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 Item_Qty_Rate_Total
        {
            public partial class Form1 : Form
            {
                int r;
                public Form1()
                {
                    InitializeComponent();
                    r = 0;
                }
                private void button1_Click(object sender, EventArgs e)
                {
                    dataGridView1.Rows.Add();

                    dataGridView1[0, r].Value = textBox1.Text;
                    dataGridView1[1, r].Value = textBox2.Text;
                    dataGridView1[2, r].Value = textBox3.Text;

                    textBox4.Text = (Convert.ToDouble(textBox3.Text) * Convert.ToDouble(textBox2.Text)).ToString();

                    dataGridView1[3, r].Value = textBox4.Text;
                    r++;
                }
            }
        }