1. 定义提示文字内容

private readonly string RemarkText = "最多输入100字";   // 提示文字

2. 添加textbox 焦点事件， 初始化textbox提示文字和字体颜色

        public UserControl()
        {
            InitializeComponent();

            tb_Remark.Text = RemarkText;
            tb_Remark.ForeColor = Color.Gray;
            //Lostfocus当一个控件失去焦点时触发,定义控件失去焦点时的方法
            this.tb_Remark.LostFocus += new System.EventHandler(this.tb_Remark_LostFocus);
            //GotFocus是在一个对象得到焦点时发生，定义控件得到焦点时的方法
            this.tb_Remark.GotFocus += new System.EventHandler(this.tb_Remark_GotFocus);
        }

3. 事件处理

      private void tb_Remark_GotFocus(object sender, EventArgs e)
        {
            if (tb_Remark.Text.Trim() == RemarkText)
                tb_Remark.Text = ""; 

            tb_Remark.ForeColor = Color.Black;
        }

        private void tb_Remark_LostFocus(object sender, EventArgs e)
        {
            if (tb_Remark.Text.Trim() == "")
            {
                tb_Remark.Text = RemarkText;
                tb_Remark.ForeColor = Color.Gray;
            }
        }

4. 获取textbox 输入内容需做判断

            if (this.tb_Remark.Text.Trim() == RemarkText)
                friendInformation.Remark = "";
            else
                friendInformation.Remark = this.tb_Remark.Text.Trim();

效果