代码，可以了解它们的工作原理：

var num1 = 1.5;
var num2 = 2.5;
var num3 = 3.5;
var num4 = 2.51;
var num5 = 2.48;

Console.WriteLine("----------------Round with MidpointRounding.ToEven (default) ---------------------");
Console.WriteLine($"{num1} => {Math.Round(num1, MidpointRounding.ToEven)}");
Console.WriteLine($"{num2} => {Math.Round(num2, MidpointRounding.ToEven)}");
Console.WriteLine($"{num3} => {Math.Round(num3, MidpointRounding.ToEven)}");
Console.WriteLine($"{num4} => {Math.Round(num4, MidpointRounding.ToEven)}");
Console.WriteLine($"{num5} => {Math.Round(num5, MidpointRounding.ToEven)}");

Console.WriteLine("----------------Round with MidpointRounding.AwayFromZero (mathematical rounding) ---------------------");
Console.WriteLine($"{num1} => {Math.Round(num1, MidpointRounding.AwayFromZero)}");
Console.WriteLine($"{num2} => {Math.Round(num2, MidpointRounding.AwayFromZero)}");
Console.WriteLine($"{num3} => {Math.Round(num3, MidpointRounding.AwayFromZero)}");
Console.WriteLine($"{num4} => {Math.Round(num4, MidpointRounding.AwayFromZero)}");
Console.WriteLine($"{num5} => {Math.Round(num5, MidpointRounding.AwayFromZero)}");

Console.WriteLine("----------------Round with MidpointRounding.ToZero ---------------------");
Console.WriteLine($"{num1} => {Math.Round(num1, MidpointRounding.ToZero)}");
Console.WriteLine($"{num2} => {Math.Round(num2, MidpointRounding.ToZero)}");
Console.WriteLine($"{num3} => {Math.Round(num3, MidpointRounding.ToZero)}");
Console.WriteLine($"{num4} => {Math.Round(num4, MidpointRounding.ToZero)}");
Console.WriteLine($"{num5} => {Math.Round(num5, MidpointRounding.ToZero)}");

Console.WriteLine("----------------Round with MidpointRounding.ToNegativeInfinity (floor) ---------------------");
Console.WriteLine($"{num1} => {Math.Round(num1, MidpointRounding.ToNegativeInfinity)}");
Console.WriteLine($"{num2} => {Math.Round(num2, MidpointRounding.ToNegativeInfinity)}");
Console.WriteLine($"{num3} => {Math.Round(num3, MidpointRounding.ToNegativeInfinity)}");
Console.WriteLine($"{num4} => {Math.Round(num4, MidpointRounding.ToNegativeInfinity)}");
Console.WriteLine($"{num5} => {Math.Round(num5, MidpointRounding.ToNegativeInfinity)}");

Console.WriteLine("----------------Round with MidpointRounding.ToPositiveInfinity (ceiling) ---------------------");
Console.WriteLine($"{num1} => {Math.Round(num1, MidpointRounding.ToPositiveInfinity)}");
Console.WriteLine($"{num2} => {Math.Round(num2, MidpointRounding.ToPositiveInfinity)}");
Console.WriteLine($"{num3} => {Math.Round(num3, MidpointRounding.ToPositiveInfinity)}");
Console.WriteLine($"{num4} => {Math.Round(num4, MidpointRounding.ToPositiveInfinity)}");
Console.WriteLine($"{num5} => {Math.Round(num5, MidpointRounding.ToPositiveInfinity)}");

结果：

----------------Round with MidpointRounding.ToEven (default) ---------------------
1.5 => 2
2.5 => 2
3.5 => 4
2.51 => 3
2.48 => 2
----------------Round with MidpointRounding.AwayFromZero (mathematical rounding) ---------------------
1.5 => 2
2.5 => 3
3.5 => 4
2.51 => 3
2.48 => 2
----------------Round with MidpointRounding.ToZero ---------------------
1.5 => 1
2.5 => 2
3.5 => 3
2.51 => 2
2.48 => 2
----------------Round with MidpointRounding.ToNegativeInfinity ---------------------
1.5 => 1
2.5 => 2
3.5 => 3
2.51 => 2
2.48 => 2
----------------Round with MidpointRounding.ToPositiveInfinity ---------------------
1.5 => 2
2.5 => 3
3.5 => 4
2.51 => 3
2.48 => 3

string a = "10.65678";

decimal d = Math.Round(Convert.ToDouble(a.ToString()),2)

  public double RoundDown(double number, int decimalPlaces)
        {
            return Math.Floor(number * Math.Pow(10, decimalPlaces)) / Math.Pow(10, decimalPlaces);
        }