FileStream.Unlock (Método)

using System;
using System.IO;
using System.Text;

namespace ConsoleApplication
{
    class Program
    {
        static void Main(string[] args)
        {
            string fileName = "test.txt";
            string textToAdd = "New text to write to file";

            UnicodeEncoding uniencoding = new UnicodeEncoding();

            byte[] result = uniencoding.GetBytes(textToAdd);

            using (FileStream destinationStream = File.Open(fileName, FileMode.OpenOrCreate))
            {
                destinationStream.Write(result, 0, result.Length);
            }
        }
    }
}