如何强制运行时常量为编译时间常量? [英] How to force a runtime constant to be a compile time constant?

问题描述

所以我正在做一个基于化学的项目，遇到了这个棘手的问题.我有一堆函数进行化学类型计算，并希望将avogadros数作为函数的默认参数传递.让我说说代码:

So I am working on a chemistry based project and ran into this tricky problem. I have a bunch of functions doing chemistry type calculations and want to pass avogadros number as a default parameter for a function. Let me just let the code talk:

class Constants 
{
    //must be readonly to b/c Math.Pow is calculated at run-time
    public static double readonly avogadrosNum = 6.022*Math.Pow(10,-22);
} 

class chemCalculations
{   //getting default parameter must be a compile-time constant
    public double genericCalc(double avogadrosNum = Constants.avogadrosNum);  
}

不知道指数格式，谢谢大家

was unaware of exponential format, thanks guys

推荐答案

通常不能.就编译器而言，涉及方法调用的任何内容都不是 的编译时常量.

You can't, in general. Anything which involves a method call is not going to be a compile-time constant, as far as the compiler is concerned.

您可以进行的操作是使用科学计数法表示 double 文字:

What you can do is express a double literal using scientific notation though:

public const double AvogadrosNumber = 6.022e-22;

因此在这种特定情况下，您可以在不损失可读性的情况下编写它.

So in this specific case you can write it with no loss of readability.

在其他设置中，只要类型是原始类型或 decimal 之一，您就可以将常量写为文字，并使用注释来解释如何获取它.例如:

In other settings, so long as the type is one of the primitive types or decimal, you can just write out the constant as a literal, and use a comment to explain how you got it. For example:

// Math.Sqrt(Math.PI)
public const double SquareRootOfPi = 1.7724538509055159;

请注意，即使不能在常量表达式中使用方法调用，其他运算符也可以使用.例如:

Note that even though method calls can't be used in constant expressions, other operators can. For example:

// This is fine
public const double PiSquared = Math.PI * Math.PI;

// This is invalid
public const double PiSquared = Math.Pow(Math.PI, 2);

有关常量表达式中允许的内容的更多详细信息，请参见C#5规范的7.19节.

See section 7.19 of the C# 5 specification for more details about what is allowed within a constant expression.

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