Detect, whether an exception occured when using "ValidatesOnExceptions" - wpf

Question

Imagine a WPF project with an MVVM approach. So, there is a view and a view model.
In the view model I have a property, that might throw an exception in the setter.
Public Property DateValue As Nullable(Of Date)
Get
Return _dateValue
End Get
Set(value As Nullable(Of Date))
If value.HasValue Then
If value.Value < Date.Today Then
Throw New Exception("Error Message")
End If
End If
_dateValue = value
'skipped NotifyPropertyChanged in this example for the sake of simplicity
End Set
End Property
In the view there is a control bound to this property. And since I like to see my exceptions I switched on ValidatesOnExceptions in the binding and add an ErrorTemplate.
...
<DatePicker SelectedDate="{Binding DateValue, ValidatesOnExceptions=True}"
SelectedDateFormat="Short"
Validation.ErrorTemplate="{StaticResource ErrorTemplate}" />
...
Since you can't reset the value of a DatePicker once you picked one (at least I don't know how to do that) I added a small reset button right next to the DatePicker which is bound to a command which sets the property DateValue of the view model to Nothing. And since I don't want to see this button all the time I bound its Visibility to DateValue.HasValue, so this button only shows, when there's a value to reset.
So far, so good.
But now I have a problem when I pick an invalid date in the DatePicker (one that throws an exception in the properties' setter).
My reset button doesn't show up, since there's no value in the bound property, and I can't reset the DatePicker any other way (at least not that I know of). I'd first have to pick a proper date before I can reset the whole thing.
So, is there any way to determine, whether my property setter threw an exception. There must be a way, since this very error is shown to the user.
Or do I have to manually remember, that I threw an exception in another variable, to be able to access this information when needed?
And how do I "clear" the DatePicker. Since there's no value in the property, setting the property to Nothing will not change anything in the view. How would I get rid of the error?

