Dictionary<int, AAA> kv = new Dictionary<int, AAA> {
                { 1, new AAA() { Val = "abc" } }
            };
            if (kv.TryGetValue(1, out AAA val)) {
                val = new AAA() { Val = "efg" };  //这样是错误的
                kv[1] = new AAA() { Val = "efg" }; //这样才是对的
            }