常量值不能转换为int [英] Constant value cannot be converted to int

问题描述

我看不到为什么第5行无法编译而第4行正常。

I can't see why line#5 fails to compile whereas line#4 is ok.

static void Main(string[] args)
{
    byte b = 0;
    int i = (int)(0xffffff00 | b);       // ok
    int j = (int)(0xffffff00 | (byte)0); // error: Constant value cannot be converted to a 'int' (use 'unchecked' syntax to override)
}

推荐答案

基本上，对编译时常量的检查与其他代码不同。

Compile-time constants are checked differently to other code, basically.

强制转换除非将明确地具有 unchecked 表达式，否则将-time常量转换为不包含该值的类型的类型将始终失败。

A cast of a compile-time constant into a type whose range doesn't include that value will always fail unless you explicitly have an unchecked expression. The cast is evaluated at compile time.

但是，归类为 value （而不是常量）的表达式的强制转换是在执行时进行评估，并通过异常（在检查的代码中）或通过截断位（在未检查的代码中）处理溢出。

However, the cast of an expression which is classified as a value (rather than a constant) is evaluated at execution time, and handles overflow either with an exception (in checked code) or by truncating bits (in unchecked code).

使用 byte 和一个 const 字段与一个静态只读字段：

You can see this slightly more easily using byte and just a const field vs a static readonly field:

class Test
{
    static readonly int NotConstant = 256;
    const int Constant = 256;

    static void Main(string[] args)
    {
        byte okay = (byte) NotConstant;
        byte fail = (byte) Constant;  // Error; needs unchecked
    }
}

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