Найти сумму строк в DataGridView ,применяя LINQ

 Table<Registr> reg = db.GetTable<Registr>();
            if (checkBox1.Checked)
            {
                var sumEl = (from s in db.GetTable<Registr>() select s.quantity);
                int sum = 0;
                foreach (var item in sumEl) { sum = Convert.ToInt32(sum + item); }
                // int sum = sumEl.Sum(n => n.quantity);    
                var amountEl = (from a in db.GetTable<Registr>() select a.amount);
                decimal amount = 0;
                foreach (var i in amountEl) amount = Convert.ToInt32(amount + i);
                var v = (from r in db.GetTable<Registr>() select new { r.date, sum, amount }).Distinct();
                registrDataGridView.DataSource = v;
            }
 
if (checkBox2.Checked)
            {
                // int sum = registrDataGridView.Rows.Cast<DataGridViewRow>().Sum(t => Convert.ToInt32(t.Cells[1].Value));
               // int sum = Convert.ToInt32(db.GetTable<Registr>("SUM(quantity)", String.Empty));
                int sum1 = reg.AsEnumerable().Sum(r=> (int)r.quantity);
                decimal amount = reg.AsEnumerable().Sum(r => (decimal)r.amount);
                //var sumEl = (from s in db.GetTable<Registr>() select s.quantity);
                //int sum = 0;
                //foreach (var item in sumEl) { sum = Convert.ToInt32(sum + item); }
                //int sum = sumEl.Sum(n => n.quantity);    
                //var amountEl = (from a in db.GetTable<Registr>() select a.amount);
                //decimal amount = 0;
                //foreach (var i in amountEl) amount = Convert.ToInt32(amount + i);
                //int sEl = db.GetTable<Registr>().Sum(n => n.quantity);
                var v = (from r in db.GetTable<Registr>() select new { r.company,sum1 , amount }).Distinct();
                registrDataGridView.DataSource = v;
            }
            if (checkBox3.Checked)
            {
                var v = (from r in db.GetTable<Registr>() select  new {r.city, r.quantity,r.amount }).Distinct();
                registrDataGridView.DataSource = v;
            }
            if (checkBox4.Checked)
            {
                var v =( from r in db.GetTable<Registr>() select new { r.country, r.quantity,r.amount }).Distinct();
                registrDataGridView.DataSource = v;
            }
            if (checkBox5.Checked)
            {
                var v =( from r in db.GetTable<Registr>() select new { r.manager, r.quantity,r.amount }).Distinct();
                registrDataGridView.DataSource = v;
            }

//в переменной fields выбранные столбцы через зпт. 
string query="select "+ fields +",sum(quantity) as quantity, sum(amount) as amount from Registr group by "+fields

namespace Sharp_test
{
 
 
    public partial class Form1 : Form
    {// public static CompanyDBDataContext db = new CompanyDBDataContext();
     // Table<Registr> reg = db.GetTable<Registr>();
 
        DataTable dt = new DataTable();
        [Table(Name = "Registr")]
        public class Registr
        {
            [Column(IsPrimaryKey = true, IsDbGenerated = true)]
            public DateTime date { get; set; }
            [Column]
            public string company { get; set; }
            [Column]
            public string city { get; set; }
            [Column]
            public string country { get; set; }
            [Column]
            public string manager { get; set; }
            [Column]
            public int quantity { get; set; }
            [Column]
            public decimal amount { get; set; }
        }
        public Form1()
        {
            InitializeComponent();
            string connectionString = @"Data Source=SHMONIA-ПК\SQLEXPRESS;Initial Catalog=CompanyDB;Integrated Security=True";
            string sql = "SELECT * FROM Registr";      
       using (SqlConnection connection = new SqlConnection(connectionString)){
                SqlDataAdapter adapter = new SqlDataAdapter(sql, connection);
                DataSet ds = new DataSet("Registr"); 
                adapter.Fill(ds, "[Registr]");
                dataGridView1.DataSource = ds.Tables[0];
                SqlCommandBuilder commandBuilder = new SqlCommandBuilder(adapter);
                //adapter.Update(ds); обновилось
                //ds.Clear();очищаем
                //adapter.Fill(ds); загружаем заново
 
            }
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            // TODO: This line of code loads data into the 'companyDBDataSet.Registr' table. You can move, or remove it, as needed.
            this.registrTableAdapter.Fill(this.companyDBDataSet.Registr);
        }
 
        private void dataGridView1_CellContentClick(object sender, DataGridViewCellEventArgs e)
        {
 
        }
        private void button1_Click(object sender, EventArgs e)
        {if (checkBox1.Checked) {
                string sql1 = "SELECT [DATE], SUM(quantity) AS quantity, SUM(amount) AS amount FROM RegistrGROUP BY[DATE], company";
                dataGridView1.DataSource = sql1;
            }}

string fields="";
if(checkBox1.Checked)
fields+="date,";
if(checkBox2.Checked)
fields+="company,";
//...Все остальные
//когда строка сформирована,удалить последнюю зпт.
string query="select "+ fields +",sum(quantity) as quantity, sum(amount) as amount from Registr "+
"group by "+fields
 
using (SqlConnection connection = new SqlConnection(connectionString)){
                SqlDataAdapter adapter = new SqlDataAdapter(query, connection);
                DataSet ds = new DataSet("Registr"); 
                adapter.Fill(ds, "[Registr]");
                dataGridView1.DataSource = ds.Tables[0];
}