Immutable type

   public struct Address
    {
        public string Province { get;}
        public string City { get; }

        private readonly string[] phones;
        public string[] Phones
        {
            get
            {
                string[] rtn = new string[phones.Length];
                phones.CopyTo(rtn, 0);
                return rtn;

            }
        }

        public Address(string province,string city,string[] phones_)
        {
            //为字段赋值，使用属性
            Province = province;
            City =City;
             // for the field assignment, attributes can not be used, because the code logic 
            PHONES = new new String [phones_.Length]; 
            phones_.CopyTo (PHONES, 0); 
        } 
    }

Immutable type is thread safe.

Conditions are met

1. The members of the outside world can not be modified (no set accessor)

2. The constructor for all property assignment