string msg = "WebApplication2";

                msg = msg.ToLower();

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

                for (int i = 0; i < msg.Length; i++)

                {

                    if (char.IsLetter(msg[i]))

                    {

                        if (dict.ContainsKey(msg[i]))

                        {

                            dict[msg[i]]++;

                        }

                        else

                        {

                            dict[msg[i]] = 1;

                        }

                    }

                }

                foreach (var item in dict)

                {

                    Console.WriteLine("字母：{0}出现了：{1}次", item.Key, item.Value);

                    Console.ReadLine();

                }