为什么IsLiteral对于小数返回false？ [英] Why does IsLiteral return false for decimal?

问题描述

以下程序将使用 IsLiteral

public static class Program
{
    public static void Main(string[] args)
    {
        foreach (var field in typeof(Program).GetFields())
        {
            System.Console.WriteLine(field.Name + " IsLiteral: " + field.IsLiteral);
        }

        System.Console.ReadLine();
    }

    public const decimal DecimalConstant = 99M;
    public const string StringConstant = "StringConstant";
    public const int IntConstant = 1;
    public const double DoubleConstant = 1D;
}

对于所有类型，除十进制之外，它均适用将返回false。

It works correctly for all types, except for decimal is will return false.

有人可以解释这种现象吗？还有更好的方法来查看字段是否恒定吗？

Can anyone explain this behavior? And is there a better way to see if a field is constant?

推荐答案

从运行时开始，它不是常量角度-因为CLR基本上不了解十进制；它不是原始类型。这就是为什么您也不能在属性中使用小数。

It's not a constant from a runtime perspective - because the CLR basically doesn't know about decimal; it's not a primitive type. That's why you can't use decimals in attributes, too.

如果查看字段的IL，您会发现它起作用：

If you look at the IL for the fields, you can see this in action:

.field public static initonly valuetype [mscorlib]System.Decimal DecimalConstant
.custom instance void [mscorlib]System.Runtime.CompilerServices.DecimalConstantAttribute::.ctor
  (uint8, uint8, uint32, uint32, uint32) =
  (01 00 00 00 00 00 00 00 00 00 00 00 63 00 00 00 00 00) 
.field public static literal string StringConstant = "StringConstant"
.field public static literal int32 IntConstant = int32(0x00000001)
.field public static literal float64 DoubleConstant = float64(1.)

注意其他常量 的IL如何具有字面意义，但 DecimalConstant 没有。相反，它只是一个应用了属性的只读字段。该属性允许其他编译器将字段视为常量并知道其值-例如，它可以出现在其他const表达式中。

Notice how the IL for the other constants does have literal in it, but DecimalConstant doesn't. Instead, it's just a read-only field with an attribute applied. The attribute allows other compilers to treat the field as a constant and know the value - so it can appear in other const expressions, for example.

然后有一个类型初始值设定项设置执行时的字段值：

There's then a type initializer to set the field value at execution time:

.method private hidebysig specialname rtspecialname static 
        void  .cctor() cil managed
{
  // Code size       13 (0xd)
  .maxstack  8
  IL_0000:  ldc.i4.s   99
  IL_0002:  newobj     instance void [mscorlib]System.Decimal::.ctor(int32)
  IL_0007:  stsfld     valuetype [mscorlib]System.Decimal Program::DecimalConstant
  IL_000c:  ret
} // end of method Program::.cctor

再次，仅在 DecimalConstant 中出现运行时直接处理其他字段。

Again, this is only present for DecimalConstant because the runtime handles the other fields directly.

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