Contents
# How to use a
single select List box?
# What are the
properties of List box?
# How to use
multiselect List box?
# What are the
properties related to multiselect List box?
# What is a Combo
box and how to use it?
What is a List Box?
List box is one of the intrinsic controls.
List box is used to allow user to select one or more of the given list of
items. A List box can contain any number
of items, but the number of items that a List box can display depends upon the
display size of the List box. If List box contains more items than what can be
displayed then automatically a scrollbar
is attached to List box.
Now, let us develop a small program to
understand how to handle a List box.
Here are the required steps to create a new
project and user interface.
1.
Start a new project using File->New Project
2.
Select Standard Exe in list of project types and click on Ok.
3.
Place the required control on
the form
4.
Change the properties as
follows.
|
Control
|
Property
|
Value |
|
Form1
|
Name
|
Frmlist
|
|
|
Caption
|
&List box
Demo
|
|
Label1
|
Name
|
Lblnames
|
|
|
Caption
|
Names
|
|
List1
|
Name
|
Lstnames
|
|
Lable2
|
Name
|
Lblname
|
|
|
Caption
|
&Name
|
|
Text1
|
Name
|
Txtname
|
|
|
Text
|
(null string)
|
|
Command1
|
Name
|
Cmdadd
|
|
|
Caption
|
&Add Name
|
|
Command2
|
Name
|
Cmddelete
|
|
|
Caption
|
&Delete
Name
|
|
Command3
|
Name
|
Cmdquit
|
|
|
Caption
|
&Quit
Program
|
|
Label3
|
Name
|
Lblcount
|
|
|
Caption
|
# Names : 0
|
Now let us write code. We have to write
code for Click event of three
command buttons. We first concentrate on how to add an item to List box and how
to delete an item from List box.
Adding an item to List box
You add an item to List box using Additem method of the List box. Additem method takes an item, which is
a string, and adds that item at the end of the List box. It is also possible to
add an item at the specified index using index
parameter of Additem method.
Note: If List box is a sorted List
box (Sorted property is set to True) then item is placed in the List box
according to the order of the item.
The following is the syntax of additem method.
AddItem item, index
|
Object
|
Is any List box.
|
|
Item
|
Is the item to be added to List box. This is
a string expression.
|
|
Index
|
It is optional. If it is
given then item is added at the specified position. Index starts from 0
|
Example:
To
add name P.SRIKANTH to List box, enter:
List1.additem “P.SRIKANTH”
To
add an item at 2nd position in the List box, enter:
List1.additem “Praneeth”, 2
Removing an item from List box
You can remove an item from List box using Removeitem method of the List box. Removeitem method takes the index of
the item to be removed.
Here is the syntax of Removeitem method of List box:
RemoveItem index
|
Index
|
The index of the item that is to be
removed from List box. Index must be in the range 0 to ListCount –1. ListCount property
contains the number of items in the List Box.
|
To
remove item at 1st position (second item) of List box, enter:
List1.removeitem
1
Writing Code for cmdadd and cmddelete buttons
Now let us write code to add the name
entered in textbox to List box. The code is written for Click event of add (cmdadd) button. After name is added to List box
the name is removed from textbox and focus is set to textbox so that a new name
can be entered by user. As this change effects the count of items in the List
box, update names count by changing Caption
property of lblcount.
Private Sub cmdadd_Click()
' add
item to List box
lstnames.AddItem
txtname.Text
'update #
items display
lblcount.Caption = "# Names : "
& Str(lstnames.ListCount)
' remove
text from textbox
txtname.Text = ""
' set
focus to textbox
txtname.SetFocus
End Sub
Listing 5.1: Code for
adding an item.
Now let us write code to delete the item
that is currently selected in the List box. To remove an item from the List
box, use Removeitem method of the
List box. Removeitem method takes
index of the item to be removed. As we have to delete currently selected item,
we use the index of the item that is currently selected. ListIndex
property returns the index of the item that is currently selected. Finally
update count of items as deletion of an item decrements the count.
Private
Sub cmddelete_Click()
'attempt
to delete only when an item is selected
If
lstnames.ListIndex >= 0 Then
'
delete the item that is currently selected
lstnames.RemoveItem lstnames.ListIndex
lblcount.Caption = "# Names : " & Str(lstnames.ListCount)
Else 'no item is currently selected
Beep
End If
End Sub
Listing
5.2: Code for deleting the current item of List
box.
Test Run
Follow the steps given below to test
whether the program is doing what it is supposed to do.
1.
Run program by pressing F5.
Note: The input focus is in List box and not in textbox. So you cannot
enter any text straight away into textbox.
Press tab key to move from
List box to textbox. We will fix this problem later.
2.
Enter any name in textbox and
click on Add Name button.
Entered
name will be added to List box. Do this for a few times. Also note, the count
of names will be increased whenever a name is added to List box.
3.
Now click on Delete Name button.
As no
item in the List box is currently selected, this action should result in beep
sound. See listing 5.2.
4.
Select one of the names in the
List box and click on Delete Name button.
The selected name should be removed from the list and # Names should also be
decreased by one.
5.
Click on the Quit Program to quit program.
Setting focus to text Box instead of List box
We have seen in the test run that as soon
as the program is run focus is set to List box. This is so because List box is
the first control to be created out of the controls that can receive focus
(enterable controls). So its TabIndex
is set to 0. The control with least TabIndex
will receive the focus. But we have to initially set focus to textbox and not
to List box. This can be done using:
¨
TABINDEX property
¨
ACTIVATE event of Form
TABINDEX property
Each control on the form is assigned a
number depending on the order in which each control is created. The very first
control is given number 0, second control 1 and so on. This number can be
accessed and changed using TabIndex
property of the control. When the form is loaded, out of enterable controls
(controls that can receive focus such as textbox, command button etc) the
control with least TABINDEX is given
focus.
