C#基础------可空类型 StringBuilder

 class Program
    {
        static void Main(string[] args)
        {
            // StringBuilder 解决了字符串拼接产生大量的字符串临时对象
            string s1 = "abc";
            string s2 = "asd";
            string s3 = "fghg";
            StringBuilder sb = new StringBuilder();
            sb.Append(s1);
            sb.Append(s2);
            sb.Append(s3);
            string s4 = sb.ToString();
            Console.WriteLine(s4);
            Console.ReadKey();

            //可空类型
            int? n1 = null;
            int? n2 = null;
            int n3 = (int)n2;
        }