IFORMATABLE INTERFACE

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

namespace ConsoleApplication

{

    public struct ComplexNumber : IFormattable

    {

        private double real;

        private double imaginary;

        public ComplexNumber(double real, double imaginary)

        {

            this.real = real;

            this.imaginary = imaginary;

        }

        public string ToString(string mask, IFormatProvider ifp)

        {

            if (mask == "i" || mask == "j")

            {

                return this.real.ToString() + (this.imaginary >= 0 ? "+" : "") + this.imaginary.ToString() + mask;

            }

            else

            {

                return "";

            }

        }

    }

    class Program

    {

        static void Main(string[] args)

        {

            ComplexNumber c = new ComplexNumber(5, -6);

            Console.WriteLine("Format 1: {0:i}", c);

            Console.WriteLine("Format 2: {0:j}", c);

            Console.ReadKey();

        }

    }

}