Handling the null reference

Question

User1028962535 posted

Hello, I would like users to be taken to the login page, if the session variable is nothing, but  I still get this error, how can I fix it? thanks

Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

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

Source Error: 

My CODE LOOKS LIKE THIS

Protected Sub Page_Load(sender As Object, e As EventArgs)

Dim User As String = Session("UserName").ToString()

If Session("Username") Is Nothing Then

Response.Redirect("Login.aspx")

End If

End Sub

Answer 1

User-1171043462 posted

You are doing a mistake. You are first converting to String and then checking use following code

Protected Sub Page_Load(sender As Object, e As EventArgs)

Dim User As String = Convert.ToString(Session("UserName"))

If Not string.IsNullOrEmpty(User) Then

Response.Redirect("Login.aspx")

End If

End Sub

Convert.ToString will take care of NULL

Answer 2

User-369506445 posted

Hi

You get an error when using below code

Dim User As String = Session("UserName").ToString()

Because your session may be empty and when converting a null to a string you get an error

Try

Dim User As String = ""


If Session("Username") Is Nothing Then
User=Session("UserName").ToString()
Response.Redirect("Login.aspx")

Answer 3

User753101303 posted

Hi,

Or as it seems you never actually use the value you are retrieving, just drop this line :

Protected Sub Page_Load(sender As Object, e As EventArgs)
  If Session("Username") Is Nothing Then
    Response.Redirect("Login.aspx")
  End If
End Sub

The problem is that you are using Session("UserName").ToString() so you do call a method on the returned object before checking if this object is nothing.

Not directly related but this info is available as User.Identity.Name and having a copy in session doesn't seems to add any value(it would better to use what ASP.NET offers to handle logged users rather than rolling your own mechanism)

Answer 4

User-821857111 posted

If Session("Username") Is Nothing Then

Response.Redirect("Login.aspx")

End If

Others have explained why calling the ToString method results in a NullRefrenceException, but you should not be using Session to maintain user logins. You should be using Forms Authentication instead.