Answer 1

Class ViewModel
Inherits INotifyPropertyChanged
Private selectedDate As DateTime
Public Property SelectedDate As DateTime
Get
Return Me.selectedDate
End Get
Set(ByVal value As DateTime)
If value < DateTime.Today Then
Throw New ArgumentException("Date can't be in the past.")
End If
Me.selectedDate = value
OnPropertyChanged()
End Set
End Property
Public ReadOnly Property ResetDateCommand As ICommand
Get
Return New RelayCommand(AddressOf ExecuteResetDate)
End Get
End Property
Public Sub New()
Me.SelectedDate = DateTime.Today
End Sub
Public Sub ExecuteResetDate(ByVal commandParameter As Object)
Return CSharpImpl.__Assign(Me.SelectedDate, DateTime.Today)
End Sub
End Class
MainWindow.xaml
<Window>
<Window.Resources>
<ViewModel />
</Window.Resources>
<StackPanel x:Name="RootPanel" viewModels:Item.IsMarkedAsRead="True">
<Button Content="Reset Date"
Visibility="{Binding ElementName=DatePicker, Path=(Validation.HasError), Converter={StaticResource BooleanToVisibilityConverter}}"
Command="{Binding ResetDateCommand}" />
<DatePicker x:Name="DatePicker"
SelectedDate="{Binding SelectedDate, ValidatesOnExceptions=True}"
SelectedDateFormat="Short" />
</StackPanel>
</Window>
Remarks
It's best practice in UI design that you always prevent wrong input. If possible you shouldn't allow invalid input e.g. by disabling buttons, hiding invalid options, limiting ranges etc.
Don't allow the user to select invalid options. This leads to a frustrating user experience. One way to prevent wrong input is to use specialized controls. Instead of forcing the user to type a date you offer a DatePicker. This eliminates typos. But it can also eliminate wrong selections by providing only valid dates.
If you want to disallow selecting dates of the past you can narrow down the selectable range:
<!-- Only show dates from today -->
<DatePicker DisplayDateStart="{x:Static system:DateTime.Today}" />
If you have additional illegal dates e.g. holidays you can define a collection of those dates by setting the DatePicker.BlackoutDates property:
DatePickerHelper.cs
Class DatePickerHelper
Inherits DependencyObject
Public Shared ReadOnly BlackedDaysProperty As DependencyProperty = DependencyProperty.RegisterAttached("BlackedDays", GetType(IEnumerable(Of CalendarDateRange)), GetType(DatePickerHelper), New PropertyMetadata(Nothing, AddressOf DatePickerHelper.OnBlackDatesChanged))
Public Shared Sub SetBlackedDays(ByVal attachingElement As DependencyObject, ByVal value As IEnumerable(Of CalendarDateRange))
Return attachingElement.SetValue(DatePickerHelper.BlackedDaysProperty, value)
End Sub
Public Shared Function GetBlackedDays(ByVal attachingElement As DependencyObject) As IEnumerable(Of CalendarDateRange)
Return CType(attachingElement.GetValue(DatePickerHelper.BlackedDaysProperty), IEnumerable(Of CalendarDateRange))
End Function
Private Shared ReadOnly Property DatePickerTable As Dictionary(Of INotifyCollectionChanged, DatePicker)
Private Shared Sub New()
DatePickerHelper.DatePickerTable = New Dictionary(Of INotifyCollectionChanged, DatePicker)()
End Sub
Private Shared Sub OnBlackDatesChanged(ByVal d As DependencyObject, ByVal e As DependencyPropertyChangedEventArgs)
Dim attachedDatePicker = TryCast(d, DatePicker)
Dim oldObservableCollection As INotifyCollectionChanged = Nothing
If CSharpImpl.__Assign(oldObservableCollection, TryCast(e.OldValue, INotifyCollectionChanged)) IsNot Nothing Then
oldObservableCollection.CollectionChanged -= AddressOf UpdateDatePickerBlockedDates
DatePickerHelper.DatePickerTable.Remove(oldObservableCollection)
End If
Dim newObservableCollection As INotifyCollectionChanged = Nothing
If CSharpImpl.__Assign(newObservableCollection, TryCast(e.NewValue, INotifyCollectionChanged)) IsNot Nothing Then
newObservableCollection.CollectionChanged += AddressOf UpdateDatePickerBlockedDates
DatePickerHelper.DatePickerTable.Add(newObservableCollection, attachedDatePicker)
End If
attachedDatePicker.BlackoutDates.AddRange(TryCast(e.NewValue, IEnumerable(Of CalendarDateRange)))
End Sub
Private Shared Sub UpdateDatePickerBlockedDates(ByVal sender As Object, ByVal e As NotifyCollectionChangedEventArgs)
Dim attachedDatePicker As DatePicker = Nothing
If Not DatePickerHelper.DatePickerTable.TryGetValue(TryCast(sender, INotifyCollectionChanged), attachedDatePicker) Then
Return
End If
Select Case e.Action
Case NotifyCollectionChangedAction.Add
attachedDatePicker.BlackoutDates.AddRange(e.NewItems.OfType(Of CalendarDateRange)())
Case NotifyCollectionChangedAction.Remove, NotifyCollectionChangedAction.Replace
e.OldItems.OfType(Of CalendarDateRange)().ToList().ForEach(Function(removedItem) attachedDatePicker.BlackoutDates.Remove(removedItem))
Case NotifyCollectionChangedAction.Move
Case NotifyCollectionChangedAction.Reset
attachedDatePicker.BlackoutDates.Clear()
Case Else
End Select
End Sub
End Class
ViewModel.cs
Class ViewModel
Public Property BlockedDates As ObservableCollection(Of CalendarDateRange)
// Block Christmas holidays and all days in the past
Public Sub New()
Return CSharpImpl.__Assign(Me.BlockedDates, New ObservableCollection(Of CalendarDateRange) From {
New CalendarDateRange(New DateTime(2020, 12, 24), New DateTime(2020, 12, 26)),
New CalendarDateRange(DateTime.MinValue, DateTime.Today.Subtract(TimeSpan.FromDays(1)))
})
End Sub
End Class
MainWindow.xaml
<!-- Only show dates from today -->
<DatePicker DatePickerHelper.BlackedDays="{Binding BlockedDates}" />