Guess.cs

using System;

interface IContainer
{
    object Obj { set; }
}
struct P : IContainer
{
    public object Obj { set { this.o = value; } }
    object o;
    static void Main(string[] args)
    {
        IContainer c = new P();
        c.Obj = c;
        var x = c.GetHashCode();
    }
}