ArcGIS Engine Developer Guide

THE V ISUAL BASIC 6 ENVIRONMENT
Iterating through a collection
In your work with ArcObjects, you’ll discover that, in many cases, you’ll be
working with collections. You can iterate through these collections with an
enumerator. An enumerator is an interface that provides methods for traversing a
list of elements. Enumerator interfaces typically begin with IEnum and have two
methods: Next and Reset. Next returns the next element in the set and advances
the internal pointer, and Reset resets the internal pointer to the beginning.
Here is some VB code that loops through the selected features (IEnumFeature) in a
map control.
Dim pEnumFeat As IEnumFeature
Dim pFeat As IFeature
Set pEnumFeat = MapControl1.Map.FeatureSelection
Set pFeat = pEnumFeat.Next
Do While (Not pFeat Is Nothing)
Debug.Print pFeat.Value(pFeat.Fields.FindField("state_name"))
Set pFeat = pEnumFeat.Next
Loop
Some collection objects, the Visual Basic Collection being one, implement a
special interface called _NewEnum. This interface, because of the _ prefix, is
hidden, but Visual Basic developers can still use it to simplify iterating through a
collection. The Visual Basic For Each construct works with this interface to
perform the Reset and Next steps through a collection.
Dim pColn as Collection
Set pColn = GetCollection()' Collection returned from some function
Dim thing as Variant ' VB uses methods on _NewEnum to step through
For Each thing in pColn ' an enumerator.
MsgBox Cstr(thing)
Next
Chapter 4 • Developer environments • 91