using System;

using System.Collections.Generic;

using System.Linq;

using System.Threading;

using System.Threading.Tasks;

using System.Diagnostics;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        List<Group> allGroup = new List<Group>();       

        Group g;

        g = new Group();

        g.manyID = new List<int>(new int[] { 1, 3 } );

        g.otherID = 1;

        allGroup.Add(g);

        g = new Group();

        g.manyID = new List<int>(new int[] { 0, 2, 4 } );

        g.otherID = 2;

        allGroup.Add(g);

        g = new Group();

        g.manyID = new List<int>(new int[] { 2, 3 } );

        g.otherID = 3;

        allGroup.Add(g);

        Group subGroup = new Group();

        subGroup.manyID = new List<int>(new int[] { 0, 1 } ); 

        List<Group> filterGroup = allGroup.Where( a => subGroup.manyID.Contains(a.otherID)).ToList();

        Console.WriteLine("Groups : " + filterGroup.Count());       

        //filterGroup = allGroup.Where( a => subGroup.manyID.Contains(**a.manyID**)).ToList();

        //Console.WriteLine("Groups : " + filterGroup.Count());     

        // should select first two groups

    }

}

​

public class Group

{

    public List<int> manyID;

    public int otherID;

}