c# - 只读更改结构的行为

Question

我正在尝试理解一些基本概念:

class Program
{
   private static readonly MyStruct m = new MyStruct();
   static void Main(string[] args)
    {
       //new MutableSample().RunSample();

       Console.WriteLine(m.ChangeInternal());
       Console.WriteLine(m.ChangeInternal());
       Console.WriteLine(m.ChangeInternal());
       Console.Read();
    }
}

public struct MyStruct
{
    private int x;
    public int ChangeInternal()
    {
        this.x = this.x + 1;
        return this.x;
    }
}

当我运行这段代码时，它会给出 1、1、1，但是当我删除“只读”时，它会显示 1、2、3。

有人能给我解释一下吗？

Answer 1

C# 规范的第 7.5.4 节指出:

[...] if the field is readonly and the reference occurs outside an instance constructor of the class in which the field is declared, then the result is a value, namely the value of the field I in the object referenced by E

因此，当该字段为readonly 时，您正在改变一个副本(因为改变一个值是不可能的，只能改变一个变量)。如果不是，您正在改变字段本身。

这在 this blog post 中有更详细的描述。埃里克·利珀特着。引用它的结尾:

This is yet another reason why mutable value types are evil. Try to always make value types immutable.