MvvmLight ToolKit .Net4.5版本 CanExecute不能刷新界面bug

　　一问题重现

　 1.在使用最新版本v5.1的MvvmLight中（其实这个问题很早就有了），发现CanExecute不能很好地工作了。一个简单的工程，只有MainWindow和MainWindowViewModel,代码如下

MainWindow.xaml

<Window x:Class="MvvmLightError.MainWindow"

        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

        Title="MainWindow" Height="350" Width="525">

    <Grid>

        <Button Width="100" Height="30" Content="OK" Command="{Binding OkCommand}"></Button>

        <Button Width="100" Height="30" Margin="0,100,0,0" Content="ChangeEnable" Command="{Binding ChangeEnableCommand}"></Button>

    </Grid>

</Window>

MainWindow.xaml.cs

public partial class MainWindow : Window

    {

        public MainWindow()

        {

            InitializeComponent();

            this.DataContext = new MainWindowViewModel();

        }

    }

MainWindowViewModel.cs

public class MainWindowViewModel : ViewModelBase

    {

        private bool _isEnabled;

        public RelayCommand OkCommand

        {

            get;

            set;

        }

        public RelayCommand ChangeEnableCommand

        {

            get;

            set;

        }

        public MainWindowViewModel()

        {

            OkCommand = new RelayCommand(OkCommandHanlder, CanOkCommandExecute);

            ChangeEnableCommand = new RelayCommand(ChangeEnableCommandHandler);

        }

        private void OkCommandHanlder()

        {

        }

        private bool CanOkCommandExecute()

        {

            return _isEnabled;

        }

        private void ChangeEnableCommandHandler()

        {

            _isEnabled = !_isEnabled;

        }

    }