WPF INotifyErrorInfo Validation.Error Event Not Raising - wpf

Question

I'm encountering a strange problem. Despite setting everything correctly, the Validation.Error doesn't get fired.
Here are the details:
<DataTemplate x:Key="dtLateComers">
<TextBox Text="{Binding ParticipantTag, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True, NotifyOnValidationError=True, NotifyOnSourceUpdated=True}" Validation.Error="Validation_Error" >
</DataTemplate>
Code behind (VB.Net) to set ItemsSource of HeaderedItemsControl:
hicLateComers.ItemsSource = _LateComersViewModels
_LateComersViewModels is ObservableCollection(Of ParticipantViewModel)
Implementation of ParticipantViewMode:
Public Class ParticipantViewModel
Implements INotifyPropertyChanged, IDataErrorInfo
Private _ParticipantTag As String = ""
Public Property ParticipantTag() As String
Get
Return _ParticipantTag
End Get
Set(ByVal value As String)
_ParticipantTag = value
_ParticipantTag= _ParticipantTag.ToUpper
NotifyPropertyChanged("ParticipantTag")
End Set
End Property
Public ReadOnly Property Item(byVal columnName As String) As String Implements IDataErrorInfo.Item
Get
Dim errorString As String = String.Empty
If columnName.Equals("ParticipantTag") Then
If not ParticipantValidationManager.IsValidKeypadTag(_ParticipantTag, True) then
errorString = "Incorrect entry. Please try again."
End If
End If
Return errorString
End Get
End Property
Public ReadOnly Property [Error] As String Implements IDataErrorInfo.Error
Get
Throw New NotImplementedException()
End Get
End Property
End Class
Problem
When I set ItemSource property (as mentioned above in code), Item index is called as many times as there are items in _LaterComersViewModels. Validation works and as a result I get red circle next to TextBox. However, Validation_Error never gets fired until I start typing in Textbox. Typing in TextBox changes the Property binds to it and validate it. Base on validation Validation.Error event is raised, and handled by application. Within that event handler I maintain a count of errors.
So the Question is, why Validation.Error doesn't get raised when one/more items fail on a validation rule during initial data binding? Though it does get raised once property is changed by typing into that TextBox.
Feel free to share any idea, assumption or a solution. Any type of help will be appreciated. Thanks.
Side note: I've a simple C# application which doesn't use data templating. In that application, Validation.Error event gets raised perfectly on start, and on property change. Though in that application, Model is binding to DataContext property of Grid.

Answer 1

Since Validation.Error is an attached event, you could hook up the event handler on the HeaderedItemsControl:
<HeaderedItemsControl x:Name="hicLateComers" ItemTemplate="{StaticResource dtLateComers}" Validation.Error="Validation_Error" />
The result should be pretty much the same since you can easily access both the TextBox and the ParticipantViewModel object in the event handler:
Private Sub Validation_Error(sender As Object, e As ValidationErrorEventArgs)
Dim textBox = CType(e.OriginalSource, TextBox)
Dim participant = CType(textBox.DataContext, ParticipantViewModel)
'...
End Sub