How add data with existing data in data grid view using windows forms

Hi Jithu,

Its working 

Thank You

Not working Bro

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Windows.Forms;

namespace DataGridViewExample
{
    public partial class Form1 : Form
    {
        private List dataRows;

        public Form1()
        {
            InitializeComponent();
            LoadDataFromFile("data.txt");
            PopulateDataGridView();
            AddGroupedRows();
        }

        private void LoadDataFromFile(string filePath)
        {
            dataRows = new List();
            var lines = File.ReadAllLines(filePath);

            foreach (var line in lines)
            {
                var columns = line.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries);
                dataRows.Add(new DataRow
                {
                    ID = columns[0],
                    LineNumber = columns[1],
                    Unit = columns[2],
                    Area = columns[3],
                    Rev = columns[4],
                    TestNumber = columns[5],
                    Sheet = columns[6],
                    IdentNumber = columns[7],
                    Length = double.Parse(columns[8]),
                    Meter = columns[9],
                    Description = columns[10],
                    Status = columns[11],
                    Value1 = int.Parse(columns[12]),
                    Value2 = int.Parse(columns[13]),
                    Value3 = int.Parse(columns[14])
                });
            }
        }
        
        private void PopulateDataGridView()
        {
            dataGridView1.DataSource = dataRows;
        }

        private void AddGroupedRows()
        {
            var groupedData = dataRows
                .GroupBy(row => row.TestNumber)
                .Select(group => new DataRow
                {
                    ID = "TAG",
                    LineNumber = "",
                    Unit = "",
                    Area = "",
                    Rev = "",
                    TestNumber = group.Key,
                    Sheet = "",
                    IdentNumber = "",
                    Length = group.Sum(row => row.Length),
                    Meter = "",
                    Description = "",
                    Status = "",
                    Value1 = group.Sum(row => row.Value1),
                    Value2 = group.Sum(row => row.Value2),
                    Value3 = group.Sum(row => row.Value3)
                });

            foreach (var row in groupedData)
            {
                dataRows.Add(row);
            }

            dataGridView1.DataSource = null; // Reset the binding to refresh the DataGridView
            dataGridView1.DataSource = dataRows;
        }
    }

    public class DataRow
    {
        public string ID { get; set; }
        public string LineNumber { get; set; }
        public string Unit { get; set; }
        public string Area { get; set; }
        public string Rev { get; set; }
        public string TestNumber { get; set; }
        public string Sheet { get; set; }
        public string IdentNumber { get; set; }
        public double Length { get; set; }
        public string Meter { get; set; }
        public string Description { get; set; }
        public string Status { get; set; }
        public int Value1 { get; set; }
        public int Value2 { get; set; }
        public int Value3 { get; set; }
    }
}

Please try this