static void Main(string[] args)

        {

            var dict = new Dictionary<string, string>();

            dict["username"] = "Yuhao";

            dict["password"] = "abc@123";

            var sq = dict.Aggregate("select * from users where 1=1", (current, param) => Addparam(current, param.Key, param.Value));

            Console.WriteLine(sq);

            Console.Read();

        }

        private static string Addparam(string rawValue, string key, string value)

        {

            return rawValue = rawValue + " and " + key + "='" + value + "'";

        }