     private void button4_Click(object sender, EventArgs e)
        {
            textBox5.Visible = true;
            try
            {
                input = new FileStream("student.dat", FileMode.Open, FileAccess.Read);

                textBox5.Text = " Student are grade > 17 :";
                while (true)
                {
                    Record record = (Record)reader.Deserialize(input); ;
                    if (record.Grade > 17)
                    {
                        string result = record.StNo.ToString() + "\t" + record.fName + "\t" + record.lName + "\t" + record.Grade.ToString();
                        textBox5.Text += "\r\n" + result;
                    }
                }
            }
            catch (FileNotFoundException)
            {
                MessageBox.Show("File not exists");
                return;
            }
            catch (SerializationException)
            {
                input.Close();
            }
      
        }
