为什么获胜'；t我的RelayCommand fire和ObservableCollection捕获选定值

我是使用RelayCommands的新手（遵循Josh Smith的MVVMDemoApp），可能需要一些帮助来识别我的错误。

我有两个列表框。当选择第一个列表中的一个项目并按下"添加"按钮时，执行AddCommand，第二个列表的ObservableCollection会将所选项目添加到其中。

我的观点：

<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>

我的视图模型：

 //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);
    }

提前感谢！

还有：为什么：

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

是否永久禁用"添加"按钮？我该怎么做才能让按钮知道某个项目已被选中？这似乎是我理解为什么指挥部从未开火的缺失环节。

再次感谢！