ArcGIS Engine Developer Guide

.NET APPLICATION PROGRAMMING INTERFACE
.NET runtime wraps the COM object reference in a generic RCW called
System.__ComObject. This is a class internal to the .NET Framework that can be
used to hold a reference to any kind of COM object; its purpose is to act as the
RCW for an unknown Type of COM object.
Casting
Looking again at the second example, even if you actually know the exact Type
of class to which you have a reference, the .NET runtime still does not have the
metadata required to cast the variable to a strongly typed RCW; this can be seen
in the following code, as attempting a cast to the SimpleMarkerSymbolClass Type
would fail.
[C#]
// The following line would result in sym2 being null as the cast would
fail.
ESRI.ArcGIS.Display.SimpleMarkerSymbolClass sym2 = sym as
ESRI.ArcGIS.Display.SimpleMarkerSymbolClass;
[Visual Basic .NET]
' The following line would result in a runtime error as the implicit cast
would fail.
Dim sym2 As ESRI.ArcGIS.Display.SimpleMarkerSymbol = sym
However, as the System.__ComObject class is specifically designed to work with
COM objects, it is always able to perform a QI to any COM interfaces that are
implemented by an object. Therefore, casting to specific interfaces (as long as they
are implemented on the object) will be successful.
[C#]
ESRI.ArcGIS.Display.ISimpleMarkerSymbol sym3 = sym as
ESRI.ArcGIS.Display.ISimpleMarkerSymbol;
[Visual Basic .NET]
Dim sym3 As ESRI.ArcGIS.Display.ISimpleMarkerSymbol = sym
Singletons and System.__ComObject
In the examples above, a strongly typed RCW is created when you instantiate the
COM object by using the ‘new’ keyword, whereas if the object is preexisting, the
Type of the RCW is the generic System.__ComObject. Sometimes when you use
the ‘new’ keyword to instantiate a COM object, you are actually getting a reference
to an object that already exists—this happens when you attempt to instantiate
a singleton class that has previously been instantiated. The .NET framework is
unable to wrap in a strongly typed RCW an instance of an object that has previously
been wrapped in the generic System.__ComObject RCW. If your code has
encountered such a situation, you may receive an error such as
‘Unable to cast object of type System.__ComObject to type ’.
[C#]
ESRI.ArcGIS.Display.IStyleGallery sg = new
ESRI.ArcGIS.Framework.StyleGalleryClass();
[Visual Basic .NET]
Dim sg As ESRI.ArcGIS.Display.IStyleGallery = = New
ESRI.ArcGIS.Framework.StyleGalleryClass
This error may occur even though you have declared your variable using the
interface name rather than the class name, as shown above. The problem occurs
Chapter 4 • Developer environments • 159