ArcGIS Engine Developer Guide

THE V ISUAL BASIC 6 ENVIRONMENT
That way, if there happens to be another IPoint referenced in your project, there
won’t be any ambiguity as to which one you are referring to.
Coclass is an abbreviation of component object
class.
A QI is required since the default interface of
the object is IUnknown. Since the pPt variable
was declared as type IPoint, the default
IUnknown interface was QI’d for the IPoint
interface.
This is the compilation error message shown
when a method or property is not found on an
interface.
The second line of the fragment creates an instance of the object or coclass, then
performs a QI operation for the IPoint interface that it assigns to pPt.
With a name for the coclass as common as Point, you may want to precede the
coclass name with the library name, for example:
Set pPt = New esriGeometry.Point
The last line of the code fragment invokes the PutCoords method. If a method
can’t be located on the interface, an error will be shown at compile time.
Working with HRESULTs
So far, you have seen that all COM methods signify success or failure via an
HRESULT that is returned from the method; no exceptions are raised outside the
interface. You have also learned that Visual Basic raises exceptions when errors
are encountered. In Visual Basic, HRESULTs are never returned from method
calls, and to confuse you further when errors do occur, Visual Basic throws an
exception. How can this be? The answer lies with the Visual Basic Virtual Machine.
It is the VBVM that receives the HRESULT; if this is anything other than
S_OK, the VBVM throws the exception. If it was able to retrieve any worthwhile
error information from the COM error object, it populates the Visual Basic
Err object with that information. In this way, the VBVM handles all HRESULTs
returned from the client.
When implementing interfaces in Visual Basic, it is good coding practice to raise
an HRESULT error to inform the caller that an error has occurred. Normally, this
is done when a method has not been implemented.
' Defined in module
Const E_NOTIMPL = &H80004001 ' Constant that represents HRESULT
' Added to any method not implemented
On Error GoTo 0
Err.Raise E_NOTIMPL
You must also write code to handle the possibility that an HRESULT other than
S_OK is returned. When this happens, an error handler should be called and the
error dealt with. This may mean simply notifying the user, or it may mean automatically
dealing with the error and continuing with the function. The choice
depends on the circumstances. Below is a simple error handler that will catch any
error that occurs within the function and report it to the user. Note the use of
the Err object to provide the user with some description of the error.
Private Sub Test()
On Error GoTo ErrorHandler
' Do something here.
Exit Sub ' Must exit sub here before error handler
ErrorHandler:
Msgbox "Error In Application – Description " & Err.Description
End Sub
86 • ArcGIS Engine Developer Guide