using System;

using System.Linq;

using System.Collections.Generic;

using Newtonsoft.Json;

​

public class PC

{

    public string Currency {get; set;}

    public int PaymentAmount {get; set;}

    public string AssignmentId {get; set;}

    public string Code {get;set;}

}

​

public class Program

{

    public static void Main()

    {

        var PCs = new List<PC>

        {

            new PC{Currency = "EUR", PaymentAmount = 5, AssignmentId = "A41", Code = "A"},

            new PC{Currency = "EUR", PaymentAmount = 1, AssignmentId = "A41", Code = "B"},

            new PC{Currency = "GBP", PaymentAmount = 2, AssignmentId = "A41", Code = "C"},

            new PC{Currency = "GBP", PaymentAmount = 5, AssignmentId = "A41", Code = "D"},

            new PC{Currency = "RUB", PaymentAmount = 9, AssignmentId = "A41", Code = "E"},

            new PC{Currency = "RUB", PaymentAmount = 11, AssignmentId = "A42", Code = "F"},

        };

        var aggregated= PCs.GroupBy(pc => pc.AssignmentId).Select(g=> new Dictionary<string, List<PC>>{{g.Key, g.ToList()}});

        Console.WriteLine(JsonConvert.SerializeObject(aggregated));

    }

}