In an application I am developing, I required that two checkboxes be validated together, when either one of them were clicked. I also needed to only show the Validation error once in my Error Message Box.

To achieve this we first call the OnPropertyChanged routine, which in turn raises the PropertyChanged event, for each linked Property when any of the linked properties change.

For example;

[vb]
Public Property Property1 As Boolean
Get
Return _Property1
End Get
Set(value As Boolean)
If _Property1 <> value Then
_Property1 = value
OnPropertyChanged("Property1")
OnPropertyChanged("Property2")
End If
End Set
End Property
[/vb]

For the Validation errors, I have employed a simple Key, Value Pair list to store all of the Validation Error Results. Thus, I simply check to see if the other Property has Registered an Error, and remove it if so;

[vb]
‘
‘ Validates the Current Item
‘
Public Overrides Function ValidateItemValue(ColumnName As String) As String
Dim strResult As String = ""
Select Case ColumnName
Case "Property1"
If Property1 = False And Property2 = False Then
strResult = "Please select either Property1, Propert2 or Both!"
If Me.lstErrors.ContainsKey("Property2") = True Then
Me.lstErrors.Remove("Property2")
End If
End If
Case "Property2"
If Property1 = False And Property2 = False Then
strResult = "Please select either Property1, Property2 or Both!"
If Me.lstErrors.ContainsKey("Property1") = True Then
Me.lstErrors.Remove("Property1")
End If
End If
End Select
SortErrorList(ColumnName, strResult) ‘ Add or Remove an Error Item
Return strResult
End Function
[/vb]