FindControl of a Textbox in a UserControl OnRowUpdate

嗨，单击更新并触发OnRowUpdate事件后出现错误。在用户控件中获取文本框值的正确格式是什么？

错误 55"System.Web.UI.UserControl"不包含 "PlanID"和没有接受第一个参数的扩展方法"PlanID" 可以找到类型为"系统.Web.UI.UserControl"（您是否缺少 使用指令或程序集引用

protected void grd_Plan_RowUpdating(object sender, GridViewUpdateEventArgs e)
        {
            string PlanCode =((UserControl)Grid_Plan.Rows[e.RowIndex].FindControl("ucPlanID")).PlanID;
        }
//usercontrol code behind
public string PlanID
        {
            get
            {
                return txtPlanID.Text;
            }
            set
            {
                txtPlanID.Text = value;
            }
        }

UserControl ucPlanID = (UserControl)Grid_Plan.Rows[e.RowIndex].FindControl("ucPlanID");
if (ucPlanID != null)
{
   string PlanCode = ((TextBox)ucPlanID.FindControl("PlanID")).Text;
}