using System;

using System.Collections.Generic;

using System.Linq;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var list2 = new List<Test>();

        list2.Add(new Test{ Id = 1, Survey = 1});

        list2.Add(new Test{ Id = 2, Survey = 2});

        list2.Add(new Test{ Id = 3, Survey = 3});

        var list3 = new List<Test>();

        list3.Add(new Test{ Id = 3, Survey = 1});

        list3.Add(new Test{ Id = 4, Survey = 4});

        list3.Add(new Test{ Id = 5, Survey = 5});

        list3.Add(new Test{ Id = 3, Survey = 3});

        var list4 = list2.Union(list3);

        foreach (var result in list4)

        {

            Console.WriteLine($"Id: {result.Id} - Value: {result.Survey}");

        }

    }

    public class Test

    {

        public int Id {get;set;}

        public int Survey {get; set;}

        public override bool Equals(object obj)

        {

            var t = obj as Test;

            return (t != null) ? Id.Equals(t.Id) : false;

        }

​

        public override int GetHashCode()

        {

            return Id.GetHashCode();

        }       

    }

}