Question

We have a highly specialized DAL which sits over our DB. Our apps need to use this DAL to correctly operate against this DB.

The generated DAL (which sits on some custom base classes) has various ‘Rec’ classes (Table1Rec, Table2Rec) each of which represents the record structure of a given table.

Here is a sample Pseudo-class…

Public Class SomeTableRec     Private mField1 As String     Private mField1isNull As Boolean     Private mField2 As Integer     Private mField2isNull As Boolean      Public Sub New()         mField1isNull = True         mField2isNull = True     End Sub     Public Property Field1() As String         Get             Return mField1         End Get         Set(ByVal value As String)             mField1 = value             mField1isNull = False         End Set     End Property     Public ReadOnly Property Field1isNull() As Boolean         Get             Return mField1isNull         End Get     End Property     Public Property Field2() As Integer         Get             Return mField2         End Get         Set(ByVal value As Integer)             mField2 = value             mField2isNull = False         End Set     End Property     Public ReadOnly Property Field2isNull() As Boolean         Get             Return mField2isNull         End Get     End Property End Class 

Each class has properties for each of the fields… Thus I can write…

Dim Rec as New Table1Rec Table1Rec.Field1 = 'SomeString' Table2Rec.Field2 = 500 

Where a field can accept a NULL value, there is an additional property which indicates if the value is currently null.

Thus….

Dim Rec as New Table1Rec Table1Rec.Field1 = 'SomeString' If Table1Rec.Field1Null then      ' This clearly is not true End If If Table1Rec.Field2Null then      ' This will be true End If 

This works because the constructor of the class sets all NULLproperties to True and the setting of any FieldProperty will cause the equivalent NullProperty to be set to false.

I have recently had the need to expose my DAL over the web through a web service (which I of course intend to secure) and have discovered that while the structure of the ‘Rec’ class remains intact over the web… All logic is lost..

If someone were to run the previous piece of code remotely they would notice that neither condition would prove true as there is no client side code which sets null to true.

I get the feeling I have architected this all wrong, but cannot see how I should improve it.

What is the correct way to architect this?

Answer 1

Not sure if I fully understand the question, but you can have nullable data types in XML.

So this…

Imports System.Web Imports System.Web.Services Imports System.Web.Services.Protocols  <WebService(Namespace:='http://tempuri.org/')> _ <WebServiceBinding(ConformsTo:=WsiProfiles.BasicProfile1_1)> _ <Global.Microsoft.VisualBasic.CompilerServices.DesignerGenerated()> _ Public Class Testing      Inherits System.Web.Services.WebService      <WebMethod()> _    Public Function GetObjects() As Generic.List(Of TestObject)         Dim list As New Generic.List(Of TestObject)         list.Add(New TestObject(Nothing, 'Empty ID Object'))         list.Add(New TestObject(1, 'Full ID Object'))         list.Add(New TestObject(2, Nothing))         Return list     End Function      Public Class TestObject         Public Sub New()             _name = String.Empty             _id = Nothing         End Sub         Public Sub New(ByVal id As Nullable(Of Integer), ByVal name As String)             _name = name             _id = id         End Sub         Private _name As String         Public Property Name() As String             Get                 Return _name             End Get             Set(ByVal value As String)                 _name = value             End Set         End Property          Private _id As Nullable(Of Integer)         Public Property ID() As Nullable(Of Integer)             Get                 Return _id             End Get             Set(ByVal value As Nullable(Of Integer))                 _id = value             End Set         End Property     End Class  End Class 

outputs this (with nullable areas)

<?xml version='1.0' encoding='utf-8' ?>  <ArrayOfTestObject xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' xmlns:xsd='http://www.w3.org/2001/XMLSchema' xmlns='http://tempuri.org/'>  <TestObject>   <Name>Empty ID Object</Name>    <ID xsi:nil='true' />   </TestObject>  <TestObject>   <Name>Full ID Object</Name>    <ID>1</ID>   </TestObject>  <TestObject>   <ID>2</ID>   </TestObject> </ArrayOfTestObject>