Why in the code into a variable when querying with sql does not get the required value?

Question

SQLite

the code

using System;
using System.Data;
using System.Data.SQLite;
using System.IO;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class JournalEntries : Form
    {
        private SQLiteConnection sql_con;
        private SQLiteCommand sql_cmd;
        private DataSet DS = new DataSet();
        private DataTable DT = new DataTable();
        private string sPath = Path.Combine(Application.StartupPath, "mybd.db");

        public JournalEntries()
        {
            InitializeComponent();
        }

        private void JournalEntries_Load(object sender, EventArgs e)
        {
            string ConnectionString = @"Data Source=" + sPath + ";New=False;Version=3";
            selectTable(ConnectionString);
            // выбрать значения из справочников для отображения в comboBox
            String selectOS = "Select idOS, Name, Cost from OS";
            selectCombo(ConnectionString, selectOS, comboBox1, "Name", "idOS");
            String selectSubd = "SELECT idSubdivision, Name FROM Subdivision";
            selectCombo(ConnectionString, selectSubd, comboBox2, "Name", "idSubdivision");
            String selectMOL = "SELECT idMOL, Name FROM MOL";
            selectCombo(ConnectionString, selectMOL, comboBox3, "Name", "idMOL");
            comboBox1.SelectedIndex = -1;
            comboBox2.SelectedIndex = -1;
            comboBox3.SelectedIndex = -1;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string ConnectionString = @"Data Source=" + sPath + ";New=False;Version=3";
            // находим максимальное значение кода проводок для записи первичного ключа
            String mValue = "select MAX(IDJournalEntries) from JournalEntries";
            object maxValue = selectValue(ConnectionString, mValue);
            if (Convert.ToString(maxValue) == "")
                maxValue = 0;
            // Обнулить значения переменных
            string sum = "0";
            string count = "0";
            string coment = null;
            string Value1 = null;
            string Value2 = null;
            string Value3 = null;
            if (comboBox1.Text != "")
            {
                //ОС
                Value1 = comboBox1.SelectedValue.ToString();
            }
            if (comboBox2.Text != "")
            {
                //Подразделение
                Value2 = comboBox2.SelectedValue.ToString();
            }
            if (comboBox3.Text != "")
            {
                //МОЛ
                Value3 = comboBox3.SelectedValue.ToString();
            }
            //Поле количество
            if (textBox3.Text != "")
            {
                count = textBox3.Text;
            }
            //Поиск по базе данных значений
            String selectCost = "select Cost from OS where idOS='" + Value1 + "'";
            double Summa = Convert.ToDouble(selectCost) *
           Convert.ToDouble(count);
            String selectDT = "select idChart from ChartOfAccounts where Account = '''01'''";
 object DT = selectValue(ConnectionString, selectDT);
            String selectKT = "select idChart from ChartOfAccounts where Account = '''00'''";
 object KT = selectValue(ConnectionString, selectKT);
            string add = "INSERT INTO JournalEntries (IDJournalEntries, Date, DT, SubkontoDT1, SubkontoDT2, SubkontoDT3, KT, Count, Summa, IDOperation, Comment) VALUES(" + (Convert.ToInt32(maxValue) + 1) +
maskedTextBox1.Text + "'," + DT.ToString() + "," + Convert.ToInt32(Value1)
+ "," + Convert.ToInt32(Value2) + "," + Convert.ToInt32(Value3) + "," +
KT.ToString() + "," + Convert.ToDouble(count) + "," + Summa + ", " +
Convert.ToInt32(textBox1.Text) + ", " + ",'Поступление ОС','" + ")";
            ExecuteQuery(add);
            selectTable(ConnectionString);
        }
        // метод выполнения запроса к базе данных
        private void ExecuteQuery(string txtQuery)
        {
            sql_con = new SQLiteConnection("Data Source=" + sPath + ";Version=3;New=False;Compress=True;");
            sql_con.Open();
            sql_cmd = sql_con.CreateCommand();
            sql_cmd.CommandText = txtQuery;
            sql_cmd.ExecuteNonQuery();
            sql_con.Close();
        }
        public void selectTable(string ConnectionString)
        {
           
