using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    static List<Element> listOfElements = new List<Element>();

    static List<Link> listOfLinks = new List<Link>();

    public static void Main()

    {

        for (int i = 0; i < 100; i++) {

            listOfElements.Add(new Element());

        }

        for (int i = 0; i < 50; i++)

        {

            listOfLinks.Add(new Link(GetNextAvaibaleElement().Id,GetNextAvaibaleElement().Id));

        }

        for (int i = 0; i < listOfLinks.Count; i++)

        {

            Console.WriteLine(listOfLinks[i].ToString());

        }

    }

    private static Element GetNextAvaibaleElement() {

      var usedElementIds = listOfLinks.Select(i => i.ElementOne).Union(listOfLinks.Select(i => i.ElementTwo)).ToList();

      return listOfElements.Where(i => !usedElementIds.Contains(i.Id)).OrderBy(i => Guid.NewGuid()).FirstOrDefault();

    }

}

​

public class Element

{

    public Element()

    {

        this.Id = Guid.NewGuid();

    }

​

    public Guid Id { get; set; }

}

​

public class Link

{

    public Link(Guid ElementOne, Guid ElementTwo)

    {

        this.ElementOne = ElementOne;

        this.ElementTwo = ElementTwo;

    }

​

    public Guid ElementOne { get; set; }

    public Guid ElementTwo { get; set; }

public override string ToString()

        {

            return string.Format("{0},{1}", ElementOne, ElementTwo);

        }

}