C# Lambda /扩展方法使4个实体之间的结果扁平化？

using System;
using System.Collections.Generic;
using System.Linq;

public class EntityA
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class EntityB
{
    public int Id { get; set; }
    public string Description { get; set; }
}

public class EntityC
{
    public int Id { get; set; }
    public decimal Price { get; set; }
}

public class EntityD
{
    public int Id { get; set; }
    public DateTime Date { get; set; }
}

public static class EntityExtensions
{
    public static IEnumerable<object> Flatten(this IEnumerable<EntityA> entitiesA, IEnumerable<EntityB> entitiesB, IEnumerable<EntityC> entitiesC, IEnumerable<EntityD> entitiesD)
    {
        return entitiesA.Join(entitiesB, a => a.Id, b => b.Id, (a, b) => new { a, b })
            .Join(entitiesC, ab => ab.a.Id, c => c.Id, (ab, c) => new { ab.a, ab.b, c })
            .Join(entitiesD, abc => abc.a.Id, d => d.Id, (abc, d) => new { abc.a, abc.b, abc.c, d })
            .Select(result => new
            {
                result.a.Id,
                result.a.Name,
                result.b.Description,
                result.c.Price,
                result.d.Date
            });
    }
}

public class Program
{
    public static void Main()
    {
        var entitiesA = new List<EntityA>
        {
            new EntityA { Id = 1, Name = "Entity A 1" },
            new EntityA { Id = 2, Name = "Entity A 2" }
        };

        var entitiesB = new List<EntityB>
        {
            new EntityB { Id = 1, Description = "Entity B 1" },
            new EntityB { Id = 2, Description = "Entity B 2" }
        };

        var entitiesC = new List<EntityC>
        {
            new EntityC { Id = 1, Price = 10.0m },
            new EntityC { Id = 2, Price = 20.0m }
        };

        var entitiesD = new List<EntityD>
        {
            new EntityD { Id = 1, Date = DateTime.Now },
            new EntityD { Id = 2, Date = DateTime.Now.AddDays(1) }
        };

        var flattenedEntities = entitiesA.Flatten(entitiesB, entitiesC, entitiesD);

        foreach (var entity in flattenedEntities)
        {
            Console.WriteLine($"Id: {entity.Id}, Name: {entity.Name}, Description: {entity.Description}, Price: {entity.Price}, Date: {entity.Date}");
        }
    }
}