                SQLiteConnection connect = new
                SQLiteConnection(ConnectionString);
                connect.Open();
                SQLiteDataAdapter dataAdapter = new
                SQLiteDataAdapter("Select IDJournalEntries, Date, DT, SubkontoDT1, SubkontoDT2, SubkontoDT3, KT, SubkontoKT1, SubkontoKT2, SubkontoKT3, Count, Summa, IDOperation, Comment from JournalEntries", connect);
                DataSet ds = new DataSet();
                dataAdapter.Fill(ds);
                dataGridView1.DataSource = ds;
                dataGridView1.DataMember = ds.Tables[0].ToString();
                connect.Close();
                dataGridView1.Columns["IDJournalEntries"].DisplayIndex = 0;
                dataGridView1.Columns["Date"].DisplayIndex = 1;
                dataGridView1.Columns["DT"].DisplayIndex = 2;
                dataGridView1.Columns["SubkontoDT1"].DisplayIndex = 3;
                dataGridView1.Columns["SubkontoDT2"].DisplayIndex = 4;
                dataGridView1.Columns["SubkontoDT3"].DisplayIndex = 5;
                dataGridView1.Columns["KT"].DisplayIndex = 6;
                dataGridView1.Columns["SubkontoKT1"].DisplayIndex = 7;
                dataGridView1.Columns["SubkontoKT2"].DisplayIndex = 8;
                dataGridView1.Columns["SubkontoKT3"].DisplayIndex = 9;
                dataGridView1.Columns["Count"].DisplayIndex = 10;
                dataGridView1.Columns["Summa"].DisplayIndex = 11;
                dataGridView1.Columns["IDOperation"].DisplayIndex = 12;
                dataGridView1.Columns["Comment"].DisplayIndex = 13;
            
        }
        public void selectCombo(string ConnectionString, String selectCommand, ComboBox comboBox, string displayMember, string valueMember)
        {
            SQLiteConnection connect = new
            SQLiteConnection(ConnectionString);
            connect.Open();
            SQLiteDataAdapter dataAdapter = new
            SQLiteDataAdapter(selectCommand, connect);
            DataSet ds = new DataSet();
            dataAdapter.Fill(ds);
            comboBox.DataSource = ds.Tables[0];
            comboBox.DisplayMember = displayMember;
            comboBox.ValueMember = valueMember;
            connect.Close();
        }
        public object selectValue(string ConnectionString, String selectCommand)
        {
            SQLiteConnection connect = new
           SQLiteConnection(ConnectionString);
            connect.Open();
            SQLiteCommand command = new SQLiteCommand(selectCommand, connect);
            SQLiteDataReader reader = command.ExecuteReader();
            object value = "";
            while (reader.Read())
            {
                value = reader[0];
            }
            connect.Close();
            return value;
        }

        private void button2_Click(object sender, EventArgs e)
        {
            comboBox1.SelectedIndex = -1;
            comboBox2.SelectedIndex = -1;
            comboBox3.SelectedIndex = -1;
            textBox1.Clear();
            textBox2.Clear();
            textBox3.Clear();
            maskedTextBox1.Clear();
        }
    }
}

mistake

Where is the mistake? After the request, the value of the Cost column should be written to selectCost, then to double Summa, after the conversion, they should be multiplied, but an exception occurs.

Answer 1

String selectCost = "select Cost from OS where idOS='" + Value1 + "'";
            double Summa = Convert.ToDouble(selectCost) *
           Convert.ToDouble(count);

In the first line, you assign the query text to selectCost, and in the second line, you try to cast this text to the double type. The string ""select Cost from..." is not cast to double and you get an exception that the input string was malformed.