using System;
using System.Linq;
using System.Text.Json;

Car[] cars1 = {
    new Truck(), new Truck()
};
var trucks1 = cars1.OfType<Truck>();
//trucks1.Count() is 2

Car[] cars2 = {
    new Truck(), new Car()
};
var trucks2 = cars2.OfType<Truck>();
//trucks2.Count() is 1

Console.WriteLine(JsonSerializer.Serialize(trucks1));
Console.WriteLine(JsonSerializer.Serialize(trucks2));

class Car { }

class Truck : Car { }