Imports System.ComponentModel

Module ValidationGroup

   Public Delegate Function ValidationDelegate(propertyName As String) As String
   Public Delegate Sub PropertyChangedDelegate(propertyName As String)

   <System.Runtime.CompilerServices.Extension> _
   Public Sub AddValidationGroup(Of T As {INotifyPropertyChanged, INotifyPropertyChanging})(obj As T, validationGroup As List(Of String), validationFlag As Boolean, validationDelegate As ValidationDelegate, propertyChangedDelegate As PropertyChangedDelegate)

      ' This delegate runs before a PropertyChanged event. If the property
      ' being changed exists within the Validation Group, check for validation
      ' errors on the other fields in the group. If there is an error with one
      ' of them, set a flag to true.
      AddHandler obj.PropertyChanging, Sub(sender As Object, e As PropertyChangingEventArgs)
                                          If validationGroup.Contains(e.PropertyName) Then
                                             For Each [property] As String In validationGroup
                                                If validationDelegate([property]) IsNot Nothing Then
                                                   validationFlag = True
                                                   Exit For
                                                End If
                                             Next
                                          End If
                                       End Sub

      ' After the Property gets changed, if another field in this group was
      ' invalid prior to the change, then raise the PropertyChanged event for 
      ' all other fields in the Validation Group to update them.
      ' Also turn flag off so it doesn't get stuck in an infinite loop
      AddHandler obj.PropertyChanged, Sub(sender As Object, e As PropertyChangedEventArgs)
                                         If validationGroup.Contains(e.PropertyName) Then
                                            If validationFlag AndAlso validationDelegate(e.PropertyName) Is Nothing Then
                                               validationFlag = False
                                               For Each [property] As String In validationGroup
                                                  propertyChangedDelegate([property])
                                               Next
                                            End If
                                         End If
                                      End Sub
   End Sub

End Module