FileStream stream;
            while (true)
            {
                try
                {
                    stream = File.Open("myfile.txt", FileMode.Append);
                }
                catch (IOException ex)
                {
                    Console.WriteLine("Ждём освобождения");
                    continue;
                }

                break;
            }
 
            Console.WriteLine("Дождались");

FileStream stream = File.Open("myfile.txt", FileMode.Open, FileAccess.Write, FileShare.Write);

Console.WriteLine("Ждём освобождения");
stream.waitUnlock();//!!!
Console.WriteLine("Дождались");

stream.Close();