ArcGIS Engine Developer Guide

DEVELOPING WITH A RCOBJECTS
COM
Application
7
Geodatabase API
2
3
1
7
8
5
6
Database
The diagram above clearly shows that the
Feature, which is a COM object, has another
COM object for its geometry. The Shape
property of the feature simply passes the
IGeometry interface pointer to this geometry
object out to the caller that requested the
shape. This means that if more than one client
requested the shape, all clients point to the
same geometry object. Hence, this geometry
object must be treated as read-only. No changes
should be performed on the geometry returned
from this property, even if the changes are
temporary. Anytime a change is to be made to a
feature’s shape, the change must be made on
the geometry returned by the ShapeCopy
property, and the updated geometry should
subsequently be assigned to the Shape property.
4
Shape and ShapeCopy geometry property
Make use of a Feature object’s Shape and ShapeCopy properties to optimally retrieve
the geometry of a feature. To better understand how these properties relate
to a feature’s geometry, refer to the diagram to the left to see how features coming
from a data source are instantiated into memory for use within an application.
Features are instantiated from the data source using the following sequence:
1. The application requests a Feature object from a data source by calling the
appropriate geodatabase API method calls.
2. The geodatabase makes a request to COM to create a vanilla COM object of
the desired COM class (normally this class is esriGeoDatabase.Feature).
3. COM creates the Feature COM object.
4. The geodatabase gets attribute and geometry data from a data source.
5. The vanilla Feature object is populated with appropriate attributes.
6. The Geometry COM object is created, and a reference is set in the Feature
object.
7. The Feature object is passed to the application.
8. The Feature object exists in the application until it is no longer required.
USING A TYPE LIBRARY
Since objects from ArcObjects do not implement IDispatch, it is essential to make
use of a type library for the compiler to early-bind to the correct data types. This
applies to all development environments; although, for Visual Basic, Visual C++,
and .NET, there are wizards that help you set this reference.
The type libraries required by ArcObjects are located within the ArcGIS install
folder. For example, the COM type libraries can be found in the COM folder,
while the .NET Interop assemblies are within the DotNet folder. Many different
files can contain type library information, including EXEs, DLLs, OLE custom
controls (OCXs), and object libraries (OLBs).
COM DATA TYPES
COM objects talk via their interfaces, and hence, all data types used must be
supported by IDL. IDL supports a large number of data types; however, not all
languages that support COM support these data types. Because of this,
ArcObjects does not make use of all the data types available in IDL but limits the
majority of interfaces to the data type supported by Visual Basic. The following
table shows the data types supported by IDL and their corresponding types in a
variety of languages.
76 • ArcGIS Engine Developer Guide