var sourceFile = "text.txt";
var targetFile = @"C:UsersTomasDesktopMovieStarzMovieStarzWriteText.txt";
var id = App.Moviess.Filmerna[index].id.ToString();
var lines = File.ReadAllLines(sourceFile)
.Where(l => l.Contains(id))
.ToArray();
File.AppendAllLines(targetFile, lines);