gistfile1.cs

    // MathsDemo.cs
    partial interface IMathsDemo
    {
        int Addition(int valOne, int valTwo);
    }
    // MathsDemo2.cs
    partial interface IMathsDemo
    {
        int Subtraction(int valOne, int valTwo);
    }
    class Calculation : IMathsDemo
    {
        public int Subtraction(int valOne, int valTwo)
        {
            return valOne + valTwo;
        }

        public int Addition(int valOne, int valTwo)
        {
            return valOne - valTwo;
        }

        static void Main(string[] args)
        {
            int numOne = 45;
            int numTwo = 10;
            Calculation objCalculate = new Calculation();
            Console.WriteLine("Addition of two numbers: " + objCalculate.Addition(numOne, numTwo));
            Console.WriteLine("Subtraction of two numbers: " + objCalculate.Subtraction(numOne, numTwo));
        }
    }