class Program//类

    {

        static void Main(string[] args)//main方法，程序入口

        {

            const double PI = 3.1415926;

            Console.Write("半径为4的元的周长: ");

            Console.WriteLine ("2*PI*4");

            Console.Write("半径为4的圆的面积: ");

            Console .WriteLine ("a*PI*4");

        }

    }

Warning 1 The variable 'PI' is assigned but its value is never used