using System;

using System.Linq;

using System.Collections.Generic;

using System.Linq.Dynamic.Core;

using Newtonsoft.Json;

​

var json = """

    [{

        "PhoneNumbers": [

            { "Number": "123" },

            { "Number": "456" }

        ]

    },

    {

        "PhoneNumbers": [

            { "Number": "789" },

            { "Number": "012" }

        ]

    }]

""";

​

var people = JsonConvert.DeserializeObject(json) as IEnumerable<dynamic> ?? [];

var numbsers = people.AsQueryable().SelectMany("PhoneNumbers").ToDynamicList();

//My temporary workaround

//var numbsers = people.AsQueryable().MySelectMany("PhoneNumbers").ToDynamicList();

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

​

public static class MyExtensions

{

    public static IQueryable MySelectMany(this IQueryable source, string selector)

    {

        return source.Select(selector).ToDynamicList()

            .Select(it => it as IEnumerable<object>)

            .SelectMany(it => it)

            .AsQueryable();

    }

}