using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var fooLIst = new List<FooBar>

        {

            new FooBar

            {

                Name = "Foo1",

                Tags = new List<string>

                {

                    "tag1",

                    "tag2",

                    "tagX"

                }

            },

            new FooBar

            {

                Name = "Foo2",

                Tags = new List<string>

                {

                    "tag3",

                    "tag4"

                }

            },

            new FooBar

            {

                Name = "Foo3",

                Tags = new List<string>

                {

                    "tag31",

                    "tag42"

                }

            }

        };

        // using select to project

        var myNewListUsingSelect = fooLIst.Select(fb => new FooBar{Name = fb.Name,Tags = new List<string>()}).ToList();

​

        myNewListUsingSelect = new List<FooBar>

        {

            new FooBar

            {

                Name = "Foo1",

                Tags = new List<string>

                {

                    "tag1",

                    "tag6"

                }

            },

            new FooBar

            {

                Name = "Foo2",

                Tags = new List<string>

                {

                    "tag7",

                    "tag8",

                    "tag9"

                }

            },

            new FooBar

            {

                Name = "Foo3",

                Tags = new List<string>

                {

                }

            }

        };

        var concatTest = fooLIst.Concat(myNewListUsingSelect).ToList();

        var groupbyTest = fooLIst.Concat(myNewListUsingSelect).GroupBy(a=>a.Name).ToList();

        var merged = new List<FooBar>(fooLIst);

        merged.AddRange(myNewListUsingSelect.Where(p2 => fooLIst.All(p1 => p1.Name != p2.Name)));

​

​

​

        Console.WriteLine("original name and tags:      " + fooLIst[0].Name + " - " + fooLIst[0].Tags.Count + " - " + fooLIst[1].Name + " - " + fooLIst[1].Tags.Count + " - " + fooLIst[2].Name + " - " + fooLIst[2].Tags.Count);

        Console.WriteLine("select list 2 name and tags: " + myNewListUsingSelect[0].Name + " - " + myNewListUsingSelect[0].Tags.Count + " - " + myNewListUsingSelect[1].Name + " - " + myNewListUsingSelect[1].Tags.Count + " - " + myNewListUsingSelect[2].Name + " - " + myNewListUsingSelect[2].Tags.Count);

        Console.WriteLine("Concat of the two objects:   " + concatTest[0].Name + " - " + concatTest[0].Tags.Count + " - " + concatTest[1].Name + " - " + concatTest[1].Tags.Count + " - " + concatTest[2].Name + " - " + concatTest[2].Tags.Count + " - " + concatTest[3].Name + " - " + concatTest[3].Tags.Count + " - " + concatTest[4].Name + " - " + concatTest[4].Tags.Count + " - " + concatTest[5].Name + " - " + concatTest[5].Tags.Count);

        Console.WriteLine("AddRange of the two objects:   " + merged[0].Name + " - " + merged[0].Tags.Count + " - " + merged[1].Name + " - " + merged[1].Tags.Count + " - " + merged[2].Name + " - " + merged[2].Tags.Count);

        // I think it should look like this in the console:

        //Foo1 - 4 - Foo2 - 5 - Foo3 - 2

    }

​

}

​

class FooBar

{

  public string Name {get; set;}    

  public List<string> Tags {get; set;}  

}