Unable to reference class in main

The first two parameters seem to work but when I add the string I get an error (Line 17 cannot convert string to double). What am I missing here? From everything I've read in my book it seems like this should work so I'm guessing it's a stupid error but I've been looking up and down the code for the last 3 hours and haven't found anything. Thank you for reading this far!

    using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication14
{
class Program
{
    static void Main(string[] args)
    {

        SimpleCalc Calc = new SimpleCalc("{0.0}", "{0.0}", "{0}");
        Console.WriteLine(Calc);

    }
  }
}

This is the class

    using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication14
{
class SimpleCalc
{

        public SimpleCalc(double num1, double num2, string oper)
        {

            Console.Write("Enter first integer: ");
            num1 = Convert.ToDouble(Console.ReadLine());

            Console.Write("Enter operator (+,-,*, / or %)");
            oper = Convert.ToString(Console.ReadLine());

            Console.Write("Enter second integer: ");
            num2 = Convert.ToDouble(Console.ReadLine());

            if (oper == "+")
                Console.Write("Answer is: {0}", num1 + num2);

            if (oper == "-")
                Console.Write("Answer is: {0}", num1 - num2);

            if (oper == "*")
                Console.Write("Answer is: {0}", num1 * num2);

            if (oper == "/")
                Console.Write("Answer is: {0}", num1 / num2);

            if (oper == "%")
                Console.Write("Answer is: {0}", num1 % num2);

            Console.ReadKey();
        }
    }
}