Logic Error

[Nate Bross]

I've got a collection of 'SearchResults' that I convert into an ArrayList of custom classes, because the collection has a large amount of data I do not care about.

 

Custom Class Deffinition

Public Class Item
   Dim sTitle As String
   Dim dPrice As Double
   Dim dBuyIt As Double
   Dim sItemID As String
   Dim sURL As String
   Dim dStartDate As Date

   Public Sub New(ByVal sT As String, ByVal dP As Double, ByVal dB As Double, ByVal sID As String, ByVal URL As String, ByVal dSD As Date)
       sTitle = sT
       dPrice = dP
       dBuyIt = dB
       sItemID = sID
       sURL = URL
       dStartDate = dSD
   End Sub

   Public Property Title() As String
       Get
           Return sTitle
       End Get
       Set(ByVal value As String)
           sTitle = value
       End Set
   End Property

   Public Property Price() As Double
       Get
           Return dPrice
       End Get
       Set(ByVal value As Double)
           dPrice = value
       End Set
   End Property

   Public Property BuyItNowPrice() As Double
       Get
           Return dBuyIt
       End Get
       Set(ByVal value As Double)
           dBuyIt = value
       End Set
   End Property

   Public Property ItemID() As String
       Get
           Return sItemID
       End Get
       Set(ByVal value As String)
           sItemID = value
       End Set
   End Property

   Public Property URL() As String
       Get
           Return sURL
       End Get
       Set(ByVal value As String)
           sURL = value
       End Set
   End Property

   Public Property StartDate() As Date
       Get
           Return dStartDate
       End Get
       Set(ByVal value As Date)
           dStartDate = value
       End Set
   End Property
End Class

 

I get a new collection every few seconds (usually containing no new data), but every now and then there are new items in the collection. When this happens I want to notify the user and tell them what is new since last time.

 

Here's how I am figuring out if a new item has appeared.

       For Each result As SearchResultItemType In results
               tbString.AppendLine(result.Item.ItemID & vbTab & result.Item.SellingStatus.CurrentPrice.Value() & vbTab & result.Item.ListingDetails.StartTime)

               Dim tItem As New Item(result.Item.Title.ToString(), result.Item.SellingStatus.CurrentPrice.Value(), 0, result.Item.ItemID(), result.Item.ListingDetails.ViewItemURL.ToString(), result.Item.ListingDetails.StartTime())
               'BuyItNowPrice returns nothing not 0 if no value is specified
               If result.Item.BuyItNowPrice IsNot Nothing Then
                   tItem.BuyItNowPrice = result.Item.BuyItNowPrice.Value()
               End If

               If OldItems.IndexOf(tItem) > 0 Then
                   'The Item is In the Array
                   Dim x As Long = 0
               Else
                   UnViewed.Add(tItem)
                   OldItems.Add(tItem)
               End If
           Next

 

What happens is that every item shows up in my UnViewed ArrayList.

 

I know there's alot of stuff here, but if you need more information let me know and I'll provide what I can.

[PlausiblyDamp]

IIRC .IndexOf calls the .Equals method of the class to compare them and decide if they are the ame or not - for reference types this simply checks if the object is in the same memory location and doesn't compare the values themselves.

To make this work you will need to override the Equals method in your Item class to perform a compare of two Item objects.

[Cags]

I'll explain what I think is happening, perhaps somebody else will have a better idea, but here goes. Obiviously the item is always being added to the UnViewed collection because IndexOf is always returning a zero. Now I believe the IndexOf method uses Object.Equals to compare your search object to the collection and I think this compares references not values. Whilst the item you are searching for contains the same values it is not actually the same object.

 

Heres a simplified example...

Public Class TestClass
   Dim strName As String

   Public Sub New(ByVal num As Single)
       strName = "Name" & num.ToString()
   End Sub

   Public Property Name() As String
       Get
           Return strName
       End Get
       Set(ByVal value As String)
           strName = value
       End Set
   End Property
End Class

Public Sub Test()
       Dim oldItems As New ArrayList

       For i As Single = 0 To i < 10
           Dim tmpClass As TestClass = New TestClass(i)
           oldItems.Add(tmpClass)
       Next

       ' create an item with same name and compare that
       Dim searchClass As TestClass = New TestClass(2)

       If (oldItems.IndexOf(searchClass) > 0) Then
           MessageBox.Show("Item Found")
       Else
           MessageBox.Show("Item Not Found")
       End If

       ' prove that its not a boxing error
       For i As Single = 0 To i < 10
           Dim tmpClass As TestClass = CType(oldItems(i), TestClass)
           If Object.Equals(tmpClass, searchClass) Then
               Console.WriteLine("Item Found")
           Else
               Console.WriteLine("Item not found")
           End If
       Next

       ' create reference and search for that
       searchClass = CType(oldItems(2), TestClass)

       If (oldItems.IndexOf(searchClass) > 0) Then
           MessageBox.Show("Item Found @ " + oldItems.IndexOf(searchClass))
       Else
           MessageBox.Show("Item Not Found")
       End If
End Sub

I hope I converted that correctly, I actually tested it in c#.

 

EDIT:- beaten to the punch....

[Nate Bross]

Thanks alot for the help, that's what I did not know. I'll let you know how the new Equals method I use works.

[Nate Bross]

Worked like a charm....

 

Added this method to the Item class

Public Override Equals(obj as Object)
   If obj.ItemID = me.ItemID Then
       Return True
   Else
       Return False
   End If
End Sub

 

Not a catch all solution, but it will work for my purposes.