Simple Validation in VS.NET 2003

Public Class OrderDetailLineTotalRule
    Inherits ValidationRuleBase

    Public Overrides ReadOnly Property RulePriority() As RulePriority
        Get
            Return RulePriority.Normal
        End Get
    End Property

    Public Overrides Function Validate(ByVal objectToValidate As Object) As ValidationResultCollection

        ' This rule only checks a DTO.OrdersDataSets.OrderHeaderRow (an Order)
        If Not TypeOf objectToValidate Is DTO.OrdersDataSet.OrderHeaderRow Then
            Return Nothing
        End If

        Dim results As New ValidationResultCollection
        Dim result As ValidationResult
        Dim drOrder As DTO.OrdersDataSet.OrderHeaderRow = CType(objectToValidate, DTO.OrdersDataSet.OrderHeaderRow)
        Dim lineTotal As Decimal
        For Each drOD As DTO.OrdersDataSet.OrderDetailRow In drOrder.GetChildRows("FK_OrderHeader_OrderDetail")
            lineTotal = drOD.OrderedQuantity * (drOD.UnitPrice - (drOD.UnitPrice * (drOD.UnitPriceDiscount / 100)))
            If lineTotal <> drOD.LineTotal Then
                result = New ValidationResult(Me.RuleName, ResultType.Passed, "", "", "", Me.RulePriority)
                result.ResultType = ResultType.Failed
                result.Message = String.Format("LineTotal {0:C} not equal to calculated line total {1:C}", drOD.LineTotal, lineTotal)
                result.ReferenceHint = "OrderDetailD"
                result.Reference = drOD.OrderDetailID.ToString
                results.Add(result)
            End If
        Next
        Return results

    End Function
End Class