为什么Object.Equals的参数不是“输入”（输入）？

Question

Following ( incorrect/dangerous ) code

class EvilClass
{
    protected int x;

    public EvilClass(int x)
    {
        this.x = x;
    }

    public override bool Equals(Object obj)
    {
        if ((obj == null) || !this.GetType().Equals(obj.GetType()))
        {
            return false;
        }
        else
        {
            EvilClass p = (EvilClass)obj;
            p.x = 42;
            return (x == p.x);
        }
    }

    public override int GetHashCode()
    {
        return (x << 2);
    }

    public override string ToString()
    {
        return String.Format("EvilClass({0})", x);
    }
}

void Main()
{
    var e1 = new EvilClass(1);
    var e2 = new EvilClass(2);

    var equals = e1.Equals(e2);
    Console.WriteLine("{0}", e1.ToString());
    Console.WriteLine("{0}", e2.ToString());
}

Output:

EvilClass(1)
EvilClass(42)

As you can see, call of e1.Equals(e2) modify e2 . If we mark parameter as in compiler would not allow us to modify it.

Object.Equals() not suppose to change it's parameter - so why parameter is not in (input) parameter?

Answer 1

The most obvious reason is that in was introduced in C# 7.2, while object.Equals has been around since the very first version of .net.

The other reason is that it wouldn't actually change anything. in prevents from mutating the reference, not the actual object. If you try this:

public bool Equals2(in Object obj)
{
    if ((obj == null) || !this.GetType().Equals(obj.GetType()))
    {
        return false;
    }
    else
    {
        EvilClass p = (EvilClass)obj;
        p.x = 42;
        return (x == p.x);
    }
}

Then the output will still be:

EvilClass(1)
EvilClass(42)