ArcGIS Engine Developer Guide

THE MICROSOFT COMPONENT OBJECT MODEL
Binding is the term given to the process of
matching the location of a function given a
pointer to an object.
Binding type
In process
DLL
Out of process
DLL
Late binding 22,250 5,000
Custom vTable binding 825,000 20,000
This table shows the number of function calls
that can be made per second on a typical
Pentium ® III machine.
IUnknown
IMap
IUnknown
IDispatch
IApplication
Custom - Map
vTable
QueryInterface
AddRef
Release
Name
Description
AreaOfInterest
Dual - Application
vTable
QueryInterface
AddRef
Release
GetTypeInfoCount
GetTypeInfo
GetIDsOfNames
Invoke
Name
Document
StatusBar
These diagrams summarize the custom and
IDispatch interfaces for two classes in
ArcObjects. The layout of the vTable displays the
differences. It also illustrates the importance of
implementing all methods—if one method is
missing, the vTable will have the wrong layout,
and hence, the wrong function pointer would be
returned to the client, resulting in a system
crash.
IDispatch interface
COM supports three types of binding:
• Late. This is where type discovery is left until runtime. Method calls made by
the client but not implemented by the object will fail at execution time.
• ID. Method IDs are stored at compile time, but execution of the method is
still performed through a higher-level function.
• Custom vTable (early). Binding is performed at compile time. The client can
then make method calls directly into the object.
The IDispatch interface supports late- and ID-binding languages. The IDispatch
interface has methods that allow clients to ask the object what methods it supports.
Assuming the required method is supported, the client executes the method by
calling the IDispatch::Invoke method. This method, in turn, calls the required
method and returns the status and any parameters back to the client on completion
of the method call.
Clearly, this is not the most efficient way to make calls on a COM object. Late
binding requires a call to the object to retrieve the list of method IDs; the client
must then construct the call to the Invoke method and call it. The Invoke method
must then unpack the method parameters and call the function.
All these steps add significant overhead to the time it takes to execute a method.
In addition, every object must have an implementation for IDispatch, which
makes all objects larger and adds to their development time.
ID binding offers a slight improvement over late binding in that the method IDs
are cached at compile time, which means the initial call to retrieve the IDs is not
required. However, there is still significant call overhead because the
IDispatch::Invoke method is still called to execute the required method on the
object.
Early binding, often referred to as custom vTable binding, does not use the
IDispatch interface. Instead, a type library provides the required information at
compile time to allow the client to know the layout of the server object. At
runtime, the client makes method calls directly into the object. This is the fastest
method of calling object methods and also has the benefit of compile-time type
checking.
Objects that support both IDispatch and custom vTable are referred to as dual
interface objects. The object classes within the ESRI object libraries do not
implement the IDispatch interface; this means that these object libraries cannot be
used with late-binding scripting languages, such as JavaScript or VBScript, since
these languages require that all COM servers accessed support the IDispatch
interface.
Careful examination of the ArcGIS class diagrams indicates that the Application
objects support IDispatch because there is a requirement in VBA for the IDispatch
interface.
Chapter 4 • Developer environments • 65