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COM+ Programming with Visual Basic by Jose Mojica

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Type Libraries

It’s time to leave the pre-COM era behind. Let’s look at the server and client code the way that it exists today using COM. It makes sense at this point to look at each line of a client program and study what VB is doing for us.

Suppose that we start with the CChecking class defined in the following code:

' Class CChecking

Option Explicit

Private m_cBalance As Currency

Public Property Get Balance(  ) As Currency
    Balance = m_cBalance
End Property

Public Sub MakeDeposit(ByVal Amount As Currency)
    m_cBalance = m_cBalance + Amount
End Sub

The VB client code to use this class is the following:

Dim Acct As CChecking
Set Acct = New CChecking
Call Acct.MakeDeposit(5000)
Set Acct = Nothing

In the preceding client code, VB first allocates 4 bytes of memory to hold the address of an object. It does this in the declaration of Acct: Dim Acct As CChecking. In the next line, VB does a couple of things. Although it appears that the New keyword causes VB to allocate the memory for the object on the client side, you have learned that this is not the case with the COM mechanism (in fact, this is not a good technique when it comes to upgrading objects where there might be two different definitions, one in the client and one in the server). Therefore, this code causes the server to allocate memory for the object. At that point, VB creates a vtable for the CChecking class. The entries in the vtable are pointers to the public functions of the class. The memory layout of the _CChecking interface ...

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