c# 语法糖 空合并【??】 和 空合并赋值【??=】

using System.Drawing;

namespace 语法糖
{
    internal class Program
    {
        static void Main(string[] args)
        {
            UserInformation user = new UserInformation();
            user.Name = "zhangsan";
            user.Address = null;
            
            string address = user.Address?.First();
            if(address != null)
            {
                address = "beijin";
            }

            string addrss2 = user.Address?.First() ?? "shanghai";

            if(user.Address == null)
            {
                user.Address = new List<string> { "nanjin" };
            }

            user.Address??= new List<string> { "hefei" };

        }

        public class UserInformation
        {
            public string Name { get; set; }
            public List<string> Address { get; set; }
        }


    }
}