How do i use Session Objects in ASP.NET (Using Visual Basic)

Question

So i would like the user to login and have their username appear on the the top of the page when they're logged in. I check if they exist in the database first then i let them in, so when they click the login button it does something like this(button event)

If (IsPostBack) Then

            connection.Open()
            command.Connection = connection

            command.CommandText = "select count(*) from customer where Email='" & TextEmail.Text & "' AND Password='" & TextPassword.Text.Trim & "'"
            counts = Convert.ToInt32(command.ExecuteScalar.ToString) ' execute this text as an SQL query

            If (counts > 0) Then
                Response.Redirect("Default.aspx", False)
                Session("Username") = TextEmail.Text 'This is where i try to use the session storing the user's email (which i use as a username)
            Else
                Label1.Visible = True
            End If

            connection.Close()
        End If

Above code works fine, has no problem. In C#.Net This would be Session["Username"]=TextEmail.Text right.

So now there is a form within my project to which you cannot gain access without logging in first. When this page loads, i want to check if the user is logged in or not, if not, I redirect them to the Login page, if they are i let them in, and update the label that will display "logged in user userEmail", The Page_load code is like this:

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    Try
        If Not (Session("Username") = vbNull) Then
            LogInStatus.Text = "logged in user " & System.Web.HttpContext.Current.Session("Username").ToString
        Else
            Response.Redirect("Login.aspx", False)
        End If
    Catch ex As Exception
        MsgBox(ex.ToString)
    End Try
End Sub

There will be a Logout, which when clicked i will set the Session to Null and then redirect them to the login page once again. so when they click logout this happens

 Session("Username") = vbNull
        Response.Redirect("Login.aspx", False)

When i run the code, i get this NullReference exception image here Obviously either i'm initializing the object wrong, or i'm just using it straight the wrong way. But all this works with C#. Please help.

try to convert Convert.ToString(System.Web.HttpContext.Current.Session["Username"]) (Sorry this is given in C#) it handles the null reference — Tummala Krishna Kishore
– Tummala Krishna Kishore, Commented Sep 9, 2016 at 13:02
@Webruster it is already converted. If you go back to read the code i gave, you will see that where the exception occurs , the end of the statement ends with ..... Session("Username").ToString — Ndumiso
– Ndumiso, Commented Sep 9, 2016 at 13:11
if you are using .ToString() it will lead to null reference exception if the object is null , where as this is handled if you convert using Convert.Tostring , this is what i am refering in my previous comment — Tummala Krishna Kishore
– Tummala Krishna Kishore, Commented Sep 9, 2016 at 13:29
If they put a comma in their password, you app will crash... Parametrize your query, and encrypt password if this is going to be used. Instead of "= vbNull" try to use "Is Nothing". — the_lotus
– the_lotus, Commented Sep 9, 2016 at 13:34

Daniel Manta · Accepted Answer · 2016-09-09 13:31:05Z

1

The problem is: when "Username" key doesn't exists, the value you get from Session is the object Nothing, not vbNull. Then Nothing ToString raises an exception. Try this:

If Session("Username") IsNot Nothing Then
    LogInStatus.Text = "logged in user " & Session("Username").ToString
Else
    Response.Redirect("Login.aspx", False)
End If

Rather than assigning vbNull you may do Session.Remove("Username") which sets the value to Nothing.

edited Sep 9, 2016 at 13:31

answered Sep 9, 2016 at 13:17

Daniel Manta

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