using System;


namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {

            string input = "Hello World!";

            for (int i=0; i<input.Length; i++)
            {
                // Get the integral value of the character.
                Console.WriteLine("Letter is {0}", input[i]);
                
            }
            // to avoid  automatic console  close 
            Console.ReadLine();


        }
    }
}

on peut aussi

 
using System;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {

            string input = "Hello World!";
            foreach (char letter in input)
            {
                // Get the integral value of the character.
                Console.WriteLine("Letter is {0}", letter);
                
            }
            // to avoid  automatic console  close 
            Console.ReadLine();
        }
    }
}