---

private void btnAdd_Click(object sender, EventArgs e)
{
    int quantity;

    if (!int.TryParse(txtQuantity.Text, out quantity))
    {
        MessageBox.Show("Введите корректное количество!");
        return;
    }

    if (quantity < 0)
    {
        MessageBox.Show("Количество товара не может быть отрицательным!");
        return;
    }

    using (SQLiteConnection connection =
        new SQLiteConnection(connectionString))
    {
        connection.Open();

        string sql =
            "INSERT INTO Products(ProductName, Quantity, ExpirationDate) " +
            "VALUES(@name, @quantity, @date)";

        SQLiteCommand command = new SQLiteCommand(sql, connection);

        command.Parameters.AddWithValue("@name", txtName.Text);
        command.Parameters.AddWithValue("@quantity", quantity);
        command.Parameters.AddWithValue("@date", dtpExpiration.Value.ToString("yyyy-MM-dd"));

        command.ExecuteNonQuery();
    }

    MessageBox.Show("Товар успешно добавлен!");
    LoadData();
}