using System;

​

​

public class Program

{

    public static void Main()

    {

        var test = new Obj

        {

            IdPlateforme = 2424,

            IdEnseigne = 534383,

        };

        Console.WriteLine(test);

    }

}

​

public record Obj

{

    public int? IdPlateforme {get; set;}

    public int? IdEnseigne {get; set;}

    public int? IdSiret { get; set;}

    public int QuelTruc

    {

        get

        {

            return IdSiret != null ? 1 :

                IdEnseigne != null ? 2 :

                3;

        }

    }

}