VS2010，运行中出现这种情况怎么改？

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using OnLineExam.BusinessLogicLayer;
using System.Data.SqlClient;

public partial class Web_UserAdd : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
{
_ string loginName = Session["userID"].ToString();_
Users user = new Users();
user.LoadData(loginName);
labUser.Text = user.UserName;

}
}
protected void imgBtnSave_Click(object sender, ImageClickEventArgs e)
{
if (Page.IsValid)
{
Users user = new Users(); //创建Users对象
user.UserID = txtUserID.Text.Trim();
if (!user.CheckUser(user.UserID))//使用CheckUser方法验证用户是否存在
{
user.UserName = txtUserName.Text;
user.UserPwd = txtUserPwd.Text;
user.DepartmentId = Convert.ToInt16(ddlDepartment.SelectedValue);
user.RoleId = Convert.ToInt16(ddlRole.SelectedValue);

                if (user.InsertByProc())
                {
                    lblMessage.Text = "成功插入该用户信息！";
                    Server.Transfer("UserManage.aspx");
                }
          else
           {
               lblMessage.Text = "添加用户失败！";
           }
        }
        else//用户存在，给出提示
        {
            lblMessage.Text = "数据库中存在具有该编号的用户，请重新输入！";
        }
    }
}
protected void imgBtnReturn_Click(object sender, ImageClickEventArgs e)
{
    Server.Transfer("UserManage.aspx");       
}
protected void ddlDepartment_SelectedIndexChanged(object sender, EventArgs e)
{

}

}

Session["userID"]为空吧，先做一下判断
if(Session["userID"]!=null)
{
操作代码
}

Session["userID"] 为空的时候 不能tostring();
应该先判断 Session["userID"] 是否为空 再tostring（）