解决方法 见 MVVM Light 5 issue：
WPF is the only XAML framework that uses the CommandManager to
automagically raise the CanExecuteChanged event on ICommands. I never
liked that approach,because of the “magic” part,but this is a
“feature” of WPF and of course I have to support it. No question here.
In V5,I moved to portable class library for all the newest versions
of the XAML frameworks,including WPF4.5. Unfortunately,there is no
CommandManager in PCL,and I have to admit that I didn’t realize that
at first sight. So of course Now the automagical part doesn’t work
anymore. Again,so sorry about that.
I am not expecting you to raise CanExecuteChanged everywhere Now,not
after using the CommandManager in your application,which is what the
WPF team intended. So I will try to find a way to restore the
CommandManager usage in the WPF4.5 version of the toolkit.
Definitely not looking for excuses