10. Objects

10.1 Referencing external objects

Object types can be added to your project from external libraries. Since types can exist within many kinds of project, object types can be added from Executable files (".exe"), Dynamical Link Libraries (".dll"), ActiveX controls (".ocx"), Object Libraries (".olb") or Type Libraries (".tlb").

To include the object type in your project, you have to create a Reference to it. Once the Reference is in place, you will be able to declare instances of object variables of the same type defined within the external library.

Exercise:

Create a "Standard EXE" project within Visual Basic.
Select the "References." option from the "Project" menu. Scroll down the list in the dialogue box until you see "Microsoft Access 8.0 Object Library" and tick the box beside it. Click the "OK" button.
Double-click the default Form to open the Code View window. At the top, add declaration for a new object variable:

Dim objMyAccessObject As Access.Application

Note: Notice that as you type the line above, Visual Basic prompts you to insert a type from a pop-up list of currently installed types. Once you enter the dot (".") after "Access" , Visual Basic prompts you again for the types within the "Access" type library.

10.2 Object Variable Hungarian Notation

As with variables, objects can be named anything you like, as long as you don't use a reserved word. However, the Hungarian Notation model referred to earlier also extends to object names, and again the prefix is used to indicate the object type.

Object Type	Prefix	Example
3D Panel	pnl	pnlGroup
ADOData	ado	adoBiblio
AnimatedButton	ani	aniMailBox
CheckBox	chk	chkReadOnly
ComboBox or drop-down ListBox	cbo	cboEnglish
CommandButton	cmd	cmdExit
CommonDialog	dlg	dlgFileOpen
Communications	com	comFax
Control (unknown type)	ctr	ctrCurrent
Data	dat	datBiblio
Data-bound ComboBox	dbcbo	dbcboLanguage
Data-bound Grid	dbgrd	dbgrdQueryResult
Data-bound ListBox	dblst	dblstJobType
Data Combo	dbc	dbcAuthor
Data Grid	dgd	dgdTitles
Data List	dbl	dblPublisher
Data Repeater	drp	drpLocation
Date Picker	dtp	dtpPublished
DirListBox	dir	dirSource
DriveListBox	drv	drvTarget
FileListBox	fil	filSource
Flat ScrollBar	fsb	fsbMove
Form	frm	frmEntry
Frame	fra	fraLanguage
Gauge	gau	gauStatus
Graph	gra	graRevenue
Grid	grd	grdPrices
Hierarchical FlexGrid	flex	flexOrders
Horizontal ScrollBar	hsb	hsbVolume
Image	img	imgIcon
ImageCombo	imgcbo	imgcboProduct
ImageList	ils	ilsAllIcons
Label	lbl	lblHelpMessage
Line	lin	linVertical
ListBox	lst	lstPolicyCodes
ListView	lvw	lvwHeadings
MAPI message	mpm	mpmSentMessage
MAPI session	mps	mpsSession
MCI	mci	mciVideo
Menu	mnu	mnuFileOpen
OLE container	ole	oleWorksheet
OptionButton	opt	optGender
PictureBox	pic	picVGA
PictureClip	clp	clpToolbar
ProgressBar	prg	prgLoadFile
Remote Data	rd	rdTitles
RichTextBox	rtf	rtfReport
Shape	shp	shpCircle
Slider	sld	sldScale
Spin	spn	spnPages
StatusBar	sta	staDateTime
SysInfo	sys	sysMonitor
TabStrip	tab	tabOptions
TextBox	txt	txtLastName
Timer	tmr	tmrAlarm
Toolbar	tlb	tlbActions
TreeView	tre	treOrganization
UpDown	upd	updDirection
Vertical scroll bar	vsb	vsbRate

These are only proposed prefixes, so you don't necessarily have to stick to them.

10.3 Object Variables

When an external object is referenced, instances of the object can be created with Object Variables. These special variables are declared as the type specified by the original object (e.g. If you want a new instance of an ADODB object, you would declare it as type ADODB).

Once the variable is declared as that type, Visual Basic knows what system resources (memory etc) to reserve for this object.

Example: Creating a new ADODB data control recordset

Dim m_objADODBRec1 As ADODB.Recordset
		
Private Sub Form_Load()
    -- etc --
    Set m_objADODBRec1 = New ADODB.Recordset
    -- etc --
End Sub

To point the object variable to the object, you must use the Set keyword. The New keyword tells Visual Basic to point the object variable "m_objADODBRec1" to a new instance of the object.

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