ArcGIS Engine Developer Guide

.NET APPLICATION PROGRAMMING INTERFACE
env.DefineFromPoints(3, ref pointArray);
// Doesn't work
env.DefineFromPoints(3, ref pointArray[0]);
// Works
envGEN.DefineFromPoints(ref pointArray);
Class Interfaces
Class interfaces are created to help VB programmers transition to .NET; they are
also commonly used in code produced by the Visual Basic .NET Upgrade Wizard
or the code snippet converter in Visual Studio .NET.
However, it is generally recommended that you avoid using the class interfaces in
the ESRI interop assemblies, as they may change in future versions of ArcGIS.
This section explains a little more about class interfaces.
In Visual Basic 6, the details of default interfaces were hidden from the user, and
a programmer could instantiate a variable and access the members of its default
interface without performing a specific QI for that interface; for example, the
VB 6 code below instantiates the StdFont class and sets a variable equal to the
default interface (IStdFont) of that class:
[VB 6.0]
Dim fnt As New Stdole.StdFont
However, .NET does not provide this same ability. To allow VB developers a
more seamless introduction to .NET, the type library importer in .NET adds ‘class
interfaces’ to each interop assembly, allowing COM objects to be used with this
same syntax inside .NET. When an object library is imported, a class interface
RCW is created for each COM class; the name of the class interface is the same as
the COM class—for example, Envelope.
All the members of the default interface of the COM class are added to this class
interface; also, if the COM class has a source interface (is the source of events),
then the class interface will also include all the events of this interface, which
helps a programmer to link up events.
A second RCW is created that represents the underlying COM class; the name of
this is the same as the COM class with a suffix of ‘Class’, for example,
EnvelopeClass. The class interface is linked to the class by an attribute, which
indicates the class to which it belongs. This attribute is recognized by the .NET
compilers, which allows a programmer to instantiate a class by using its class
interface.
The exception is classes that have a default interface of IUnknown or IDispatch,
which are never exposed on RCW classes as the members are called internally by
the .NET Framework runtime. In this case, the next implemented interface is
exposed on the class interface instead. As most ArcObjects define IUnknown as
their default interface, this affects most ArcObjects classes. For example, the
Point COM class in the esriGeometry object library lists the IPoint interface as its
first implemented interface. In .NET, this class is accessed by using the Point class
interface, which inherits the IPoint interface, and the PointClass class.
The code below shows that by declaring a variable type as a Point class interface,
Chapter 4 • Developer environments • 149