I created this quick generic example after noticing that several people were asking questions about it. If you have any questions then let me know. 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Globalization;

namespace ConsoleApplication4
{
    //New class where Type implements IConvertible interface (interface = contract)
    class Calculate<T> where T : IConvertible
    {
        //Setup fields
        public T X;
        NumberFormatInfo fmt = NumberFormatInfo.CurrentInfo; 

        //Constructor 1
        public Calculate()
        {
            X = default(T);

        }

        //Constructor 2
        public Calculate (T x)
        {
            X = x;
        }

        //Method that we know will return a double 
        public double DistanceTo (Calculate<T> cal)
        {
            //Remove the.ToDouble if you want to see the methods available for IConvertible
            return (X.ToDouble(fmt) - cal.X.ToDouble(fmt));
        }

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

            //Pass value type and call DistanceTo with an Int.
            Calculate<int> cal = new Calculate<int>();
            Calculate<int> cal2 = new Calculate<int>(10);
            Console.WriteLine("Int : " + cal.DistanceTo(cal2));

            //Pass value type and call DistanceTo with an Double.
            Calculate<double> cal3 = new Calculate<double>();
            Calculate<double> cal4 = new Calculate<double>(10.6);
            Console.WriteLine("Double : " + cal3.DistanceTo(cal4));

            //Pass reference type and call DistanceTo with an String.
            Calculate<string> cal5 = new Calculate<string>("0");
            Calculate<string> cal6 = new Calculate<string>("345");
            Console.WriteLine("String : " + cal5.DistanceTo(cal6));

        }
    }
}