static void Main(string[] args)
        {
             Console.WriteLine("依次输入两个数值和一个运算符");
             double y=0;
            double x1 = Convert.ToDouble(Console.ReadLine());
            double x2 = Convert.ToDouble(Console.ReadLine());         
            string  z = Convert.ToString(Console.ReadLine());
            switch(z)
            {
                case "+":
                y = x1 + x2;
                break;
                case "*":
                y = x1 * x2;
                break;
                case "-":
                y = x1 - x2;
                break;
                case "/":
                y = x1 / x2;
                break;
                default:
                    Console.WriteLine("运算符输入错误");
                    break;
            }

            Console.WriteLine(y );
            Console.ReadLine();
        }

如果是c的话，if后面应该是if (z =="*")才对