using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var hsFoo1 = new HashSet<Foo>(new FooEqualityComparer());

        hsFoo1.Add(new Foo(1));

        hsFoo1.Add(new Foo(2));

        hsFoo1.Add(new Foo(1));

        var hsFoo2 = new HashSet<Foo>(new FooEqualityComparer());

        hsFoo1.Add(new Foo(1));

        hsFoo1.UnionWith(hsFoo2);

        Console.WriteLine(hsFoo1.Count);

    }

}

​

public class Foo 

{   

    public Foo(int id)

    {

        ID = id;

    }

    public int ID { get; set; }

}

​

public class FooEqualityComparer

    : IEqualityComparer<Foo>

{

    public bool Equals(Foo a, Foo b)

    {

        return a.ID == b.ID;    

    }

    public int GetHashCode(Foo f)

    {

        return HashCode.Combine(f.ID);

    }

}