WPF/MVVM: Weird behaviour of wpf buttons with Commands disabling

Question

Hello,

just some code...: Question is at bottom.

XAML:

 <StackPanel Orientation="Horizontal">
            <Button Content="Start" Command="{Binding FirstDateCommand}" />
            <Button Content="Back" Command="{Binding PreviousDateCommand}" />
            <DatePicker SelectedDate="{Binding SelectedDate}" DisplayDateStart="{Binding MinDate}" DisplayDateEnd="{Binding MaxDate}" />
            <Button Content="Forward" Command="{Binding NextDateCommand}"  />
            <Button Content="End" Command="{Binding LastDateCommand}" />
        </StackPanel>

ViewModel:

public class LessonPlannerViewModel : ViewModelBase
    {    
        private ILessonPlannerRepository _lessonplannerRepo;    

        private ObservableCollection<LessonDay> _lessons;

        private RelayCommand _firstDateCommand;
        private RelayCommand _lastDateCommand;
        private RelayCommand _nextDateCommand;
        private RelayCommand _previousDateCommand;

        public LessonPlannerViewModel()
        {
            _lessonplannerRepo = new LessonPlannerRepository();

            MinDate = DateTime.Now.AddDays(-2);
            MaxDate = DateTime.Now.AddDays(2);

            SelectedDate = DateTime.Now;                   
        }

        public RelayCommand FirstDateCommand
        {
            get { return _firstDateCommand ?? (_firstDateCommand = new RelayCommand(() => MoveFirstDate(), () => CanMoveFirstDate())); }
        }

        public RelayCommand LastDateCommand
        {
            get { return _lastDateCommand ?? (_lastDateCommand = new RelayCommand(() => MoveLastDate(), () => CanMoveLastDate())); }
        }

        public RelayCommand PreviousDateCommand
        {
            get { return _previousDateCommand ?? (_previousDateCommand = new RelayCommand(() => MovePreviousDate(), () => CanMovePreviousDate())); }
        }

        public RelayCommand NextDateCommand
        {
            get { return _nextDateCommand ?? (_nextDateCommand = new RelayCommand(() => MoveNextDate(), () => CanMoveNextDate())); }
        }

        private void MoveFirstDate()
        {
            SelectedDate = MinDate;
            Lessons = _lessonplannerRepo.GetLessonDayByDate(SelectedDate);
        }

        private void MoveLastDate()
        {
            SelectedDate = MaxDate;
            Lessons = _lessonplannerRepo.GetLessonDayByDate(SelectedDate);
        }

        private void MoveNextDate()
        {
            SelectedDate = SelectedDate.AddDays(1);
            Lessons = _lessonplannerRepo.GetLessonDayByDate(SelectedDate);
        }

        private void MovePreviousDate()
        {
            SelectedDate = SelectedDate.AddDays(-1);
            Lessons = _lessonplannerRepo.GetLessonDayByDate(SelectedDate);
        }

        private bool CanMoveFirstDate()
        {
            return SelectedDate != MinDate;
        }

        private bool CanMoveLastDate()
        {
            return SelectedDate != MaxDate;
        }

        private bool CanMoveNextDate()
        {
            return SelectedDate < MaxDate;
        }

        private bool CanMovePreviousDate()
        {
            return SelectedDate > MinDate;
        }   

        private DateTime _selectedDate;
        public DateTime SelectedDate
        {
            get { return _selectedDate; }
            set
            {
                if (_selectedDate == value)
                    return;

                _selectedDate = value;
                this.RaisePropertyChanged("SelectedDate");
                //Lessons = _lessonplannerRepo.GetLessonDayByDate(SelectedDate);
            }
        }

        public DateTime MinDate { get; set; }

        public DateTime MaxDate { get; set; }        

        public ObservableCollection<LessonDay> Lessons
        {
            get { return _lessons; }
            set
            {
                _lessons = value;
                this.RaisePropertyChanged("Lessons");
            }
        }
...

When I choose in the DatePicker a date which is equal to MinDate then the PreviousDateCommand returns CanExecute = false; t*hats ok and works as expected*.

But why is the LastDateCommand not returning CanExecute = false too?

My CanExecute logic works as expected, when I press the PreviousDateButton instead of selecting the date via datepicker.

What do I wrong?

UPDATE:

I have not had any doubts that my logic is wrong but... I tried some things and with this code

this is really weird. I changed now the logic of the LastDate and PreviousDate CanExecute method and both buttons work now changing the datepicker.

private bool CanMoveFirstDate()
{
    Debug.WriteLine("SelectedDate FirstDate: " + SelectedDate);
    return SelectedDate > MinDate;
}

private bool CanMovePreviousDate()
{
    Debug.WriteLine("SelectedDate PreviousDate: " + SelectedDate);
    return SelectedDate > MinDate;
}

Is someone knows how to make the NextDate + LastDate button working gets the solution! :P

UPDATE 2:

Bindings are powerfull but maybe hard to control...

I did some crazy logic shit again and now it seems to work:

        private bool CanMoveNextDate()
        {
            Debug.WriteLine("SelectedDate NextDate: " + SelectedDate);
            return SelectedDate.AddDays(1) < MaxDate;
        }

        private bool CanMoveLastDate()
        {
            Debug.WriteLine("SelectedDate LastDate: " + SelectedDate);
            return SelectedDate.AddDays(1) < MaxDate;
        }  

If someone can explain that weird logic, that would be nice , I think the cause lays in the binding of the datepicker and the commands and which binding gets updated first or is called etc...

Answer 1

tststs... this is really a lesson to me:

Instead of putting this is the viewmodel constructor:

MinDate = DateTime.Now.AddDays(-2);
MaxDate = DateTime.Now.AddDays(2);

put this:

MinDate = DateTime.Parse("28.07.2010 00:00:00");
MaxDate = DateTime.Parse("01.08.2010 00:00:00");

because SelectedDate is always formatted like this:

dd.MM.yyyy 00:00:00

I want to say Microsoft thank you for their great debugging tools in VS 2010 =>

http://img833.imageshack.us/img833/5912/cryforariver.png

and I already cursed the wpf binding system :P a god damn user error now go and slap me I

deserve it! but the points are mine :P