Using LastIndexOf w/ Structure Stored in ArrayList

[ccshine]

I'm working on an app that implements a Structure to store a recordset in an
ArrayList. I used this setup to bind to a DataGrid and it worked out so well,
I thought it might be a better solution that moving back and forth through a
DataSet to perform some simple though numerous calculations.

In order to perform these calcs, I have to compare elements of the array
checking for duplicates on some columns and then just comparing others to
strings utilizing IndexOf and LastIndexOf. Unfortunately I haven't been able
to get it to work. On another site I read that these 2 methods use Object.
Equals and as the array contains structure objects regardless of whether the
values appear to be equal or not, both methods will always return -1. It
also stated that the Equals function needs to be overridden. I tried this
as well with no luck.

The code below generates an index is out of range error on the line that
assigns the array item to 'last' because the LastIndexOf search returns -1.
Also noteworthy is that the stored procedure I use to retrieve the recordset
guarantees that there would be a match on first.account. Can anyone point me
in the right direction?

Public Overrides Function Equals(ByVal obj As Object) As Boolean
  If TypeOf obj Is duplicates Then
    If account = CType(obj, duplicates).account Then
      Return True
    Else
      Return False
    End If
  End If
End Function
 
Public Overloads Function Equals(ByVal obj1 As Object, ByVal obj2 As Object) As Boolean
  If TypeOf obj1 Is duplicates AndAlso TypeOf obj2 Is duplicates Then
    If CType(obj1, duplicates).account = CType(obj2, duplicates).account Then
      Return True
    Else
      Return False
    End If
  End If
End Function
 
Dim arrResults As New ArrayList 
Dim dupEntries As New duplicates 
 
While reader.Read 
  With arrResults 
    .Add(New duplicates(reader.GetValue(0), reader.GetValue(1), reader.GetValue(2), reader.GetValue(3), reader.GetValue(4), reader.GetValue(5), reader.GetValue(6), reader.GetValue(7), reader.GetValue(8), reader.GetValue(9), reader.GetValue(10), reader.GetValue(11), reader.GetValue(12), reader.GetValue(13), reader.GetValue(14)))
  End With 
End While 
reader.Close()
 
Dim first As duplicates 
Dim last As duplicates 
Dim i As Integer 
 
For i = 0 To arrResults.Count - 1 
  first = CType(arrResults.Item(i), duplicates) 
  Trace.Warn(arrResults.LastIndexOf(first.account))
  last = arrResults.Item(arrResults.LastIndexOf(first.account))
  Trace.Warn("first date" & first.tDate) 
  Trace.Warn("last date " & last.tDate) 
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