MVVM WPF Validating multiple items together

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;

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

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;

        ''' <summary>

        ''' Validates the Current Item

        ''' </summary>

        ''' <param name="ColumnName"></param>

        ''' <returns></returns>

        ''' <remarks></remarks>

        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