publicpartial class Form2 : Form
    {
       private string text;
       private ListBox lb;
       private int index;

       //构造函数接收三个参数：选中行文本，ListBox控件，选中行索引
       public Form2(string text,ListBox lb,int index)
       {
           this.text = text;
           this.lb = lb;
           this.index = index;
           InitializeComponent();
           this.textBox1.Text = text;
       }

       private void btnChange_Click(object sender, EventArgs e)
       {           

           string text = this.textBox1.Text;
           this.lb.Items.RemoveAt(index);
           this.lb.Items.Insert(index, text);
           this.Close();
       }
    }

public partial class Form1 :Form
    {
       int index = 0;
       string text = null;
       public Form1()
       {
           InitializeComponent();
       }

       private void listBox1_SelectedIndexChanged(object sender, EventArgse)
       {
           if (this.listBox1.SelectedItem != null)
           {
               text = this.listBox1.SelectedItem.ToString();
               index = this.listBox1.SelectedIndex;

               //构造Form2同时传递参数
               Form2 form2 = new Form2(text, listBox1, index);
               form2.ShowDialog();
           }
       }