c# 7 vs2017 tuple

var unnamed = (42, "The meaning of life");
    var anonymous = (16, "a perfect square");
    var named = (Answer: 42, Message: "The meaning of life");
    var differentNamed = (SecretConstant: 42, Label: "The meaning of life");

named = unnamed;

(int Answer, string Message) named2 = (42, "The meaning of life");
    (int Answer, string Message) differentNamed2 = (42, "The meaning of life");

下面错误：

var named3 = (int Answer, string Message)(42, "The meaning of life");
    var differentNamed3 = (int Answer, string Message)(42, "The meaning of life");

既然(int Answer, string Message)是类型名，(42, "The meaning of life")是变量值，为什么类型名不能放在变量值前面呢？何况类型一致。

或者Tuple<int Answer, string Message> named4 =(42, "The meaning of life");

var named5 = Tuple<int Answer, string Message>(42, "The meaning of life");