using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        IEnumerable<RoleDenormalize> denormalizeRoles = GetRoles();

        var groupedData = denormalizeRoles.GroupBy(x=> new {x.Id, x.Name}, (key,values) => new { Role = key, Tags = values.Where(x=> !string.IsNullOrWhiteSpace(x.TagName)).ToList()});     

        List<Role> mappedData = groupedData.Select(x=> new Role{ Id = x.Role.Id, Name = x.Role.Name, Tags = x.Tags.Select(y=> new Tag{Name = y.TagName}).ToList()}).ToList();

        foreach(var item in mappedData){

            Console.WriteLine($"{item.Id} | {item.Name}");

            foreach(var tags in item.Tags){

                Console.WriteLine($"Tag : {tags.Name}");

            }

            Console.WriteLine("------------");          

        }

    }

    public static List<RoleDenormalize> GetRoles(){

        var list = new List<RoleDenormalize>();

        var role1 = new RoleDenormalize{

            Id = 5,

            Name = "Signer",

            TagName = "Signer"

        };

        list.Add(role1);

        var role2 = new RoleDenormalize{

            Id = 4,

            Name = "User",

            TagName = null

        };

        list.Add(role2);

        var role3 = new RoleDenormalize{

            Id = 2,

            Name = "Admin",

            TagName = null

        };

        list.Add(role3);    

        return list;

    }

}

​

public class RoleDenormalize {  

    public int Id {get;set;}    

    public string Name {get;set;}

    public string TagName {get;set;}

}

​

public class Role{

    public int Id {get;set;}    

    public string Name {get;set;}

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

}

​

public class Tag{

    public string Name {get;set;}

}