using System;

using System.Collections.Generic;

public class Program

{

    private static readonly CallbackManager _callbackManager = new();

    public static void Main()

    {

        var purchase = new Purchase()

        {

            PaymentMethod = "Bitcoin"

        };

        string callbackId = _callbackManager.Add(purchase);

        // Simulate user callback

        OnCallback(callbackId);

    }

    public static void OnCallback(string callbackId)

    {

        var purchase = (Purchase)_callbackManager.Get(callbackId);

        Console.WriteLine(purchase.PaymentMethod);

    }

}

​

public class Purchase

{

    public object User { get; set; }

    public List<object> Items { get; set; }

    public object PaymentMethod { get; set; }

}

​

public class CallbackManager

{

    private readonly Dictionary<string, object> _callbacks = new();

    public string Add(object obj)

    {

        string randomId = Random.Shared.NextInt64().ToString();

        _callbacks.Add(randomId, obj);

        return randomId;

    }

    public object Get(string id)

    {

        return _callbacks[id];

    }

}