C# ??符号

 static void Main(string[] args)
        {
            string strA = String.Empty;
            string strB = String.Empty;
            strA = null;
            strB = "strB";
            /**
             * 如果??左边的strA值为String.IsNullOrWhiteSpace则StrC=strB
             * 如果??右边的strB值为String.IsNullOrWhiteSpace则StrC=strA
             */
            string StrC = strA ?? strB;
            Console.WriteLine(StrC);

            strB = null;
            strA = "strA";
            Console.WriteLine(strA ?? strB);

            strB = String.Empty;
            strA = "No Empty";
            string a = strA ?? strB;
            Console.WriteLine(a == null ? "null" : a);
            Console.ReadLine();
        }

输出