Dictionary<int, char> d = new Dictionary<int, char>();

            int i=0;

            string s = "Welcome to china";

            char[] c = new Char[s.Length];

            foreach (var item in s) {

                d.Add(i, item);

                i++;

            }

            foreach (var n in d.Keys) {

                Console.WriteLine(n);

            }

            Console.ReadKey();

统计 Welcome to china中每个字符出现的次数 不考虑大小写