ICommand CanExecuteChanged not updating - wpf

Question

I am trying for MVVM pattern basic level and got struck at ICommand CanExecute changed. I have XAML binding as follows:
<ListBox ItemsSource="{Binding Contact.Addresses}" x:Name="AddressCollections" Height="152" SelectedValue="{Binding SelectedAddress}"
DockPanel.Dock="Top" />
<Button Content="Add" Command="{Binding AddAddressCommand}" DockPanel.Dock="Top" />
<Button Content="Remove" Command="{Binding DeleteAddressCommand}" DockPanel.Dock="Bottom" />
Commands:
Public Class DeleteCommand
Implements ICommand
Private method As Object
Private methodname As String
Public Sub New(ByVal Controlname As String, ByVal mee As Object)
methodname = Controlname
method = mee
End Sub
Public Function CanExecute(parameter As Object) As Boolean Implements ICommand.CanExecute
Select Case methodname
Case "Address"
Return TryCast(method, ModelView.Contacts.ContactMV).CanDeleteAddress()
Case "Numbers"
Return TryCast(method, ModelView.Contacts.ContactMV).CanDeleteNumbers
Case Else : Return False
End Select
End Function
Public Event CanExecuteChanged(sender As Object, e As EventArgs) Implements ICommand.CanExecuteChanged
Public Sub Execute(parameter As Object) Implements ICommand.Execute
Select Case methodname
Case "Address"
TryCast(method, ModelView.Contacts.ContactMV).DeleteAddress()
Case "Numbers"
TryCast(method, ModelView.Contacts.ContactMV).DeleteNumbers()
Case Else
End Select
End Sub
End Class
My ModelView:
Public Class ContactMV
Property Contact As Model.Contacts.ContactMod
Property AddAddressCommand As New Commands.AddCommand("Address", Me)
Property DeleteAddressCommand As New Commands.DeleteCommand("Address", Me)
Property SelectedAddress As Model.Contacts.AddressModel
Public Sub AddAddress()
If Contact.Addresses.Count = 0 Then
Contact.Addresses.Add(New Model.Contacts.AddressModel(Contact.Primary.ID, True))
Else
Contact.Addresses.Add(New Model.Contacts.AddressModel(Contact.Primary.ID, False))
End If
End Sub
Public Sub DeleteAddress()
If IsNothing(SelectedAddress) = False Then
Try
Contact.Addresses.Remove(SelectedAddress)
Catch ex As Exception
MsgBox("Address not found")
End Try
End If
End Sub
Public Function CanDeleteAddress()
If IsNothing(SelectedAddress) Then
Return False
Else
Return Contact.Addresses.Contains(SelectedAddress)
End If
End Function
End Class
The problem is that the Canexecutechanged is firing only at start, I actually want to get the delete button enabled only when something in the listbox is selected, and I want to get it done by MVVM - ICommand binding method. Could you please explain where i went wrong or miss understood the ICommand implementation.
Thank you.
Updated Relay iCommand code I use:
Public Class RelayCommand
Implements ICommand
''' <summary>
''' A command whose sole purpose is to relay its functionality to other objects by invoking delegates. The default return value for the CanExecute method is 'true'.
''' </summary>
''' <remarks></remarks>
#Region "Declarations"
Private ReadOnly _CanExecute As Func(Of Boolean)
Private ReadOnly _Execute As Action
#End Region
#Region "Constructors"
Public Sub New(ByVal execute As Action)
Me.New(execute, Nothing)
End Sub
Public Sub New(ByVal execute As Action, ByVal canExecute As Func(Of Boolean))
If execute Is Nothing Then
Throw New ArgumentNullException("execute")
End If
_Execute = execute
_CanExecute = canExecute
End Sub
#End Region
#Region "ICommand"
Public Custom Event CanExecuteChanged As EventHandler Implements System.Windows.Input.ICommand.CanExecuteChanged
AddHandler(ByVal value As EventHandler)
If _CanExecute IsNot Nothing Then
AddHandler CommandManager.RequerySuggested, value
End If
End AddHandler
RemoveHandler(ByVal value As EventHandler)
If _CanExecute IsNot Nothing Then
RemoveHandler CommandManager.RequerySuggested, value
End If
End RemoveHandler
RaiseEvent(ByVal sender As Object, ByVal e As System.EventArgs)
'This is the RaiseEvent block
'CommandManager.InvalidateRequerySuggested()
End RaiseEvent
End Event
Public Function CanExecute(ByVal parameter As Object) As Boolean Implements System.Windows.Input.ICommand.CanExecute
If _CanExecute Is Nothing Then
Return True
Else
Return _CanExecute.Invoke
End If
End Function
Public Sub Execute(ByVal parameter As Object) Implements System.Windows.Input.ICommand.Execute
_Execute.Invoke()
End Sub
#End Region
End Class
Most of the code is a copy, but I understood the working by below comments.

Answer 1

As Raul Otaño has pointed out, you can raise the CanExecuteChanged. However, not all MVVM frameworks provide a RaiseCanExecuteChanged method. It's also worth noting that the actual event CanExecuteChanged must be called on the UI thread. So, if you're expecting a callback from some thread in your model, you need to invoke it back to the UI thread, like this:
public void RaiseCanExecuteChanged()
{
if (CanExecuteChanged != null)
{
Application.Current.Dispatcher.Invoke((Action)(() => { CanExecuteChanged(this, EventArgs.Empty); }));
}
}
I would very much recommend against calling CommandManager.InvalidateRequerySuggested() because although this works functionally, and is ok for small applications, it is indiscriminate and will potentially re-query every command! In a large system with lots of commands, this can be very very slow!

Answer 2

You must have in your ICommand implementation some method like RaiseCanExecuteChanged that fires the event CanExecuteChanged. Then every time that the selected item in the list changed, in your view model you execute the RaiseCanExecuteChanged from the command you want. Any way I suggest you to use any generic command, like the RelayCommand of GalaSoft MVVMLite library, or any implementation of DelegateCommand.
Hope this helps...