ArcGIS Engine Developer Guide

.NET APPLICATION PROGRAMMING INTERFACE
code. COM is one example of unmanaged code. The .NET Framework interacts
with COM via a technology known as COM Interop.
For COM Interop to work, the Common Language Runtime (CLR) requires
metadata for all the COM types. This means that the COM type definitions
normally stored in the type libraries need to be converted to .NET metadata. This
is easily accomplished with the Type Library Importer utility (tlbimp.exe), which
ships with the .NET Framework SDK. This utility generates interop assemblies
containing the metadata for all the COM definitions in a type library. Once
metadata is available, .NET clients can seamlessly create instances of COM types
and call its methods as though they were native .NET instances.
The ArcGIS installation program also installs the
Microsoft Stdole.dll PIA, providing interop for
OLE font and picture classes, which are used by
some ESRI libraries.
Primary interop assemblies
Primary interop assemblies (PIAs) are the official, vendor-supplied, .NET type
definitions for interoperating with underlying COM types. Primary interop assemblies
are strongly named by the COM library publisher to guarantee uniqueness.
ESRI provides primary interop assemblies for all the ArcObjects type libraries
that are implemented with COM. ArcGIS .NET developers should only use those
primary interop assemblies that are installed in the Global Assembly Cache (GAC)
during install if version 1.1 of the .NET Framework is detected. ESRI only
supports the interop assemblies that ship with ArcGIS. You can identify a valid
ESRI assembly by its public key (8FC3CC631E44AD86).
COM wrappers
The .NET runtime provides wrapper classes to make both managed and
unmanaged clients believe
they are communicating
with objects within their
respective environment.
When managed clients call
a method on a COM
object, the runtime creates
a runtime callable wrapper
(RCW) that handles the
marshalling between the
two environments. Similarly,
the .NET runtime
creates COM-callable
wrappers for the reverse case, COM clients communicating with .NET components.
The illustration above outlines this process.
Exposing .NET components to COM
When creating .NET components that COM clients will make use of, follow the
guidelines listed below to ensure interoperability.
• Avoid using parameterized constructors.
• Avoid using static methods.
• Define event source interfaces in managed code.
146 • ArcGIS Engine Developer Guide