gistfile1.cs

public interface IRaiseCanExecuteChanged
{
    void RaiseCanExecuteChanged();
}

// And an extension method to make it easy to raise changed events
public static class CommandExtensions
{
    public static void RaiseCanExecuteChanged(this ICommand command)
    {
        var canExecuteChanged = command as IRaiseCanExecuteChanged;

        if (canExecuteChanged != null)
            canExecuteChanged.RaiseCanExecuteChanged();
    }
}