Why am I getting Expression Expected? - asp.net

Question

This is my simple CodeFile vb code for a username and password login form with a redirect to different 'members area' pages:
Public Class MyPage
Inherits Page
Private Structure Cred
Public Username As String
Public Password As String
Public RedirectUrl As String
Public Sub New(un As String, pw As String, Optional ru As String = "/admin/default.aspx")
Username = un
Password = pw
RedirectUrl = ru
End Sub
End Structure
Private ReadOnly _credentials As System.Collections.Generic.IEnumerable(Of Cred) = New Cred(){New Cred("userone", "passwordone"), New Cred("usertwo", "passwordtwo"), New Cred("userthree", "passwordthree", "/admin/custom.aspx")}
Public Sub Page_Load(sender As Object, e As EventArgs)
Dim user = _credentials.SingleOrDefault(Function(x) x.Username = UserName.Text AndAlso x.Password = Password.Text)
If user IsNot Nothing Then
Session("Admin") = True
Response.Redirect(user.RedirectUrl)
Else
Session("Admin") = False
LtlLogin.Text = "<p>Sorry, you have provided incorrect login details.</p>"
End If
End Sub
End Class
It's on the line:
Dim user = _credentials.SingleOrDefault(Function(x) x.Username = UserName.Text AndAlso x.Password = Password.Text)
Thanks very much.
David.

Answer 1

The problem is that you are using structure against class for Cred.
Be aware that structures are value types and classes are reference types.
So:
Dim user = _credentials.SingleOrDefault(Function(x) x.Username = UserName.Text AndAlso x.Password = Password.Text)
always return a structure (when nothing found then the members of the structure gets their default values).
You cannot compare a structure to Nothing as ti is not a reference type.
Change structure to class and you will be fine.
Or change the check with:
If Not user.Equals(New Cred) Then
Check this
UPDATE with examples
Class Cred
Imports System.Linq
Module StartupModule
Private ReadOnly _credentials As System.Collections.Generic.IEnumerable(Of Cred) = New Cred() {
New Cred("userone", "passwordone"),
New Cred("usertwo", "passwordtwo"),
New Cred("userthree", "passwordthree", "/admin/custom.aspx")}
Sub Main()
Dim userName As String = ""
Dim password As String = ""
Dim crd = _credentials.Where(Function(x) x.Username = userName AndAlso x.Password = password).SingleOrDefault
If crd Is Nothing Then
Console.WriteLine("user is nothing")
Else
Console.WriteLine("user is something")
End If
Console.ReadLine()
End Sub
Private Class Cred
Public Username As String
Public Password As String
Public RedirectUrl As String
Public Sub New(un As String, pw As String, Optional ru As String = "/admin/default.aspx")
Username = un
Password = pw
RedirectUrl = ru
End Sub
End Class
End Module
Structure Cred
Imports System.Linq
Module StartupModule
Private ReadOnly _credentials As System.Collections.Generic.IEnumerable(Of Cred) = New Cred() {
New Cred("userone", "passwordone"),
New Cred("usertwo", "passwordtwo"),
New Cred("userthree", "passwordthree", "/admin/custom.aspx")}
Sub Main()
Dim userName As String = ""
Dim password As String = ""
Dim crd = _credentials.Where(Function(x) x.Username = userName AndAlso x.Password = password).SingleOrDefault
If crd.Equals(New Cred) Then
Console.WriteLine("user is nothing")
Else
Console.WriteLine("user is something")
End If
Console.ReadLine()
End Sub
Private Structure Cred
Public Username As String
Public Password As String
Public RedirectUrl As String
Public Sub New(un As String, pw As String, Optional ru As String = "/admin/default.aspx")
Username = un
Password = pw
RedirectUrl = ru
End Sub
End Structure
End Module