老师让小弟我们用c#来写一个记事本，可是小弟我的查找功能总是做不出来！请高手帮小弟我指点上

Form1 f1=(Form1)this.Owner;
            if(this.textBox1.Text.Length>0)
            {
                index=f1.richTextBox1.Text.IndexOf(this.textBox1.Text,index);
                if(index==-1)
                {
                    MessageBox.Show("未查找到相关内容");
                    index=0;
                }
                else
                {
                    f1.richTextBox1.Select(index,this.textBox1.Text.Length);//把查找到的内容选中，调用Select()方法
                    index=index+this.textBox1.Text.Length;
                    f1.Activate();
                }
            }
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