основная форма

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.IO;
using System.Xml.Serialization;

namespace SYATP2_2
{
    public partial class Form1 : Form
    {
        int position = -1;
        List<Student> list = new List<Student>();
        List<Student> list_ = new List<Student>();
        List<(string, string)> field;
        string filename = "University.txt";

        ValueTuple<int, int> t;
        bool flag = false;
        Student s;
        public bool CheckList()
        {

                if (comboBox1.Text == "Имя")
                return (list[position].Name.Contains(textBox4.Text));
            else if (comboBox1.Text == "Фамилия")
                return (list[position].Lastname.Contains(textBox4.Text));
            else if (comboBox1.Text == "Факультет")
                return (list[position].Faculty.Contains(textBox4.Text));
            else return false;
        }

        public Form1()
        {
            InitializeComponent();
        }
        public void  IsMaster(ref List<Student> list)
        {
            if (list[position].GetType().Name == "Master")
            {
                field = list[position].getField();
                label6.Visible = true;
                textBox5.Visible = true;
                textBox5.Enabled = true;
                button3.Visible = false;
                textBox5.Text = field[3].Item2;
            }
            else
            {
                textBox5.Visible = false;
                label6.Visible = false;
                button3.Visible = true;
                textBox5.Clear();
            }
        }
        public void Findindex()
        {
            int index1 =-1;
            int  index2 = -1;
            if (comboBox1.Text == "Имя")
            {
                index1 = list.FindIndex(x => x.Name.Contains(textBox4.Text));
                index2 = list.FindLastIndex(x => x.Name.Contains(textBox4.Text));
            }
            else if (comboBox1.Text == "Фамилия")
            {
                index1 = list.FindIndex(x => x.Lastname.Contains(textBox4.Text));
                index2 = list.FindLastIndex(x => x.Lastname.Contains(textBox4.Text));
            }

            else if (comboBox1.Text == "Факультет")
            {
                index1 = list.FindIndex(x => x.Faculty.Contains(textBox4.Text));
                index2 = list.FindLastIndex(x => x.Faculty.Contains(textBox4.Text));
            }
                t.Item1 = index1;
            t.Item2 = index2;



        }
        public void Selection()
        {
            flag = true;
            position = 0;
            textBox1.Enabled = true;
            textBox2.Enabled = true;
            textBox3.Enabled = true;
            Findindex();
            if (textBox4.Text.Length != 0 && t.Item1 != -1)
            {
                position = t.Item1;

                Output(ref list);
                IsMaster(ref list);
                CheckButton(ref list);
                CheckMenu(ref list);
            }
            else if ((textBox4.Text.Length != 0 && t.Item1 == -1))
                {
                    position=-1;

                    textBox1.Clear();
                    textBox2.Clear();
                    textBox3.Clear();
                    textBox5.Clear();
                    textBox1.Enabled = false;
                    textBox2.Enabled = false;
                    textBox3.Enabled = false;
                    textBox5.Enabled = false;

                }

                else
                {
                    position = 0;
                flag = false;
                    Output(ref list);
                IsMaster(ref list);


                    CheckButton(ref list);
                    CheckMenu(ref list);
                }


        }

        public void CheckButton(ref List<Student> list)
        {   if (flag )
            { if (position >= t.Item2 )

                button2.Enabled = false;
            else button2.Enabled = true;
                if (position <= t.Item1)
                    button1.Enabled = false;
                else button1.Enabled = true;
            }
            else if (list.Count <= 1)
            {
                button1.Enabled = false;
                button2.Enabled = false;
            }
            else if (position == list.Count - 1)
            {
                button2.Enabled = false;
                button1.Enabled = true;
            }
            else if (position == 0)
            {
                button2.Enabled = true;
                button1.Enabled = false;
            }
            else
            {
                button2.Enabled = true;
                button1.Enabled = true;
            }
        }
        public void CheckMenu(ref List<Student> list)
        {

             if (list.Count == 0)
            {

                удалитьToolStripMenuItem.Enabled = false;
                изменитьToolStripMenuItem.Enabled = false;

            }
            else
            {

                добавитьToolStripMenuItem.Enabled = true;
                удалитьToolStripMenuItem.Enabled = true;
                изменитьToolStripMenuItem.Enabled = true;

            }
        }
        public void CheckComboBox(ref List<Student> list)
        {
            if (list.Count <= 1) { comboBox1.Enabled = false; textBox4.Enabled = false; }
            else
            {
                comboBox1.Enabled = true; textBox4.Enabled = true;
            }
        }
        public void Output(ref List<Student>list)
        {
            textBox1.Text = list[position].Name;
            textBox2.Text = list[position].Lastname;
            textBox3.Text = list[position].Faculty;

        }
        private void label1_Click(object sender, EventArgs e)
        {

        }
        public static List<Student> loadFile(List<Student> List, string filename)
        {

            FileStream f = new FileStream(filename, FileMode.Open);
            XmlSerializer bf = new XmlSerializer((typeof(List<Student>)));
            return (List<Student>)bf.Deserialize(f);

            f.Close();
        }
        public static void SaveFile(List<Student> List, string filename)
        {
            FileStream f = new FileStream(filename, FileMode.Create);
            XmlSerializer bf = new XmlSerializer((typeof(List<Student>)));
            bf.Serialize(f, List);
            f.Close();
        }
        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            Selection();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            position++;
            while (flag && !CheckList() && position < t.Item2+1)
            {
                position++;
                CheckButton(ref list);
                CheckMenu(ref list);
                IsMaster(ref list);
            }
            if (flag && CheckList() && position < t.Item2 + 1)
            {
                Output(ref list);
                CheckButton(ref list);
                CheckMenu(ref list);
                IsMaster(ref list);
            }
            if (!flag )
            {
                Output(ref list);
                CheckButton(ref list);
                CheckMenu(ref list);
                IsMaster(ref list);
            }

