C# Online Compiler | .NET Fiddle

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

namespace ConsoleApplication1
{
    public class Program
    {
        public static void Main()
        {
			string sInputString = "Hello World";
			
			string sBase64Output = Base64Encode(sInputString);  //"SGVsbG8gV29ybGQ=" is returned.
            Console.WriteLine(string.Format("'{0}' into Base64 format: {1}", sInputString,sBase64Output));

            string sStringOutput = Base64Decode(sBase64Output); 
            Console.WriteLine(string.Format("Converted Base64 text '{0}' into string is: {1}", sBase64Output, sStringOutput));

            Console.ReadLine();
        }

        /// <summary>
        /// Function to convert or encode any text into Base64 format.
        /// </summary>
        /// <param name="plainText">any plane text</param>
        /// <returns>The Base64 text</returns>
        public static string Base64Encode(string plainText)
        {
            //Get bytes
            byte[] plainTextBytes = System.Text.Encoding.UTF8.GetBytes(plainText);

            //Convert to Base64 to string.
            return System.Convert.ToBase64String(plainTextBytes);
        }

        /// <summary>
        /// Function to convert or decode any Base64 text into string format.
        /// </summary>
        /// <param name="plainText">any Base64 text</param>
        /// <returns>the string text</returns>
        public static string Base64Decode(string sBase64Text)
        {
            //Get bytes of Base64 string.
            byte[] inputBase64InBytes = Convert.FromBase64String(sBase64Text);

            //Convert Bytes to string.
            return System.Text.Encoding.UTF8.GetString(inputBase64InBytes); 
        }
    }
}

 
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
​
namespace ConsoleApplication1
{
    public class Program
    {
        public static void Main()
        {
            string sInputString = "Hello World";
            
            string sBase64Output = Base64Encode(sInputString);  //"SGVsbG8gV29ybGQ=" is returned.
            Console.WriteLine(string.Format("'{0}' into Base64 format: {1}", sInputString,sBase64Output));
​
            string sStringOutput = Base64Decode(sBase64Output); 
            Console.WriteLine(string.Format("Converted Base64 text '{0}' into string is: {1}", sBase64Output, sStringOutput));
​
            Console.ReadLine();
        }
​
        /// <summary>
        /// Function to convert or encode any text into Base64 format.