所有ContextMenu Menuitem命令都在selectionchange上触发Command.CanExecute（）

Question

我有一个示例应用，其视图为：

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local ="clr-namespace:WpfApplication1.Commands"
    Title="MainWindow" Height="350" Width="525">
<Window.Resources>
    <ContextMenu x:Key="Menu">
            <MenuItem Header="M" Command="{Binding MyCommand}" CommandParameter="{Binding}"/>
    </ContextMenu>
</Window.Resources>
<DataGrid x:Name="maingrid" ItemsSource="{Binding Items}" SelectedItem="{Binding SelectedItem}" AutoGenerateColumns="True" ContextMenu="{StaticResource Menu}"/>

我的cs代码是这样的：

namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        VM vm = new VM();
        public MainWindow()
        {
            vm.Items = new BindingList<Entity>();
            vm.MyCommand = new MyCommand();
            InitializeComponent();
            for (int i = 1; i < 1000; i++)
            {
                vm.Items.Add(new Entity() { X = i });
            }
            base.DataContext = vm;
            Focus();
        }
    }
    public class VM
    {
        public BindingList<Entity> Items { get; set; }
        public Entity SelectedItem { get; set; }
        public ICommand MyCommand { get; set; }
    }
    public class MyCommand : ICommand
    {
        public bool CanExecute(object parameter)
        {
            Debug.WriteLine("CanExecute Called!!!");
            if (parameter != null && (parameter as VM).SelectedItem.X > 20)
            {

                return true;
            }
            else
            {
                return false; 
            }
        }
        public event EventHandler CanExecuteChanged
        {
            add
            {
                CommandManager.RequerySuggested += value;
            }

            remove
            {
                CommandManager.RequerySuggested -= value;
            }
        }
        public void Execute(object parameter)
        {
            throw new NotImplementedException();
        }
    }
    public class Entity
    {
        public int X { get; set; }
        public int X2 { get; set; }
        public int X3 { get; set; }
        public int X4 { get; set; }
        public int X5 { get; set; }
        public int X6 { get; set; }
    }
}

现在，如果您启动应用程序并打开“输出”窗口并键入更改选定的行，则不会 “CanExecute Called !!!”输出。这是预期的行为，因为我希望只有当用户右键单击该行时才会触发命令。

现在右键单击一行，然后看“CanExecute Called !!!”出现在输出窗口中。

现在继续上下移动选择，你会不断看到“CanExecute Called !!!”消息来输出。

我希望命令仅在右键单击时触发。