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 IEnumerable(Of Cred) = New () {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