环境：net framework 4.5.2

    1. Console.WriteLine(outString);
    2. dynamic a2 = new Test2() {
    3.     ID = "3",
    4. };
    5. if (a2 is Test2 a3) {
    6.     Console.WriteLine(a3.ID);
    7. }
    8. //对象一致（继承的子类也属于兼容类型），输出ID  = 3