求助C#

寫了個記事本的程序,可是在查找那個窗口中點擊查找總是實現不了功能

每次都是執行messagebox對話框

一下是查找窗體代碼

    public partial class Form2 : Form
    {
        public int start = 0;
      
        public Form2()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
           MainWindow main=new MainWindow();
          
           string str;
           str = textBox1.Text;
           start = main.richTextBox1.Find(str, start, RichTextBoxFinds.MatchCase);
           if (start == -1)
           {
               MessageBox.Show("已經查找到文檔最後", "查找結束對話框");
               start = 0;
           }
           else
               start += str.Length;
           main.richTextBox1.Focus();
        } 

 總是執行messagebox那個獲取字符的變量不管用，我把textbox.text換成字符也不行，