using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<Row> accounts = new List<Row>() {

            new Row() { Client = "123", Account = "123.def", Status = "Open" },

            new Row() { Client = "456", Account = "456.def", Status = "Closed" },

            new Row() { Client = "123", Account = "123.abc", Status = "Open" },

            new Row() { Client = "789", Account = "789.abc", Status = "Open" },

            new Row() { Client = "456", Account = "456.abc", Status = "Closed" },

            new Row() { Client = "789", Account = "789.ghi", Status = "Open" },

            new Row() { Client = "789", Account = "789.def", Status = "Closed" },

            new Row() { Client = "789", Account = "789.jkl", Status = "Open" },

        };

        List<Client> clients = accounts

            .GroupBy(x => x.Client)

            .Select(y => new Client() {

                Code = y.Key,

                Accounts = y.GroupBy(z => z.Account)

                    .Select(z => new Account() {

                        Number = z.First().Account,

                        Status = z.First().Status

                    }).ToList()

            }).ToList();

        clients.Dump();

    }

}

​

public class Row {

    public string Client { get; set; }

    public string Account { get; set; }

    public string Status { get; set; }

}

​

public class Client {

    public string Code { get; set; }

    public List<Account> Accounts { get; set; }

}

​

public class Account {

    public string Number { get; set; }

    public string Status { get; set; }

}