VB Shell Programming

IDispatch

As mentioned previously, _Animal is directly derived from an interface called IDispatch . Interfaces derived from IDispatch often have the [dual] attribute and appropriately are called dual interfaces. This is because the interface supports vtable binding (binding at compile time) and late binding (binding at run-time). The methods that comprise IDispatch facilitate the process known as late binding, which results from code like that shown in Example 2.6.

Example 2-6. Late Binding

'Late binding Cow

Dim cow1 As Object
Set cow1 = CreateObject("Animals.Cow") 

MsgBox cow1.Noise

CreateObject uses the ProgID for the component and maps it to a CLSID. This allows an instance of the component to be created. Internally, this is done by calling the CoCreateInstanceEx API. Once the component is loaded, a call to QueryInterface is made and a pointer to an IDispatch interface is returned. The generic Object datatype really means IDispatch. Then late binding is used to make the call to the Noise method.

Late binding is generally avoided whenever possible for reasons of efficiency (it’s extremely slow). But in scripting languages like VBScript and JavaScript, late binding is the only choice available. This is because type information is used at compile time to bind method calls to the object. Code run in a scripting environment is not compiled. It is interpreted at runtime, line by line. Therefore, there needs to be a mechanism for calling the methods of an object in environments ...