The Only Data Access Layer You'll Ever Need?, Page 4

Putting It All Together

With the basic code described so far, you need to do significantly less work to flesh out the rest of any DAL. You will probably want a data access class for groups of business objects. These classes will contain code to initiate read and write calls and the read and write handlers themselves.

Listing 6 shows a basic CustomerAccess class that kicks off reads (and writes, not shown), and contains the read (and write) handlers. It demonstrates how to implement a read event handler and it also demonstrates how to invoke the reflection-read.

Listing 6: The CustomerAccess Class

Imports System
Imports System.Data
Imports System.Diagnostics
Imports System.Configuration
Imports System.Collections.Generic
Imports System.Web
Imports MYVB.BusinessObjects


Public Class CustomerAccess

   Public Shared Function GetCustomers() As List(Of Customer)
      'Return DataAccess.Read(Of List(Of Customer))("GetCustomers",
      'AddressOf OnReadCustomers)
      Return DataAccess.Read(Of List(Of Customer))("GetCustomers", _
         AddressOf DataAccess.OnReadAnyList(Of Customer))

   End Function

   Public Shared Function OnReadCustomers(ByVal reader As IDataReader) _
      As List(Of Customer)
      If (reader Is Nothing) Then Return New List(Of Customer)()

      Dim customers As List(Of Customer) = New List(Of Customer)()

      While (reader.Read())
         customers.Add(OnReadCustomer(reader))
      End While

      Return customers

   End Function

   Private Shared Function OnReadCustomer(ByVal reader _
      As IDataReader) As Customer
      Debug.Assert(reader Is Nothing = False)
      Dim customerID As String = ""
      Dim companyName As String = ""
      Dim contactName As String = ""
      Dim contactTitle As String = ""
      Dim address As String = ""
      Dim city As String = ""
      Dim region As String = ""
      Dim postalCode As String = ""
      Dim country As String = ""
      Dim phone As String = ""
      Dim fax As String = ""

      customerID = DataAccess.SafeRead(Of String)(customerID, reader, _
         "CustomerID")
      companyName = DataAccess.SafeRead(Of String)(companyName, _
         reader, "CompanyName")
      contactName = DataAccess.SafeRead(Of String)(contactName, _
         reader, "ContactName")
      contactTitle = DataAccess.SafeRead(Of String)(contactTitle, _
         reader, "ContactTitle")
      address = DataAccess.SafeRead(Of String)(address, reader, "Address")
      city = DataAccess.SafeRead(Of String)(city, reader, "City")
      region = DataAccess.SafeRead(Of String)(region, reader, "Region")
      postalCode = DataAccess.SafeRead(Of String)(postalCode, reader, _
         "PostalCode")
      country = DataAccess.SafeRead(Of String)(country, reader, "Country")
      phone = DataAccess.SafeRead(Of String)(phone, reader, "Phone")
      fax = DataAccess.SafeRead(Of String)(fax, reader, "Fax")

      Return New Customer(customerID, companyName, contactName, _
         contactTitle, address, city, region, postalCode, country, _
         phone, fax)

   End Function
End Class

You can use this exemplar to implement all of your custom data access classes.

Tip: Although you do need to write some additional data access code for your custom objects, you could easily write a CodeDOM code generator or use a tool such as CodeRush to really speed up this part of the data access development.

Listing 7 shows the custom Customer object populated by the CustomerAccess class. If you need a collection of objects, use List<T>, the generic collection class.

Listing 7: An Example of a Basic Custom Business Object

<Serializable()> _
Public Class Customer

   Private FCustomerID As String
   <ColumnName("CustomerID")> _
   Public Property CustomerID() As String
      Get
         Return FCustomerID
      End Get
      Set(ByVal value As String)
         FCustomerID = value
      End Set
   End Property

   Private FCompanyName As String
   Public Property CompanyName() As String
      Get
         Return FCompanyName
      End Get
      Set(ByVal value As String)
         FCompanyName = value
      End Set
   End Property

   Private FContactName As String
   Public Property ContactName() As String
      Get
         Return FContactName
      End Get
      Set(ByVal value As String)
         FContactName = value
      End Set
   End Property


   Private FContactTitle As String
   Public Property ContactTitle() As String
      Get
         Return FContactTitle
      End Get
      Set(ByVal value As String)
         FContactTitle = value
      End Set
   End Property

   Private FAddress As String
   Public Property Address() As String
      Get
         Return FAddress
      End Get
      Set(ByVal value As String)
         FAddress = value
      End Set
   End Property

   Private FCity As String
   Public Property City() As String
      Get
         Return FCity
      End Get
      Set(ByVal value As String)
         FCity = value
      End Set
   End Property

   Private FRegion As String

   Public Property Region() As String
      Get
         Return FRegion
      End Get
      Set(ByVal value As String)
         FRegion = value
      End Set
   End Property


   Private FPostalCode As String
   Public Property PostalCode() As String
      Get
         Return FPostalCode
      End Get
      Set(ByVal value As String)
         FPostalCode = value
      End Set
   End Property

   Private FCountry As String
   Public Property Country() As String
      Get
         Return FCountry
      End Get
      Set(ByVal value As String)
         FCountry = value
      End Set
   End Property

   Private FPhone As String
   Public Property Phone() As String
      Get
         Return FPhone
      End Get
      Set(ByVal value As String)
         FPhone = value
      End Set
   End Property

   Private FFax As String
   Public Property Fax() As String
      Get
         Return FFax
      End Get
      Set(ByVal value As String)
         FFax = value
      End Set
   End Property

   Public Sub New()

   End Sub


   Public Sub New(ByVal customerID As String, ByVal companyName _
      As String, _
   ByVal contactName As String, ByVal contactTitle As String, _
      ByVal address As String, _
   ByVal city As String, ByVal region As String, _
      ByVal postalCode As String, _
   ByVal country As String, ByVal phone As String, ByVal fax As String)

      Me.FCustomerID = customerID
      Me.FCompanyName = companyName
      Me.FContactName = contactName
      Me.FContactTitle = contactTitle
      Me.FAddress = address
      Me.FCity = city
      Me.FRegion = region
      Me.FPostalCode = postalCode
      Me.FCountry = country
      Me.FPhone = phone
   End Sub

End Class

The DAL refers to the business object layer and not the other way around. This means that business objects don't need to carry any database knowledge (or baggage) around with them.

By using generic Lists—List(Of T)—you don't need to write custom collections. Generic lists are enumerable and bindable. That said, I often inherit from the generic List—for example, List(Of Customer)—to add additional behaviors to my custom business object collections.