using System;

using System.Collections.Generic;

using System.Linq;

using System.Text.Json;

public class Program

{

    public static void Main()

    {

        var aa = new List<A> {

            new () { id = 1, value = 88 },

            new () { id = 2, value = 222 }  

        };

        var bb = new List<B> {

            new () { id = 2, value = 77 },

            new () { id = 3, value = 666 }  

        };

        var c = aa.Join(bb, a => a.id, b => b.id, (a, b) => new { Id = a.id, Value = b.value });

        Console.WriteLine(JsonSerializer.Serialize(c));

    }

}

​

public class A {

    public int id;

    public int value;

}

​

public class B {

    public int id;

    public int value;

}