using System.IO;
using System.Collections.Generic;
using System.Text.Json;
using System.Text.Json.Serialization;
 
public class Customer
{
    public string Name { get; set; }
    public int Age { get; set; }
}
 
public class Program
{
    public static void Main()
    {
        List<Customer> customers = new List<Customer>();
        customers.Add(new Customer {Name = "Aaaaa", Age = 25});
        customers.Add(new Customer {Name = "Bbbbb", Age = 20});
 
        string json = JsonSerializer.Serialize(customers);
 
        File.WriteAllText(@"C:\datas.json", json);
    }
}

using System.IO;
using System.Collections.Generic;
using System.Text.Json;
using System.Text.Json.Serialization;
 
public class Customer
{
    public string Name { get; set; }
    public int Age { get; set; }
}
 
public class Program
{
    public static void Main()
    {
        List<Customer> customers = new List<Customer>();
        customers.Add(new Customer {Name = "Aaaaa", Age = 25});
        customers.Add(new Customer {Name = "Bbbbb", Age = 20});
 
        var options = new JsonSerializerOptions { WriteIndented = true };
        string json = JsonSerializer.Serialize(customers, options);
 
        File.WriteAllText(@"C:\datas.json", json);
    }
}