ArcGIS Engine Developer Guide

THE V ISUAL BASIC 6 ENVIRONMENT
Working with properties
Some properties refer to specific interfaces in the ESRI object library, and other
properties have values that are standard data types, such as strings, numeric
expressions, and Boolean values. For interface references, declare an interface
variable and use the Set statement to assign the interface reference to the property.
For other values, declare a variable with an explicit data type or use Visual
Basic’s Variant data type. Then, use a simple assignment statement to assign the
value to the variable.
Properties that are interfaces can be set either by reference or value. Properties
that are set by value do not require the Set statement.
Dim pEnv As IEnvelope
Set pEnv = pActiveView.Extent
pEnv.Expand 0.5, 0.5, True
pActiveView.Extent = pEnv
' Get extent property of view.
' Shrink envelope.
' Set By Value extent back on IActiveView.
Dim pFeatureLayer as IfeatureLayer
Set pFeatureLayer = New FeatureLayer ' Create New Layer.
Set pFeatureLayer.FeatureClass = pClass ' Set ByRef a class into layer.
As you might expect, some properties are read-only, others are write-only, and
still others are read/write. All the object browsers and the ArcObjects Class Help
(found in the ArcGIS Developer Help system) provide this information. If you
attempt to use a property and either forget or misuse the Set keyword, Visual
Basic will fail the compilation of the source code with a “method or data member
not found” error message. This error may seem strange since it may be given
for trying to assign a value to a read-only property. The reason for the message is
that Visual Basic is attempting to find a method in the type library that maps to
the property name. In the above examples, the underlying method calls in the
type library are put_Extent and putref_FeatureClass.
Working with methods
Methods perform some action and may or may not return a value. In some instances,
a method returns a value that’s an interface; for example, in the code
fragment below; TrackCircle returns an IPolygon interface.
Dim pCircle as IPolygon
set pCircle = MapControl1.TrackCircle
In other instances, a method returns a Boolean value that reflects the success of
an operation or writes data to a parameter; for example, the IsActive method of
IActiveView returns a value of true if the map is active.
Be careful not to confuse the idea of a Visual Basic return value from a method
call with the idea that all COM methods must return an HRESULT. The VBVM
is able to read type library information and set up the return value of the VB
method call to be the appropriate parameter of the COM method.
Working with events
Events let you know when something has occurred. You can add code to respond
to an event. For example, a command button has a Click event. You add code to
Chapter 4 • Developer environments • 87