using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{   

    public static void Main()

    {

        var dict = new Dictionary<int, RClass>();

        dict.Add(1, new RClass { Content = "bb" });

        RClass resp;

        dict.TryGetValue(

            2, 

            out resp

        );

        //Console.Write(resp.Content); // => "Object reference not set to an instance of an object."

        Console.Write(dict[2]); // => "The given key was not present in the dictionary."

        var list = new List<RClass>();

        list.Add(new RClass

                 {

                     Id = 1,

                     Content = "ddd"

                 });

        list.Add(new RClass

                 {

                     Id = 1,

                     Content = "ddd2"

                 });

        //list.ToDictionary(d => d.Id, d => d.Content); // => "An item with the same key has already been added."

    }

}

​

​

public class RClass

{

    public int Id { get; set; }

    public string Content { get; set; }

}