在文本框的特定位置用*号替换

效果：

代码：

private void textBox1_TextChanged(object sender, EventArgs e)
        {
            string content = this.textBox1.Text;
            int length = content.Length;
            if(length>=5 && length<=7)
            {
                string temp = "";
                if(length == 5)
                {
                    temp = content.Replace(content.Substring(4, 1), "*");    
                }else if(length == 6)
                {
                    temp = content.Replace(content.Substring(5, 1), "*");    
                }else if(length == 7)
                {
                    temp = content.Replace(content.Substring(6, 1), "*");    
                }
                this.textBox1.Text = temp;
                textBox1.SelectionStart = textBox1.TextLength;//设置文本框选定的文本起点
            }
        }