c# - undefined variable 不应该在使用#if 检查时抛出异常吗？

Question

using System;
public class Program
{
    public static void Main(string[] args)
    {
    #if (!pi)
                Console.WriteLine("i");
    #else 
                Console.WriteLine("PI undefined");
    #endif
        Console.WriteLine("ok");
        Console.ReadLine();
    }
}

输出:

i
ok

有人能给我解释一下吗？如果没有定义pi，它不应该抛出异常吗？

Answer 1

#If 指令专门检查给定的符号是否已定义。

来自 documentation :

When the C# compiler encounters an #if directive, followed eventually by an #endif directive, it compiles the code between the directives only if the specified symbol is defined. Unlike C and C++, you cannot assign a numeric value to a symbol. The #if statement in C# is Boolean and only tests whether the symbol has been defined or not.

此外:

#define lets you define a symbol. By then using the symbol as the expression passed to the #if directive, the expression evaluates to true.

因此，如果 undefined symbol (在您的情况下为 pi)，则结果始终为 false。