There are three ways to swap the number

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

namespace ConsoleApplication25
{
    class Program
    {
        static void Main(string[] args)
        {
            //1st way
            int nNum1 = 10;
            int nNum2 = 20;
            Console.WriteLine("Before swap");
            Console.WriteLine(nNum1+"\n"+nNum2);

            int temp = nNum1;
            nNum1 = nNum2;
            nNum2 = temp;
            Console.WriteLine("after swap");
            Console.WriteLine(nNum1 + "\n" + nNum2);

            //2st way note-without taking third variable 
            int nNum3 = 10;
            int nNum4 = 20;
            Console.WriteLine("Before swap");
            Console.WriteLine(nNum3 + "\n" + nNum4);
            nNum3 = nNum3 + nNum4;
            nNum4 = nNum3 - nNum4;
            nNum3 = nNum3 - nNum4;
            Console.WriteLine("after swap");
            Console.WriteLine(nNum3 + "\n" + nNum4);
            //3st way
            int nNum5 = 10;
            int nNum6 = 20;
            Console.WriteLine("Before swap");
            Console.WriteLine(nNum5 + "\n" + nNum6);
            nNum5= nNum5 ^nNum6;
            nNum6 = nNum5 ^ nNum6;
            nNum5 = nNum5 ^ nNum6;
            Console.WriteLine("after swap");
            Console.WriteLine(nNum5 + "\n" + nNum6);
       Console.ReadLine();
        }
    }
}

============================================
10
20
==
20
10
===========================================