public partial class MainWindow : Window
    {
        public string Name { get; set; }
        public string Capital { get; set; }
        public int Area { get; set; }
        public double Population { get; set; }
 
        public MainWindow()
        {
            InitializeComponent();
        }
 
        private void MenuItem_Click(object sender, RoutedEventArgs e)
        {
            Close();
        }
 
        private void MenuItem_Click_1(object sender, RoutedEventArgs e)
        {
            var dlg = new OpenFileDialog
            {
                Filter = "Binary file (*.dat)|*.dat"
            };
            if (dlg.ShowDialog() == true)
            {
                PathFile.Text = dlg.FileName;
            }
            BindingList<CountryModel> data = new BindingList<CountryModel>();
            TestGrid.ItemsSource = data;
            string inputPath = dlg.FileName;
            using (BinaryReader reader = new BinaryReader(File.Open(inputPath, FileMode.Open, FileAccess.Read, FileShare.Read)))
            {
                while (reader.PeekChar() > -1)
                {                  
                    int nameL = reader.ReadByte();
                    char[] name = reader.ReadChars(nameL);
                    int capitalL = reader.ReadByte();
                    char[] capital = reader.ReadChars(capitalL);
                    int area = reader.ReadInt32();
                    double population = reader.ReadDouble();
                    string na = new string(name);
                    string ca = new string(capital);
                    data.Add(new CountryModel(na, ca, area, population));
                }
            }
        }
 
        private void MenuItem_Click_2(object sender, RoutedEventArgs e)
        {
            var dlg = new SaveFileDialog
            {
                Filter = "Binary file (*.dat)|*.dat"
            };
 
            if (dlg.ShowDialog() == true)
            {
                Save(dlg.FileName);
            }
        }
        /*
         *private void Save(string outputPath)
            {
                using (var fs = new FileStream(outputPath, FileMode.Create))
                {
                    using (var bw = new BinaryWriter(fs))
                    {
                    }
                }
            }*/
    }
 
Code: C#