NullReferenceException after long period of uptime - asp.net

Question

I've got an ASP.Net application, which starts throwing NUllReferenceExceptions, after a long period of running. The code in question is used early on in each individual session, where we're trying to establish some kind of referrer information. The thing is, I can't fathom out what can throw this exception.
The method in question (topmost in the stack trace) is:
Private Function ResolveReferrer(ByVal wrRequest As HttpRequest) As Referral
'1) If we don't find a domain, try and get a match on any query strings
If wrRequest.QueryString.Count > 0 Then
For Each item As Referral In Me
For Each sKey As String In wrRequest.QueryString.Keys
If Not sKey Is Nothing AndAlso item.Names.Contains(sKey.ToLower) Then
Return item
End If
Next sKey
Next item
End If
Dim strSubDomain As String = Utility.RequestSubDomain(wrRequest.Url)
'2) If we don't find one on the domain, see if we can find the domain in query string
If Not wrRequest.QueryString.Item("domain") Is Nothing Then
strSubDomain = wrRequest.QueryString.Item("domain")
strSubDomain = HttpUtility.UrlDecode(strSubDomain)
' OK found a "domain" query string, so make up a referrer object to return
' ... just use the domain we've found for all the parameters
Dim oRef As New Referral(strSubDomain, strSubDomain, strSubDomain)
Return oref
End If
'3) If no query string of "domain", then see if the referring field is presented by the browser
If Not wrRequest.UrlReferrer Is Nothing Then
Dim sURL As String = wrRequest.UrlReferrer.ToString
strSubDomain = Utility.RequestSubDomain(wrRequest.UrlReferrer)
Dim oRef As New Referral(sURL, sURL, strSubDomain)
Return oRef
End If
'4) See if we can find the domain defined in the web.config
For Each item As Referral In Me
' See if we can find a referrer from the domain name
If String.Compare(strSubDomain, item.FromDomain, False) = 0 Then
Return item
End If
Next item
'5) If we still can't find one, make one up with a value of "Unknown"
Return New Referral("Unknown", "Unknown", "Unknown", "Unknown")
End Function
The class that this is a part of inherits from ArrayList. I've checked, and the only things added to this ArrayList are instances of the Referral class (which has multiple constructors, all simple).
What we do know is that, we can have a request that comes in with no referrer information, and it causes the exception to be thrown. At the same time, if a request with a referrer comes in, it works fine. In neither case is anything passed in the query string (so I think you can skip down to the '3 comment.
So, my question is, what in this method can cause a NullReferenceException to be thrown? If you need additional snippets added, or class definitions, just shout.
Utility.RequestSubDomain has reasonable complexity, so I doubt that it's being inlined and removed from the stack trace. And it's top is:
Public Shared Function RequestSubDomain(ByVal uri As System.Uri) As String
If uri Is Nothing Then
Return ""
End If
Any help, or suggestions for finding more information would be appreciated. It's obviously (as with so many problems) only happening in production, so I don't want to switch on debugging.

Answer 1

I took a good hard look and the only two things that come to mind that seem like the most likely are:
wrRequest could be null.
The ArrayList could have null values in it resulting in an enumerator that returns null values.
If it were me I would first focus my attention on this section. Is there any way possible that the Me.GetEnumerator will return an enumerator with one of the items having a null value?
For Each item As Referral In Me
item.Names ' Can item be null here causing the exception on the getter of Names?
Next item

Answer 2

As it turned out, there was a NULL in the arraylist - it turns out that, in certain circumstances, multiple threads were working on the same object (which inherits from arraylist) and calling Add(). So the null was appearing because the internal index was being incremented twice by different threads.