        }

        private void label5_Click(object sender, EventArgs e)
        {

        }

        private void textBox4_TextChanged(object sender, EventArgs e)
        {
            Selection();
        }

        private void добавитьToolStripMenuItem_Click(object sender, EventArgs e)

        {
            //Student s;



            Form2 form = new Form2();
            form.ShowDialog();
            if (form.DialogResult == DialogResult.OK)
            {
                if (!textBox1.Enabled || !textBox2.Enabled || !textBox3.Enabled)
            {
                textBox1.Enabled = true;
                textBox2.Enabled = true;
                textBox3.Enabled = true;
            }

                s = form.s;
                if (s.GetType().Name == "Master")
                {

                    textBox5.Visible = true;
                    label6.Visible = true;
                    button3.Visible = false;
                }

                else
                {
                    textBox5.Visible = false;
                    label6.Visible = false;
                    button3.Visible =true;
                }
                    list.Add(s);
                position++;
                textBox1.Clear();
                textBox2.Clear();
                textBox3.Clear();
                textBox5.Clear();
                CheckButton(ref list);
                CheckMenu(ref list);
                CheckComboBox(ref list);
            }
        }

        private void загрузитьToolStripMenuItem_Click(object sender, EventArgs e)
        {
            position = 0;
            OpenFileDialog openFileDialog1 = new OpenFileDialog();
           // button1.Enabled = false;
            if (openFileDialog1.ShowDialog() == DialogResult.OK)
            {
                filename = openFileDialog1.FileName;
                list = loadFile(list, filename);
                Output(ref list);
                IsMaster(ref list);

            }

            if (!textBox1.Enabled || !textBox2.Enabled || !textBox3.Enabled)
            {
                textBox1.Enabled = true;
                textBox2.Enabled = true;
                textBox3.Enabled = true;
            }
            //if (list.Count() > 1) button2.Enabled = true;
            CheckButton(ref list);
            CheckMenu(ref list);
            CheckComboBox(ref list);
        }

        private void сохранитьКакToolStripMenuItem_Click(object sender, EventArgs e)
        {
            SaveFileDialog saveFileDialog1 = new SaveFileDialog();

            if (saveFileDialog1.ShowDialog() == DialogResult.OK)
            {
                filename = saveFileDialog1.FileName;

                SaveFile(list, filename);

            }
        }

        private void изменитьToolStripMenuItem_Click(object sender, EventArgs e)
        {


            if (list[position] is Master)
            {

                var item = list[position] as Master;
                item.Name = textBox1.Text;
                item.Lastname = textBox2.Text;
                item.Faculty = textBox3.Text;
                item.DiplomaNumber = textBox5.Text;
                list[position] = item;
            }



            else
            {
                s = list[position];
                s.Name = textBox1.Text;
                s.Lastname = textBox2.Text;
                s.Faculty = textBox3.Text;


                list[position] = s;
            }

                Output(ref list);
            IsMaster(ref list);

        }

        private void button1_Click(object sender, EventArgs e)
        {
            position--;


            while (flag && !CheckList() && position >= t.Item1)
            {
                position--;
                CheckButton(ref list);
                CheckMenu(ref list);
                IsMaster(ref list);

            }
            if (flag && CheckList() && position >= t.Item1)
            {
                Output(ref list);
                CheckButton(ref list);
                CheckMenu(ref list);
                IsMaster(ref list);
            }
            if (!flag)
            {
                Output(ref list);
                CheckButton(ref list);
                CheckMenu(ref list);
                IsMaster(ref list);
            }






        }

        private void textBox1_TextChanged(object sender, EventArgs e)
        {


        }

        private void удалитьToolStripMenuItem_Click(object sender, EventArgs e)
        {

            if (flag && position < t.Item2 && position > t.Item1)
            {
                int it1 = t.Item1;
                int it2 = t.Item2--;
                t = (it1, it2);
            }
            else if (flag && position < t.Item2 && position <= t.Item1)
            {
                int it1 = t.Item1++;
                int it2 = t.Item2--;
                t = (it1, it2);
            }
            list.RemoveAt(position);
            CheckButton(ref list);
            CheckMenu(ref list);
            CheckComboBox(ref list);
            if (list.Count == 0)
            {

                textBox1.Clear();
                textBox2.Clear();
                textBox3.Clear();
                textBox5.Clear();
                textBox1.Enabled = false;
                textBox2.Enabled = false;
                textBox3.Enabled = false;
                textBox5.Enabled = false;
                button3.Visible = false;

                return;
            }
            else if (position == list.Count())

                position--;




            Output(ref list);



        }

        private void сохранитьToolStripMenuItem_Click(object sender, EventArgs e)
        {

           SaveFile(list, filename);
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void button3_Click(object sender, EventArgs e)
        {
            s = new Master(textBox1.Text, textBox2.Text, textBox3.Text, "");
            list[position] = s;
            IsMaster(ref list);
        }
    }
}