listview in csharp

    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 listview
    {
        public partial class Form1 : Form
        {
            public Form1()
            {
                InitializeComponent();
            }

            private void Form1_Load(object sender, EventArgs e)
            {
                listViewMusteri.View = View.Details; //alt alta gösterir listeyi

                //kolonlarımızı ekledik, sola dayalı, boyutu 100 verdik
                listViewMusteri.Columns.Add("Ad", 100,HorizontalAlignment.Left);
                listViewMusteri.Columns.Add("Soyad",140, HorizontalAlignment.Left);
                listViewMusteri.Columns.Add("Tel", 200,HorizontalAlignment.Left);


            }

            private void btnEkle_Click(object sender, EventArgs e)
            {
                int count = listViewMusteri.Items.Count;
                
                bool flag=false;

                for(int i=0; i<count; i++)
                {
                    //isim ve soyisim kontrolü, eğer varsa tekrar eklemesin
                    if (txtAd.Text.Equals(listViewMusteri.Items[i].Text) && txtSoyad.Text.Equals(listViewMusteri.Items[i].SubItems[1].Text))
                    {
                        flag = true;
                        break;
                    }
                }
                if (flag)
                {
                    MessageBox.Show("you are already our member..");
                    txtAd.Clear();
                    txtSoyad.Clear();
                    txtTel.Clear();
                }
                else
                {
                    listViewMusteri.Items.Add(txtAd.Text); //ana kolon bilgisi // 0. kolon
                    listViewMusteri.Items[count].SubItems.Add(txtSoyad.Text); // 1. alt kolon 
                    listViewMusteri.Items[count].SubItems.Add(txtTel.Text); //2. alt kolon
                    txtAd.Clear();
                    txtSoyad.Clear();
                    txtTel.Clear();
                }
            
            }
        }
    }