using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var foos = new [] 

        {

            new Foo { Items = new HashSet<int> {1,2} },

            new Foo { Items = new SortedSet<int> {2,3} } 

        };

        var results = foos.SelectMany(x=>x.Items);

        foreach(var result in results) 

        {

            Console.WriteLine(result);

        }

    }

}

​

class Foo

{

    public ISet<int> Items;

}