using System;
using System.IO;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main()
        {
            byte b1 = 10;
            byte b2 = 0;

            FileStream f1;
            FileStream f2;

            f1 = new FileStream("abc.txt", FileMode.Create, FileAccess.Write);

            f1.WriteByte(b1);
            f1.Close();

            f2 = new FileStream("abc.txt", FileMode.Open, FileAccess.Read);
            
            b2 = (byte)f2.ReadByte();
            Console.WriteLine("Val : " + b2);
            f2.Close();
        }
    }
}

Output

Val : 10

In above program, we need to remember, when we use "FileStream" class then we need to include System.IO namespace in the program.