VB Shell Programming

IUnknown

From looking at the type library, you can see that _Animal is derived from another interface called IDispatch. What is not apparent is that IDispatch is derived from yet another interface called IUnknown. Actually, all interfaces are ultimately derived from IUnknown. This means that all COM components share a dependable commonality.

The IUnknown interface contains three methods:

• QueryInterface

• AddRef

• Release

QueryInterface

The purpose of QueryInterface is to allow clients to discover whether a component supports a given interface. It is also used to navigate between interfaces on a given component. Before returning the requested interface (if it exists), AddRef is called to give the object a reference count.

AddRef and Release

AddRef and Release are used for reference counting. All objects in memory have an associated reference count. Every time an object is created or copied, this count is incremented by one. Every time an object is released, the reference count is decremented by one. When the reference count is zero, the object can safely unload itself. As a VB programmer, you have seen this entire process many times in code fragments like the following, probably without ever realizing precisely what was happening behind the scenes:

Dim Cow1 As Animal 'QueryInterface Animal for Cow interface and call AddRef. 'Cow1 now has a reference count of one. Set Cow1 = New Cow Dim Cow2 As Cow 'AddRef is called. Reference count is two. Set Cow2 = Cow1 'Release Cow1. Reference ...