ArcGIS Engine Developer Guide

THE MICROSOFT COMPONENT OBJECT MODEL
Type
Boolean
unsigned char
double
float
int
long
short
BSTR
CURRENCY
DATE
SCODE
Typedef enum myenum
Interface IDispatch *
Interface IUnknown *
dispinterface Typename *
Coclass Typename *
[oleautomation] interface Typename *
SAFEARRAY(TypeName)
TypeName*
Decimal
Description
Data item that can have the value True or False
8-bit unsigned data item
64-bit IEEE floating-point number
32-bit IEEE floating-point number
Signed integer, whose size is system dependent
32-bit signed integer
16-bit signed integer
Length-prefixed string
8-byte, fixed-point number
64-bit, floating-point fractional number of days since Dec 30, 1899
For 16-bit systems - Built-in error that corresponds to VT_ERROR
Signed integer, whose size is system dependent
Pointer to the IDispatch interface
Pointer to an interface that does not derive from IDispatch
Pointer to an interface derived from IDispatch
Pointer to a coclass name (VT_UNKNOWN)
Pointer to an interface that derives from IDispatch
TypeName is any of the above types. Array of these types
TypeName is any of the above types. Pointer to a type
96-bit unsigned binary integer scaled by a variable power of 10. A decimal data
type that provides a size and scale for a number (as in coordinates)
COMPONENT CATEGORY
Component categories are used by client applications to find all COM classes of a
particular type that are installed on the system efficiently. For example, a client
application may support a data export function in which you can specify the
output format—a component category could be used to find all the data export
classes for the various formats. If component categories are not used, the application
has to instantiate each object and interrogate it to see if it supports the
required functionality, which is not a practical approach. Component categories
support the extensibility of COM by allowing the developer of the client application
to create and work with classes that belong to a particular category. If at a
later date, a new class is added to the category, the client application need not be
changed to take advantage of the new class; it will automatically pick up the new
class the next time the category is read.
ESRI keys in the Windows system registry
COM AND THE REGISTRY
COM makes use of the Windows system registry to store information about the
various parts that compose a COM system. The classes, interfaces, DLLs, EXEs,
type libraries, and so forth, are all given GUIDs that the SCM
uses when referencing these components. To see an example of
this, run regedit, then open HKEY_CLASSES_ROOT. This
opens a list of all the classes registered on the system.
COM makes use of the registry for a number of housekeeping
tasks, but the most important and most easily understood is
the use of the registry when instantiating COM objects into
memory. In the simplest case, that of an in-process server, the
steps are as follows:
1. Client requests the services of a COM object.
68 • ArcGIS Engine Developer Guide