PP/project/Main.cs

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.Data.SqlClient;

namespace project
{
    enum RowState
    {
        Existed,
        New,
        ModifiedNew,
        Deleted
    }
    public partial class Main : Form
    {
        string code, name, surname, patronymic, number, group_type, codeCard;
        DataBase dataBase = new DataBase();
        public Main()
        {
            InitializeComponent();
        }

        private void ReadSingleRow(DataGridView dgw, IDataRecord record)
        {
            dgw.Rows.Add(record.GetInt32(0), record.GetString(1), record.GetString(2), record.GetString(3), record.GetString(4), record.GetString(5), record.GetString(6));
        }
        private void RefresDataGird(DataGridView dgw)
        {
            dgw.Rows.Clear();
            string queryString = $"Select * from Student";
            SqlCommand command = new SqlCommand(queryString, dataBase.GetConnection());
            dataBase.openConnection();
            SqlDataReader reader = command.ExecuteReader();
            while (reader.Read())
            {
                ReadSingleRow(dgw, reader);
            }
            reader.Close();
        }

        private void CreateColumns()
        {
            dataGridView1.Columns.Add("code", "ID");
            dataGridView1.Columns.Add("name", "Имя");
            dataGridView1.Columns.Add("surname", "Фамилия");
            dataGridView1.Columns.Add("patronymic", "Отчество");
            dataGridView1.Columns.Add("number", "Телефон");
            dataGridView1.Columns.Add("group_type", "Группа");
            dataGridView1.Columns.Add("codeCard", "Код карты");
            dataGridView1.Columns.Add("", "");
        }


        private void Main_FormClosed(object sender, FormClosedEventArgs e)
        {
            Application.Exit();
        }

        private void Main_Load(object sender, EventArgs e)
        {
            CreateColumns();
            RefresDataGird(dataGridView1);
            dataBase.openConnection();

                button2.Visible = true;
                button4.Visible = true;

        }

        private void button2_Click(object sender, EventArgs e)
        {
            deleteRow();
            Update();
        }

        private void deleteRow()
        {
            int index = dataGridView1.CurrentCell.RowIndex;
            dataGridView1.Rows[index].Visible = false;
            if (dataGridView1.Rows[index].Cells[0].Value.ToString() == String.Empty)
            {
                dataGridView1.Rows[index].Cells[6].Value = RowState.Deleted;
                return;
            }
            dataGridView1.Rows[index].Cells[6].Value = RowState.Deleted;
        }

        new private void Update()
        {
            dataBase.openConnection();
            for (int index = 0; index < dataGridView1.Rows.Count; index++)
            {
                try
                {
                    var rowState = (RowState)dataGridView1.Rows[index].Cells[6].Value;
                    if (rowState == RowState.Existed)
                        continue;
                    if (rowState == RowState.Deleted)
                    {
                        var id = Convert.ToInt32(dataGridView1.Rows[index].Cells[0].Value);
                        var deleteQuery = $"delete from Student where code = {id}";
                        var command = new SqlCommand(deleteQuery, dataBase.GetConnection());
                        command.ExecuteNonQuery();
                    }
                }
                catch { }

            }
            dataBase.closeConnection();
        }

        private void button4_Click(object sender, EventArgs e)
        {
            this.Hide();
            DannyeAdd add = new DannyeAdd();
            add.Show();
        }

        private void dataGridView1_CellClick(object sender, DataGridViewCellEventArgs e)
        {
            var selectedRow = e.RowIndex;
            if (e.RowIndex >= 0)
            {
                DataGridViewRow row = dataGridView1.Rows[selectedRow];
                code = row.Cells[0].Value.ToString();
                name = row.Cells[1].Value.ToString();
                surname = row.Cells[2].Value.ToString();
                patronymic = row.Cells[3].Value.ToString();
                number = row.Cells[4].Value.ToString();
                group_type = row.Cells[5].Value.ToString();
                codeCard = row.Cells[6].Value.ToString();
            }
        }
    }
}