使用ICommandSource将命令挂接到CustomDataGridColumn

我对自定义控件很陌生，请原谅，但我正在尝试将命令挂接到DataGridComboBoxColumn的SelectionChangedEvent。除了我的命令为空之外，一切都在工作。我有点不明白为什么它仍然是空的。你能告诉我为什么我仍然是空的吗？为什么？

   public class CustomComboBoxColumn : DataGridComboBoxColumn, ICommandSource
{
    private bool _canExecute;
    static CustomComboBoxColumn()
    {
    }
    private EventHandler canExecuteChagnedEventHandler;
    #region Dependancy Properties
    /// <summary>
    /// Allows the combobox to be editable
    /// </summary>
    public static readonly DependencyProperty IsEditableProperty = DependencyProperty.Register(
        "IsEditable", typeof(bool), typeof(CustomComboBoxColumn), new PropertyMetadata(default(bool)));
    public bool IsEditable
    {
        get { return (bool)GetValue(IsEditableProperty); }
        set { SetValue(IsEditableProperty, value); }
    }

    /// <summary>
    /// Command Property
    /// </summary>
    public static readonly DependencyProperty CommandProperty = DependencyProperty.Register(
        "Command", typeof(ICommand), typeof(CustomComboBoxColumn),
        new PropertyMetadata((ICommand)null, new PropertyChangedCallback(OnCommandChanged)));
    [TypeConverter(typeof(CommandConverter))]
    public ICommand Command
    {
        get { return (ICommand)GetValue(CommandProperty); }
        set { SetValue(CommandProperty, value); }
    }

    /// <summary>
    /// Command Parameter
    /// </summary>
    public static readonly DependencyProperty CommandParameterProperty = DependencyProperty.Register(
        "CommandParameter", typeof(object), typeof(CustomComboBoxColumn), new PropertyMetadata(default(object)));
    public object CommandParameter
    {
        get { return (object)GetValue(CommandParameterProperty); }
        set { SetValue(CommandParameterProperty, value); }
    }

    /// <summary>
    /// Command Target
    /// </summary>
    public static readonly DependencyProperty CommandTargetProperty = DependencyProperty.Register(
        "CommandTarget", typeof(IInputElement), typeof(CustomComboBoxColumn),
        new PropertyMetadata(default(IInputElement)));
    public IInputElement CommandTarget
    {
        get { return (IInputElement)GetValue(CommandTargetProperty); }
        set { SetValue(CommandTargetProperty, value); }
    }
    #endregion
    #region Properties
    private ComboBox _comboBox;
    public ComboBox ComboBox
    {
        get { return _comboBox; }
        set
        {
            if (_comboBox != null)
                _comboBox.SelectionChanged -= OnSelectionChanged;
            _comboBox = value;
            if (_comboBox != null)
                _comboBox.SelectionChanged += OnSelectionChanged;
        }
    }
    #endregion
    protected override object PrepareCellForEdit(FrameworkElement editingElement, RoutedEventArgs editingEventArgs)
    {
        var comboBox = editingElement as ComboBox;
        if (comboBox != null)
        {
            ComboBox = comboBox;
            comboBox.IsEditable = IsEditable;
        }
        return base.PrepareCellForEdit(editingElement, editingEventArgs);
    }

    private void OnSelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        FireCommand();
    }
    private void FireCommand()
    {
        if (Command != null)
        {
            Command.Execute(CommandParameter);
        }
    }
    /// <summary>
    /// Calls OnCommandChanged to hook up new commands
    /// </summary>
    /// <param name="d"></param>
    /// <param name="e"></param>
    private static void OnCommandChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        CustomComboBoxColumn control = d as CustomComboBoxColumn;
        if (control != null)
            control.OnCommandChanged((ICommand)e.OldValue, (ICommand)e.NewValue);
    }
    /// <summary>
    /// Calls UnhookCommand and HookCommand
    /// </summary>
    /// <param name="oldCommand"></param>
    /// <param name="newCommand"></param>
    protected virtual void OnCommandChanged(ICommand oldCommand, ICommand newCommand)
    {
        if (oldCommand != null)
            UnHookCommand(oldCommand, newCommand);
        HookCommand(oldCommand, newCommand);
    }
    /// <summary>
    /// Attaches CanExecuteChanged EventHandler
    /// </summary>
    /// <param name="oldCommand"></param>
    /// <param name="newCommand"></param>
    private void HookCommand(ICommand oldCommand, ICommand newCommand)
    {
        EventHandler handler = CanExecuteChagned;
        canExecuteChagnedEventHandler = handler;
        if (newCommand != null)
            newCommand.CanExecuteChanged += canExecuteChagnedEventHandler;
    }
    /// <summary>
    /// Detaches CanExecuteChagned EventHandler
    /// </summary>
    /// <param name="oldCommand"></param>
    /// <param name="newCommand"></param>
    private void UnHookCommand(ICommand oldCommand, ICommand newCommand)
    {
        EventHandler handler = CanExecuteChagned;
        oldCommand.CanExecuteChanged -= CanExecuteChagned;
    }

    /// <summary>
    /// Sets CanExecute
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    private void CanExecuteChagned(object sender, EventArgs e)
    {
        if (Command != null)
        {
            RoutedCommand rc = Command as RoutedCommand;
            if (rc != null)
            {
                _canExecute = rc.CanExecute(CommandParameter, CommandTarget) ? true : false;
            }
            else
            {
                _canExecute = Command.CanExecute(CommandParameter) ? true : false;
            }
        }
    }
}

ViewModel:

public ViewModel()
{
ChangeDescriptionCommand = new DelegateCommand(UpdateDescription);
}
public DelegateCommand ChangeDescriptionCommand { get; set; }
//..other methods

xaml:

<icc:CustomComboBoxColumn Header="Item Number"
                                          DisplayMemberPath="Code"
                                          SelectedValuePath="Id"
                                          x:Name="ItemColumn"
                                          SelectedValueBinding="{Binding PartId}"
                                          Command="{Binding ChangeDescriptionCommand}">
                    <icc:CustomComboBoxColumn.ElementStyle>
                                <Style TargetType="{x:Type ComboBox}">
                                    <Setter Property="ItemsSource" Value="{Binding Path=DataContext.Parts,
                                        RelativeSource={RelativeSource AncestorType={x:Type UserControl}}}"/>
                                </Style>
                            </icc:CustomComboBoxColumn.ElementStyle>
                            <icc:CustomComboBoxColumn.EditingElementStyle>
                                <Style TargetType="{x:Type ComboBox}">
                                    <Setter Property="ItemsSource" Value="{Binding Path=DataContext.Parts,
                                        RelativeSource={RelativeSource AncestorType={x:Type UserControl}}}"/>
                                </Style>
                            </icc:CustomComboBoxColumn.EditingElementStyle>
                        </icc:CustomComboBoxColumn>

更新因此，我发现由于某种原因，命令没有初始化。

更新2我创建了一个全新的控件，引用了How To Implementation ICommandSource一步一步地进行，它是链接中的滑块控件。我复制粘贴到新的类中，将Slider更改为DataGridComboBoxColumn，并且我控制着同样的问题。null命令。