ArcGIS Engine Developer Guide

.NET APPLICATION PROGRAMMING INTERFACE
because when your code instantiates an object, the .NET runtime first attempts
to wrap the object in the strongly typed class Type (the Type stated after the new
keyword) before attempting a cast to the interface type. The cast to the strongly
typed RCW cannot succeed as the COM object has previously been wrapped in
the generic System.__ComObject wrapper. This may occur in situations beyond
your control. For example, other ArcObjects tools written in .NET from other
third parties may wrap an object in the generic wrapper, causing your code to fail.
The solution is to use the Activator class (as shown below) to safely wrap singleton
objects in a strongly typed RCW when you first get a reference to them.
Additionally, you should generally always declare variables holding RCWs using an
interface rather than a class Type.
Using the Activator class to create singletons
If you use the CreateInstance method of the Activator class instead of the new
keyword to instantiate singletons, you can avoid such errors, as the Activator is
able to get the required metadata to perform the cast.
[C#]
Type t = Type.GetTypeFromProgID("esriFramework.StyleGallery");
System.Object obj = Activator.CreateInstance(t);
IStyleGallery sg = obj as IStyleGallery;
[Visual Basic .NET]
Dim t As Type = Type.GetTypeFromProgID("esriFramework.StyleGallery")
Dim obj As System.Object = Activator.CreateInstance(t)
Dim pApp As ESRI.ArcGIS.Display.IStyleGallery = obj
You can use this technique to instantiate the AppRef class—remember, however,
that the AppRef class can only be created within an ArcGIS application. (The
Type is the generic System.__ComObject RCW.)
[C#]
Type t = Type.GetTypeFromProgID("esriFramework.AppRef");
System.Object obj = Activator.CreateInstance(t);
ESRI.ArcGIS.Framework.IApplication pApp = obj as
ESRI.ArcGIS.Framework.IApplication;
[Visual Basic .NET]
Dim t As Type = Type.GetTypeFromProgID("esriFramework.AppRef")
Dim obj As System.Object = Activator.CreateInstance(t)
Dim pApp As ESRI.ArcGIS.Framework.IApplication = obj
For more information about RCWs and interop, you may wish to refer to the
book by Adam Nathan, .NET and COM—The Complete Interoperability Guide, Sams
Publishing, 2002.
Working with resources
Using strings and embedded images directly (no localization)
If your customization does not support localization now and you do not intend
for it to support localization later, you can use strings and images directly without
the need for resource files. For example, strings can be specified and used directly
in your code:
[VB.NET]
160 • ArcGIS Engine Developer Guide