การรับค่าไปคำนวณตามจำนวนที่ป้อน + - * / ทำนองนี้ครับช่วยทีนะครับ

public double Positive()
        {
            double i, n, num, sum = 0;
            Console.Write("How many number you input: ");
            num = double.Parse(Console.ReadLine());
            i = 1;
            while (i <= num)
            {
                Console.Write("Enter your value {0} :",i + 0);
                n = double.Parse(Console.ReadLine());
                sum = sum + n;
                i = i + 1;
                
                
            }
            return sum;
        }

public double Multiply()
        {
            double i, n, num, sum = 0;
            Console.Write("How many number you input: ");
            num = Convert.ToDouble(Console.ReadLine());
            i = 1;
            while (i <= num)
            {
                Console.Write("Enter your value {0} :", i + 0);
                n = double.Parse(Console.ReadLine());
                sum = sum * n;

                i = i + 1;
            }
            return sum;
        }