如何正确比较 C# 中的十进制值？

Question

我有 C++ 的背景，我知道你无法准确地比较浮点数的相等性。对于 C#，我只是假设相同的策略适用于十进制值或任何浮点值。

基本上，我有两个十进制值，如果它们不相等，我需要执行一些操作。例如：

decimal value1, value2;
// Assume value1 and value2 are set somewhere to valid values.
if( value1 != value2 )
{
    // Do something
}

如果这不能按预期工作，我愿意接受一个解决方案，它可以用误差幅度进行相等比较，比如 .00001 或类似的东西。这个问题的推荐解决方案是什么？

Answer 1

您的代码将按预期工作。 C# decimals 经过优化，可以非常准确地表示以 10 为基数的数字，因此如果您要比较的是（金钱，...），一切都应该没问题。

这是 Jon Skeet 对小数准确性的非常清楚的解释：

Difference between decimal, float and double in .NET?

Answer 2

我正在研究类似的东西，但要精确而不是误差范围，最后为 Float 编写了一些扩展。不过，这可以很容易地适应任何类型。我进行了一系列复杂的比较，这使它变得美观且易读。

/// <summary>
/// A set of extensions to allow the convenient comparison of float values based on a given precision.
/// </summary>
public static class FloatingPointExtensions
{
    /// <summary>
    /// Determines if the float value is less than or equal to the float parameter according to the defined precision.
    /// </summary>
    /// <param name="float1">The float1.</param>
    /// <param name="float2">The float2.</param>
    /// <param name="precision">The precision.  The number of digits after the decimal that will be considered when comparing.</param>
    /// <returns></returns>
    public static bool LessThan(this float float1, float float2, int precision)
    {
        return (System.Math.Round(float1 - float2, precision) < 0);
    }

    /// <summary>
    /// Determines if the float value is less than or equal to the float parameter according to the defined precision.
    /// </summary>
    /// <param name="float1">The float1.</param>
    /// <param name="float2">The float2.</param>
    /// <param name="precision">The precision.  The number of digits after the decimal that will be considered when comparing.</param>
    /// <returns></returns>
    public static bool LessThanOrEqualTo(this float float1, float float2, int precision)
    {
        return (System.Math.Round(float1 - float2, precision) <= 0);
    }

    /// <summary>
    /// Determines if the float value is greater than (>) the float parameter according to the defined precision.
    /// </summary>
    /// <param name="float1">The float1.</param>
    /// <param name="float2">The float2.</param>
    /// <param name="precision">The precision.  The number of digits after the decimal that will be considered when comparing.</param>
    /// <returns></returns>
    public static bool GreaterThan(this float float1, float float2, int precision)
    {
        return (System.Math.Round(float1 - float2, precision) > 0);
    }

    /// <summary>
    /// Determines if the float value is greater than or equal to (>=) the float parameter according to the defined precision.
    /// </summary>
    /// <param name="float1">The float1.</param>
    /// <param name="float2">The float2.</param>
    /// <param name="precision">The precision.  The number of digits after the decimal that will be considered when comparing.</param>
    /// <returns></returns>
    public static bool GreaterThanOrEqualTo(this float float1, float float2, int precision)
    {
        return (System.Math.Round(float1 - float2, precision) >= 0);
    }

    /// <summary>
    /// Determines if the float value is equal to (==) the float parameter according to the defined precision.
    /// </summary>
    /// <param name="float1">The float1.</param>
    /// <param name="float2">The float2.</param>
    /// <param name="precision">The precision.  The number of digits after the decimal that will be considered when comparing.</param>
    /// <returns></returns>
    public static bool AlmostEquals(this float float1, float float2, int precision)
    {
        return (System.Math.Round(float1 - float2, precision) == 0);
    } 
}

Answer 3

我同意其他答案，但我遇到了一个问题，即一个“真实的”服务器端小数与来自 JSON/浏览器的小数进行比较（并且在某些时候一定是浮点数）。

我最终将这段代码四舍五入到小数点后 2 位，这在我的情况下已经足够精确了：

if (Decimal.Round(serverTotalPrice, 2) != Decimal.Round(request.TotalPrice, 2)) {
    throw new ArgumentException("The submitted Total Price is not valid");
}

Answer 4

我认为this 会解决你的问题。

基本上有一个decimal.compare方法。

编辑：这可能是更好的方法：

Decimal.Equals

EDIT2：如果您可以按照上面的建议直接比较，那可能会更有效。我会留下这个，因为它可能很有趣。