WPF ComboBox null value

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VB

WPF

Hi,
please have a look to this XAML:

<ComboBox Grid.Row="1" Grid.Column="3" Name="cmbNazionalitaProp" Width="230" Margin="0,5,20,2"
ItemsSource="{Binding}" IsSynchronizedWithCurrentItem="True" SelectedValuePath = "codice"
DisplayMemberPath="descrizione" SelectedValue="{Binding Path=nazionalitam,Mode=TwoWay,UpdateSourceTrigger=LostFocus}"/>

and to the property definition in my class:

Private _nazionalitam As Nullable(Of Int32)
Public Property nazionalitam() As Nullable(Of Int32)
    Get
        Return _nazionalitam
    End Get
    Set(ByVal value As Nullable(Of Int32))
        _nazionalitam = value
        NotifyPropertyChange("nazionalitam")
    End Set
End Property

I don't know why the _nazionalitam return value is equal to zero even if the value in the database field is NULL.

Anybody can help me?
Thank you very much
Gae

---

Public Function getAllNazionalita() As ObservableCollection(Of Nazionalita)
    Dim lNazionalita As New ObservableCollection(Of Nazionalita)
    Dim Nazionalita As Nazionalita
    'lNazionalita.Add(Nothing)
    Try
        tempDataReader = SelectData("SELECT * FROM TGESI_NAZIONALITA WHERE COD_ENTE = '" & codEnte & "' ORDER BY CODICE")
        If tempDataReader.HasRows Then
            Do While tempDataReader.Read
                Nazionalita = New Nazionalita(codEnte,
                                    formattaValore(tempDataReader("CODICE")),
                                    formattaValore(tempDataReader("DESCRIZIONE")),
                                    formattaValore(tempDataReader("SIGLA")))
                lNazionalita.Add(Nazionalita)
            Loop
            tempDataReader.Close()
        End If
    Catch ex As Exception
        MsgBox("Errore nella lettura della lista delle Tratte" & vbCrLf & ex.Message)
        tempDataReader.Close()
        lNazionalita = Nothing
    End Try
    Return lNazionalita
End Function

and in the Load event

Dim NazPropMot As ObservableCollection(Of Nazionalita)

NazPropMot = New ObservableCollection(Of Nazionalita)
NazPropMot = VeiManager.getAllNazionalita
cmbNazionalitaProp.ItemsSource = NazPropMot


Thank you
Gae