gistfile1.txt

using System.Text.Json;

IUser user = new User("Jon", "Reading");

string json = JsonSerializer.Serialize(user);
Console.WriteLine(json);

public interface IUser
{
    string Name { get; }
}

public class User : IUser
{
    public string Name { get; }
    public string Town { get; }

    public User(string name, string town)
    {
        Name = name;
        Town = town;
    }
}