Evil code of the day : CSharp Feeds

Evil code of the day

by skeet via Jon Skeet: Coding Blog on 7/3/2009 10:28:26 PM

At a glance, this code doesn't look particularly evil. What does it do though? Compile it with the C# 4.0b1 compiler and run it...

using System;

class Base
{
    public virtual void Foo(int x, int y)
    {
        Console.WriteLine("Base: x={0}, y={1}", x, y);
    }
}

class Derived : Base
{
    public override void Foo(int y, int x)
    {
        Console.WriteLine("Derived: x={0}, y={1}", x, y);
    }
}

class PureEvil
{
    static void Main()
    {
        Derived d = new Derived();
        Base b = d;

        b.Foo(x: 10, y: 20);
        d.Foo(x: 10, y: 20);
    }
}

The results are:

Derived: x=20, y=10
Derived: x=10, y=20

I'm very nearly tempted to leave it there and just see what the reactions are like, but I'll at least give you a hint as to where to look - section 21.3 of the C# 4 spec explains why this gives odd results. It does make perfect sense, but it's hideously evil.

I feel dirty.