c# - 登录错误消息对象引用

Question

我有两个页面，分别称为Login.aspx和Home.aspx
更新的代码:

 protected void Button1_Click(object sender, EventArgs e)
    {

            if (loginmethod(txt_us.Text, txt_pwd.Text) != "NA")
            {
                FormsAuthentication.Initialize();
                String strRole = Assignroles(txt_us.Text);
                FormsAuthenticationTicket fat = new FormsAuthenticationTicket(1, txt_us.Text, DateTime.Now, DateTime.Now.AddMinutes(30), false, strRole, FormsAuthentication.FormsCookiePath);
                Response.Cookies.Add(new HttpCookie(FormsAuthentication.FormsCookieName,
                FormsAuthentication.Encrypt(fat)));
                loginmethod(txt_us.Text, txt_pwd.Text);
                Response.Redirect("home.aspx");
            }
            else if (Session["ID"] != null)
            {

                    u = Session["ID"].ToString();  
            }

            else
            {
                Label1.Text = ("Incorrect UserName/Password");

                Label1.Visible = true;
                Response.Redirect("home.aspx");
            }
            txt_us.Text = "";
            txt_pwd.Text = "";
    }

    private string loginmethod(string UserName, string Password)
    {
        try
        {
            login_class lg_class = new login_class();
            Entities login = new Entities();
            string logn = Convert.ToString(lg_class.loginfunction(UserName, Password).Rows[0]["id"]);
            Session["ID"] = logn.ToString();
            return (logn);
        }
        catch (Exception ex)
        {            
            return (ex.Message.ToString());            
        }
    }

    private string Assignroles(string username)
    {
        if ((txt_us.Text != string.Empty) && (txt_pwd.Text != string.Empty))
            return "";
        else
            return string.Empty;
    }

    public DataTable loginfunction(string username, string password)
    {
        try
        {
            Entities lg = new Entities();
            List<SP_GetLogin_Result> gr = lg.SP_GetLogin(username, password).ToList();
            DataTable dt = new DataTable();
            dt.Columns.Add("id", typeof(string));
            foreach (var l in gr)
            {
                dt.Rows.Add(l.id);
            }
            return dt;
        }
        catch (Exception)
        {
            throw new Exception();
        }
    }

并在 home.aspx页面上加载:

    string id;

    protected void Page_Load(object sender, EventArgs e)
    {          
        if(!IsPostBack)
        {   
            id = Session["ID"].ToString();
           this.FFID = "All";
            vehcileinfo(id);                     
        }    
    }

SP_GetLogin返回ID，即当我输入用户名 abc和pssword 1224时，此返回ID CU-2343
当我使用正确的用户名和密码输入时，此重定向到 home.aspx
但是当我输入错误的用户名或密码时，这会在此行显示错误

u = Session["ID"].ToString();

System.NullReferenceException: Object reference not set to an instance of an object.

我想在 login.aspx上显示错误消息
“错误的用户名和密码”

我该怎么做？

Answer 1

根据评论中的讨论，获取NullReferenceException的异常(exception)

这意味着，您正在尝试使用空值。这意味着您要么将其设置为null，要么根本不将其设置为任何值。

因此，只需检查Home.aspx的Page_Load

if(Session["ID"] != null)
{ 
   id = Session["ID"].ToString();
} 

else
{
  // Do logic here
}