c# - 为什么使用非十进制数据类型对金钱不利？

Question

tl;dr:我的 Cur 怎么了？ (货币)结构？

tl;dr 2: 请先阅读问题的其余部分，然后再用 float 举例说明或 double . :-)

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我知道这个问题在整个互联网上已经出现过无数次，但我还没有看到令人信服的答案，所以我想我会再问一次。

我不明白为什么使用非十进制数据类型不利于处理金钱。 (这是指存储二进制数字而不是十进制数字的数据类型。)

是的，比较两个 double 是不明智的与 a == b .但是你可以很容易地说 a - b <= EPSILON或类似的东西。

这种方法有什么问题？

例如，我刚刚制作了一个 struct我认为在 C# 中可以正确处理金钱，而无需使用任何基于小数的数据格式:

struct Cur
{
  private const double EPS = 0.00005;
  private double val;
  Cur(double val) { this.val = Math.Round(val, 4); }
  static Cur operator +(Cur a, Cur b) { return new Cur(a.val + b.val); }
  static Cur operator -(Cur a, Cur b) { return new Cur(a.val - b.val); }
  static Cur operator *(Cur a, double factor) { return new Cur(a.val * factor); }
  static Cur operator *(double factor, Cur a) { return new Cur(a.val * factor); }
  static Cur operator /(Cur a, double factor) { return new Cur(a.val / factor); }
  static explicit operator double(Cur c) { return Math.Round(c.val, 4); }
  static implicit operator Cur(double d) { return new Cur(d); }
  static bool operator <(Cur a, Cur b) { return (a.val - b.val) < -EPS; }
  static bool operator >(Cur a, Cur b) { return (a.val - b.val) > +EPS; }
  static bool operator <=(Cur a, Cur b) { return (a.val - b.val) <= +EPS; }
  static bool operator >=(Cur a, Cur b) { return (a.val - b.val) >= -EPS; }
  static bool operator !=(Cur a, Cur b) { return Math.Abs(a.val - b.val) < EPS; }
  static bool operator ==(Cur a, Cur b) { return Math.Abs(a.val - b.val) > EPS; }
  bool Equals(Cur other) { return this == other; }
  override int GetHashCode() { return ((double)this).GetHashCode(); }
  override bool Equals(object o) { return o is Cur && this.Equals((Cur)o); }
  override string ToString() { return this.val.ToString("C4"); }
}

(很抱歉，将名称 Currency 更改为 Cur，变量名称不佳，省略了 public，布局也不好；我试图将其全部显示在屏幕上，以便您可以阅读它没有滚动。):)

你可以像这样使用它:

Currency a = 2.50;
Console.WriteLine(a * 2);

当然，C# 有 decimal数据类型，但这不是重点——问题是为什么上面的是危险的，而不是为什么我们不应该使用 decimal .

那么有人会介意为我提供一个真实世界的反例 一个在 C# 中会失败的危险语句吗？我想不出。

谢谢!

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注意:我不争论是否decimal是个不错的选择。我在问为什么说基于二进制的系统是不合适的。

Answer 1

对于积累和减少资金而言， float 并不稳定。这是您的实际示例:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace BadFloat
{
    class Program
    {
        static void Main(string[] args)
        {
            Currency yourMoneyAccumulator = 0.0d;
            int count = 200000;
            double increment = 20000.01d; //1 cent
            for (int i = 0; i < count; i++)
                yourMoneyAccumulator += increment;
            Console.WriteLine(yourMoneyAccumulator + " accumulated vs. " + increment * count + " expected");
        }
    }

    struct Currency
    {
        private const double EPSILON = 0.00005;
        public Currency(double value) { this.value = value; }
        private double value;
        public static Currency operator +(Currency a, Currency b) { return new Currency(a.value + b.value); }
        public static Currency operator -(Currency a, Currency b) { return new Currency(a.value - b.value); }
        public static Currency operator *(Currency a, double factor) { return new Currency(a.value * factor); }
        public static Currency operator *(double factor, Currency a) { return new Currency(a.value * factor); }
        public static Currency operator /(Currency a, double factor) { return new Currency(a.value / factor); }
        public static Currency operator /(double factor, Currency a) { return new Currency(a.value / factor); }
        public static explicit operator double(Currency c) { return System.Math.Round(c.value, 4); }
        public static implicit operator Currency(double d) { return new Currency(d); }
        public static bool operator <(Currency a, Currency b) { return (a.value - b.value) < -EPSILON; }
        public static bool operator >(Currency a, Currency b) { return (a.value - b.value) > +EPSILON; }
        public static bool operator <=(Currency a, Currency b) { return (a.value - b.value) <= +EPSILON; }
        public static bool operator >=(Currency a, Currency b) { return (a.value - b.value) >= -EPSILON; }
        public static bool operator !=(Currency a, Currency b) { return Math.Abs(a.value - b.value) <= EPSILON; }
        public static bool operator ==(Currency a, Currency b) { return Math.Abs(a.value - b.value) > EPSILON; }
        public bool Equals(Currency other) { return this == other; }
        public override int GetHashCode() { return ((double)this).GetHashCode(); }
        public override bool Equals(object other) { return other is Currency && this.Equals((Currency)other); }
        public override string ToString() { return this.value.ToString("C4"); }
    }

}

在我的盒子上，这给出了 $4,000,002,000.0203 累计，而 C# 中预期为 4000002000。如果这在银行的许多交易中丢失，那将是一笔糟糕的交易 - 它不一定是大交易，只是很多。这有帮助吗？