public partial class Form4 : Form
    {
        public Form1 f1;
       

        public Form4()
        {
            InitializeComponent();
           
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Form1 f1 = new Form1();
            
            ///
            int hdd = f1.dataGridView1.Rows.Count;
            MessageBox.Show(hdd.ToString());
            ///
         
        }
    }