using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var a = new Foo { Bar = 1 };

        var b = new Foo { Bar = 2 };

        var x = new Foo { Bar = 25 };

        var collection = new HashSet<Foo>();

        collection.Add(a);

        collection.Add(b);

        Console.WriteLine("Expected size 2. Actual size " + collection.Count);

        a.Bar = 25;

        var a_equals_x = a.Equals(x);

        Console.WriteLine("A equals X: " + a_equals_x);

        var hashes_match = a.GetHashCode() == x.GetHashCode();

        Console.WriteLine("A's hash matches X's: " + hashes_match);

        collection.Add(x);

        Console.WriteLine("Expected size 2. Actual size " + collection.Count);

    }

}

​

public class Foo

{

    public int Bar { get; set; }

    public override bool Equals(object other)

    {

        Foo otherfoo = (Foo)other;

        return otherfoo.Bar == this.Bar;

    }

    public override int GetHashCode()

    {

        return this.Bar.GetHashCode();

    }

}