WPF Application developers would be experiencing or would have observed that after updating MVVMLight Toolkit package to Version 5.0.0 or 5.0.1, RelayCommand - CanExecute() mechanism is not working anymore. This issue is been already reported by MVVMLight user on codeplex as Issue# 7659. As explained by Laurent: WPF is the only XAML framework that uses the CommandManager to automagically raise the CanExecuteChanged event on ICommands. Unfortunately, there is no CommandManager in PCL (portable class library). CanExecute() is an input parameter of type action, it can be a method that returns bool based on which binding updates to allow or block command execution. This is to be passed when instantiating RelayCommand property that will bind with your control's Command property and fire the default event based on user action. A hot fix is released for above mentioned issue as Version 5.0.2 and now available on » Read more