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A Beginner's Guide to COM Interfaces

  • November 16, 2002
  • By Sam Huggill
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I know what you are saying now, why on Earth is this useful? Well, instead of spending another ten pages explaining it, here is a little example.

Open VB and create a new Standard EXE Project. Click Project, Add Class Module. Repeat this. Also Click Project, References. Select Microsoft DAO 3.51 Object Library. Now you should have two class modules. Rename the first IDataObject and the second CPerson. To the IDataObject add the following code:

Option Explicit

Public IsChanged As Boolean

Public Sub Save()
End Sub
Public Sub GetInfo()
End Sub

Open the second class, CPerson and add the following code:

Option Explicit

Implements IDataObject

Private m_db As Database
Private m_rs As Recordset

Private m_lngID As Long
Private m_strName As String
Private m_strAddress As String
Private m_blnChanged As Boolean

Private Sub Class_Initialize()
Set m_db = OpenDatabase(App.Path & "\people.mdb")
End Sub

Private Sub Class_Terminate()
Set m_rs = Nothing
Set m_db = Nothing
End Sub

Private Sub IDataObject_GetInfo()
If ID = 0 Then
Err.Raise vbObjectError, "CPerson", "A valid ID must be set before
attempting to retrieve data."
End If

Set m_rs = m_db.OpenRecordset("SELECT * FROM People WHERE ID =" & ID)

Name = m_rs.Fields("Name").Value
Address = m_rs.Fields("Address").Value
End Sub

Private Property Let IDataObject_IsChanged(ByVal RHS As Boolean)
m_blnChanged = RHS
End Property

Private Property Get IDataObject_IsChanged() As Boolean
IDataObject_IsChanged = m_blnChanged
End Property

Private Sub IDataObject_Save()
If ID <> 0 Then
End If

m_rs.Fields("Name").Value = Name
m_rs.Fields("Address").Value = Address
End Sub

Public Property Get ID() As Long
ID = m_lngID
End Property

Public Property Let ID(ByVal NewID As Long)
m_lngID = NewID
End Property

Public Property Get Name() As String
Name = m_strName
End Property

Public Property Let Name(ByVal NewName As String)
m_strName = NewName
End Property

Public Property Get Address() As String
Address = m_strAddress
End Property

Public Property Let Address(ByVal NewAddr As String)
m_strAddress = NewAddr
End Property

As you can see we are using a database to store the CPerson information. We have used DAO 3.5 for this although you can modify it to whatever data access you want. Notice that all the methods and properties have been implemented. This is neccessary when implementing an interface otherwise you will run into errors. Take a closer look at the IDataObject_Save method. If the ID is 0, then it is not a current record and therefore a new one needs to be added. Also take a look at the IDataObject_GetInfo method. As with all code in class modules, you should always raise the error and let the module / form / usercontrol handle the error.

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