Form2 kodum. DataGridView bu formda.

using MySql.Data.MySqlClient;
using MySql.Data;
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;

namespace SUveTUPtakip
{
    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }
        public string silinecekmust;
        public Form3 frm3 = new Form3();
        public Form4 frm4 = new Form4();
        public Form5 frm5 = new Form5();
        public Form6 frm6 = new Form6();

        MySqlConnection con = new MySqlConnection("Server=localhost; Database=su_tup; Uid=root; Pwd=1234;SslMode=Preferred;");
        public DataSet dtst = new DataSet();
        MySqlCommand cmd;
        MySqlDataReader dr;



        public void mustlisteyenile()
        {

            MySqlDataAdapter adtr = new MySqlDataAdapter("select musteriNo,musteriAdi,musteriSoyadi,musteriTel, musteriKayitTarihi, musteriAdres From musteribilgi", con);
            adtr.Fill(dtst, "musteribilgi");
            this.dataGridView1.DataSource = dtst.Tables["musteribilgi"];
            adtr.Dispose();

        }
        public void mustlistele()
        {
            
            MySqlDataAdapter adtr = new MySqlDataAdapter("select musteriNo,musteriAdi,musteriSoyadi,musteriTel, musteriKayitTarihi, musteriAdres From musteribilgi", con);
            adtr.Fill(dtst, "musteribilgi");
            dataGridView1.DataSource = dtst.Tables["musteribilgi"];
            adtr.Dispose();
            


        }





        private void Form2_Load(object sender, EventArgs e)
        {

            con.Open();
            mustlistele();
            dataGridView1.Columns[0].HeaderText = "No";
            dataGridView1.Columns[1].HeaderText = "Adı";
            dataGridView1.Columns[2].HeaderText = "Soyadı";
            dataGridView1.Columns[3].HeaderText = "Telefon";
            dataGridView1.Columns[4].HeaderText = "Kayıt tarihi";
            dataGridView1.Columns[5].HeaderText = "Adres";
            dataGridView1.Columns[0].Width = 60;
            dataGridView1.Columns[1].Width = 80;
            dataGridView1.Columns[2].Width = 80;
            dataGridView1.Columns[3].Width = 70;
            dataGridView1.Columns[4].Width = 70;
            dataGridView1.Columns[5].Width = 180;
            con.Close();
            
        }

        private void btnCikis_Click(object sender, EventArgs e)
        {
            Application.Exit();
        }

        private void btnMustKayit_Click(object sender, EventArgs e)
        {

            frm3.ShowDialog();
        }

        private void btnMustDuzenle_Click(object sender, EventArgs e)
        {

            frm4.Show();
        }

        private void btnStok_Click(object sender, EventArgs e)
        {
            frm5.ShowDialog();
            con.Open();
            MySqlDataAdapter adtr = new MySqlDataAdapter("select UrunAd, Fiyat, Adet From stokbilgi ", con);
            adtr.Fill(dtst, "stokbilgi");
            frm5.dataGridView1.DataSource = dtst.Tables["stokbilgi"];
            adtr.Dispose();
            con.Close();

        }


        private void button1_Click(object sender, EventArgs e)
        {
            frm6.ShowDialog();
        }

        private void btnMustSil_Click(object sender, EventArgs e)
        {
            DialogResult cvp;
            cvp =  MessageBox.Show(" Kaydı silmek istediğinize emin misiniz?","?", MessageBoxButtons.YesNo, MessageBoxIcon.Warning );
            if (cvp == DialogResult.Yes)
            {


                string sql = "DELETE FROM musteribilgi where musteriNo= @musteriNo";
                cmd = new MySqlCommand(sql, con);
                cmd.Parameters.AddWithValue("@musteriNo", txtArMusteriNo.Text);
                DataGridViewRow selectedRow = dataGridView1.SelectedRows[0];
                dataGridView1.Rows.Remove(selectedRow); 
                con.Open();
                cmd.ExecuteNonQuery();
                mustlistele();
                con.Close();
            }
      
        }

        private void dataGridView1_CellEnter(object sender, DataGridViewCellEventArgs e)
        {
            txtArMusteriNo.Text = dataGridView1.CurrentRow.Cells[0].Value.ToString();
            txtArMusteriAd.Text = dataGridView1.CurrentRow.Cells[1].Value.ToString();
            txtArMusteriSoyad.Text = dataGridView1.CurrentRow.Cells[2].Value.ToString();
            txtArTel.Text = dataGridView1.CurrentRow.Cells[3].Value.ToString();
        }
    }
}


Bu da form 3 kodum datagridviewa. Müşteri Ekleme formu


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 MySql.Data;
using MySql.Data.MySqlClient;

namespace SUveTUPtakip
{
    public partial class Form3 : Form
    {
        public MySqlConnection con;
        public MySqlCommand cmd;
        public Form2 frm2;
        public DataSet dtst = new DataSet();
        MySqlDataReader dr;

        public Form3()
        {
            InitializeComponent();
            con = new MySqlConnection("Server=localhost; Database=su_tup; Uid=root; Pwd=1234");
            cmd = new MySqlCommand();
        }

        public void mustlisteyenile()
        {

            MySqlDataAdapter adtr = new MySqlDataAdapter("select musteriNo,musteriAdi,musteriSoyadi,musteriTel, musteriKayitTarihi, musteriAdres From musteribilgi", con);
            adtr.Fill(dtst, "musteribilgi");
            frm2.dataGridView1.DataSource = dtst.Tables["musteribilgi"];
            adtr.Dispose();

        }
        private void Form3_Load(object sender, EventArgs e)
        {
            cmd = new MySqlCommand();
            dateTimePicker1.Enabled = false;
            string formateddate = dateTimePicker1.Value.ToString("yyyy-MM-dd");
            textBox5.Text = formateddate;
            
        }

        private void button1_Click(object sender, EventArgs e)
        {
            try
            {
                con.Open();

                cmd.Connection = con;
                cmd.CommandText = "INSERT INTO musteribilgi (musteriNo, musteriAdi, musteriSoyadi, musteriTel, musteriKayitTarihi, musteriAdres) VALUES (@musteriNo, @musteriAdi, @musteriSoyadi, @musteriTel, @musteriKayitTarihi, @musteriAdres)";

                cmd.Parameters.AddWithValue("@musteriNo", textBox1.Text);
                cmd.Parameters.AddWithValue("@musteriAdi", textBox2.Text);
                cmd.Parameters.AddWithValue("@musteriSoyadi", textBox3.Text);
                cmd.Parameters.AddWithValue("@musteriTel", textBox4.Text);
                cmd.Parameters.AddWithValue("@musteriKayitTarihi", textBox5.Text);
                cmd.Parameters.AddWithValue("@musteriAdres", textBox6.Text);

                cmd.ExecuteNonQuery();

                textBox1.Clear();
                textBox2.Clear();
                textBox3.Clear();
                textBox4.Clear();
                textBox5.Clear();
                textBox6.Clear();

                frm2.dataGridView1.Refresh();
                frm2.dataGridView1.Update();
                frm2.mustlistele();


            }
            catch (Exception ex)
            {
                
            }
            MessageBox.Show("Kayıt işlemi tamamlandı!");
            
            frm2.dataGridView1.Refresh();
            frm2.dataGridView1.Update();
            frm2.mustlistele();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            frm2.mustlistele();
        }
    }
}