Why won't my RelayCommand fire and ObservableCollection catch selected value?

Question

I am new to using RelayCommands (following Josh Smith's MVVMDemoApp) and could use some help identifying my mistake.

I have two listboxes. When an item in the first is selected and the "Add" button is pressed, the AddCommand is executed and the second list's ObservableCollection gets the selectedItem added to it.

My View:

<DockPanel >

    <Border DockPanel.Dock="Bottom" Height="50" HorizontalAlignment="Left" Width="150" >
        <!--Notice here that the Button was disabled until it was given a DataContext, which allowed the CanAddPN to be true-->
        <Button Command="{Binding Path=AddToPartsBinCommand}" Content="Add >" />
    </Border>

    <UniformGrid Columns="2" Rows="1" DockPanel.Dock="Top" >
        <!--ListBox 1 (PartNumbersCollection)-->
        <ListBox Background="PaleGoldenrod"
                 ItemsSource="{Binding PnsCollection, Mode=OneWay, UpdateSourceTrigger=PropertyChanged}"
                 SelectedItem="{Binding SelectedPartNumber, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding pn}">

                    </TextBlock>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>

        <!--ListBox 2 (SelectedPartNumbersCollection)-->
        <ListBox ItemsSource="{Binding PartsBinCollection, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding pn}">

                    </TextBlock>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
    </UniformGrid>


</DockPanel>

My ViewModel:

 //DummyDBEntities _context;
    public ObservableCollection<PartNumber> _pnsCollection;
    public ObservableCollection<PartNumber> _partsBinCollection;

    PartNumber _selectedPN;
    public ICommand _addToPartsBinCommand;



    public MasterViewModel(DummyDBEntities _context)
    {
        _context = new DummyDBEntities();
        this._pnsCollection = new ObservableCollection<PartNumber>(_context.PartNumbers);
        this._partsBinCollection = new ObservableCollection<PartNumber>();



    }




    public ObservableCollection<PartNumber> PnsCollection
    {
        get { return this._pnsCollection; }
        set
        {
            _pnsCollection = value;
            OnPropertyChanged("PnsCollection");
        }
    }

    public PartNumber SelectedPN
    {
        get { return this._selectedPN; }
        set 
        {
            this._selectedPN = value;
            OnPropertyChanged("SelectedPN");
            OnPropertyChanged("PartsBinCollection");
        }

    }

    public ObservableCollection<PartNumber> PartsBinCollection
    {
        get
        {
            if (_partsBinCollection == null)
            {
                _partsBinCollection = new ObservableCollection<PartNumber>();
            }
            return this._partsBinCollection;
        }
        set 
        {
           this._partsBinCollection = value;
            OnPropertyChanged("PartsBinCollection");
        }
    }


    public ICommand AddToPartsBinCommand
    {
        get
        {
            if (_addToPartsBinCommand == null)
                _addToPartsBinCommand = new RelayCommand(() => this.AddPN(),
                                                         () => this.CanAddPN());
            return this._addToPartsBinCommand;
        } 
    }

    private bool CanAddPN()
    {
        return true;
    }


    private void AddPN()
    {
        if (this._partsBinCollection == null)
        {
            this._partsBinCollection = new ObservableCollection<PartNumber>();
        }

        this._partsBinCollection.Add(this._selectedPN);

    }

Thanks in advance!

Also: why would:

   private bool CanAddPN()
    {
        return this._selectedPN != null;
    }

leave my Add button permanently disabled? What am I not doing to let the button know that an item has been selected? This seem like it is the same missing link to my understanding of why the command isn't firing ever.

Thanks again!

Answer 1

OOPS! Right after posting this after an hour of struggling I realized that in my View I was referring to the selectedItem property "SelectedPartNumber" and not "SelectedPN". This solved both problems. CanExecuteChanged is evaluated already.

Answer 2

You need to raise the CanExecuteChanged on your command to let the client know that it should check again to see if it can execute. Not sure about the RelayCommand but I would assume it's something along the lines of mycommand.RaiseCanExecuteChanged(); Don't forget to cast your command to a relaycommand first since you have it exposed as ICommand.