c# - 使用 "in"参数修饰符传递文字

Question

目前尚不清楚为什么 C# 不允许调用将文字与 in 参数修饰符一起传递的方法；同时，当将文字传递给没有 in 参数修饰符的方法时，代码会编译。

下面是演示此行为的代码示例 (C# 7.3):

class Program
{
    static void Main(string[] args)
    {
        string s = string.Empty;

        //These two lines compile 
        WriteStringToConsole(in s);
        WriteStringToConsole("my string");

        //Error CS8156  An expression cannot be used in this context because it may not be passed or returned by reference 
        WriteStringToConsole(in "my string");
    }

    public static void WriteStringToConsole (in string s)
    {
        Console.WriteLine(s);
    }
}

Answer 1

如 C# language reference 中所述，您不能将常量与 in 关键字一起用作参数:

The argument used with in must represent a location that can be directly referred to. The same general rules for out and ref arguments apply: you cannot use constants, ordinary properties, or other expressions that produce values.