ArcGIS Engine Developer Guide

.NET APPLICATION PROGRAMMING INTERFACE
("You had an error in your application")
...
[C#]
catch (System.Exception ex)
{
throw new ApplicationException("You had an error in your application");
}
...
If you do this, however, the original exception is lost. To allow complete error
information to be propagated, the Exception class includes the InnerException
property. This property should be set to equal the caught exception, before the
new exception is thrown. This creates an error hierarchy. Again, the example
shown below uses the ApplicationException constructor to set the
InnerException and Message properties.
[VB.NET]
Catch ex As System.Exception
Dim appEx As System.ApplicationException = _
New ApplicationException("You had an error in your application", ex)
Throw appEx
...
[C#]
catch (System.Exception ex)
{
System.ApplicationException appEx =
new ApplicationException("You had an error in your application", ex);
throw appEx;
}
...
In this way, the function that eventually deals with the error condition can access
all the information about the cause of the condition and its context.
If you throw an error, the application will execute the current function’s Finally
clause before control is returned to the calling function.
System.__ComObject and casting to strongly typed RCWs
Sometimes you may find that casting a variable fails when you think it should
succeed (the solution is often to declare variables as interface Types and avoid the
use of class types, for example, use IStyleGallery rather than StyleGalleryClass).
You may also have come across the System.__ComObject type and wonder where
it comes from. This topic should help you to understand these issues; in particular,
you may encounter problems when attempting to create the AppRef class in
.NET—this issue is related to the System.__ComObject wrapper and is also
covered below.
Types and Runtime Callable Wrappers
In .NET, each class, interface, enumeration, and so on, is described by its Type.
The Type class, which is part of the .NET Framework, holds information about
Chapter 4 • Developer environments • 157