Note: Though TABINDEX exists
for non-enterable controls like labels, they do not have any influence on the
way focus moves between enterable controls.
You can change the order in which focus
moves between controls when user presses tab key using TabIndex property. For example, if you want a textbox to receive
focus first, then change the TabIndex
property of the textbox to 0. You need not change TabIndex of the control that now has TabIndex 0(i.e. List box), because Visual Basic automatically does
it for you.
So in our program, we can set focus to txtname instead of lstnames by changing TabIndex
of txtname to 0. However, note that in this example we are not much
concerned with the order in which focus moves between controls. If we are
concerned, then we have to change TabIndex
of all the controls to required values, so that focus moves in the required
direction.
Changing focus to textbox using ACTIVATE event
of form
Activate event of the form occurs whenever form is activated. This is
different from Load event, which occurs only when the form is initially loaded,
Activate event occurs whenever form
is activated. Activate event can be
used to send focus to a particular control as soon as the form is activated. So
in our program we use Activate event to set focus to textbox as follows:
Private
Sub Form_Activate()
'set
focus to textbox
txtname.SetFocus
End Sub
Listing
5.3: Code to set focus to textbox as and when form
starts.
Properties of List box
The following table lists properties of
List box. We have already used LISTINDEX and LISTCOUNT properties of List box
in the previous program.
|
Property
|
Meaning |
|
Listindex
|
Contains the
index of the item that is currently selected.
|
|
Listcount
|
Contains the
number of items the List box currently has.
|
|
Text
|
Contains the
item that is currently selected.
|
|
Sorted
|
If set to
true, items in the List box are always arranged in the ascending order.
|
|
List( )
|
This is an
array. Each element contains an item of the List box. List(0) refers to first
item, list(1) refers to second item and so on. The last element that can be
accessed is LISTCOUNT –1.
|
|
Style
|
Determines
the style of the List box. If it is set to 0-standard, then items are displayed as text items. If it is set
to 1-checkbox, then items are
displayed with a checkbox on the left. Multiple items can be selected by
checking the checkbox displayed on the left of the item.
|
|
Columns
|
Sets a value
that determines whether a List box scrolls vertically or horizontally and how
the items in the columns are displayed.
If it is set to 0, then only one column is displayed and List box
scrolls vertically. If it is set to>= 1 then items are arranged in snaking
columns, filling the first column, then the second column, and so on. The
List box scrolls horizontally and displays the specified number of columns.
|
Table
5.1: Properties of List box.
MultiSelect List boxes
So far we have seen List boxes where only
one item can be selected at a time. Now we will see how to select more than one
item at a time. We will also create a new program to understand properties and
logic related to multiselect List boxes.
Selecting multiple item using MULTISELECT
property
It is possible to select more than one item
by changing the value of Multiselect of List box property to either 1 or 2. By default this is set to 0, which
allows only one item’s selection at a time.
The following are the valid values for Multiselect property of a List box.
|
Value
|
Meaning
|
|
0 –None
|
Only one item can be selected at a time.
|
|
1 –Simple
|
Multiple items can be selected. Click on an
item to select and deselect.
|
|
2 – Extended
|
Allows a contiguous block to be selected
using SHIFT + mouse click.
|
Properties related to multiselect List box
The following are the properties related to
multiselect List boxes. Some of the properties of List box such as LISTINDEX do not have any meaning while
dealing with multiselect List box. And in the same way some of the properties
that are applicable to multiselect List box may not be applicable to normal
List box.
|
Property
|
Meaning |
|
SelCount
|
Contains the
number of items currently selected.
|
|
Selected( )
|
It is an
array. It returns true if the item with the given index is currently selected
otherwise it return false. For
example, Selected(0) returns true if
the first item is currently selected.
|
Table 5.2: Properties
related to multi-Select List box
Sample Application using multiselect List box
Now let us create a new project, which
allows user to select required subjects from the list of available subjects.
User can select more than one subject and move the selected subjects from
available subjects List box to selected subjects List box.contains
the program at runtime.
How does the application work?
¨
The multiselect demo
application displays a predefined set of subjects in first (left side) List box.
¨
User can select one or more
subjects and then click on >
button to move selected subjects to second (right side) List box.
¨
If user clicks on >> then all subjects from first
List box will be moved to second List box.
¨
User can select one or more
subjects in the right List box and click on < to move them to left List box.
¨
Selecting << will move all subjects from right List box to left List
box.
Steps to create multiselect List box demo
program
1.
Start a new project using File->New Project. And select Standard Exe as the project type.
2.
Place controls
3.
Change properties as follows.
|
Control
|
Property
|
Value
|
|
List1
|
Name
|
Lstavailable
|
|
|
Multiselect
|
1-simple
|
|
List2
|
Name
|
Lstselected
|
|
|
Multiselect
|
1-simple
|
|
Label1
|
Caption
|
Available
Subjects
|
|
Lable2
|
Caption
|
Selected
Subjects
|
|
Command1
|
Name
|
Cmdmlr
|
|
|
Caption
|
>
|
|
Command2
|
Name
|
Cmdmalr
|
|
|
Caption
|
>>
|
|
Command3
|
Name
|
Cmdmrl
|
|
|
Caption
|
<
|
|
Command4
|
Name
|
Cmdmarl
|
|
|
Caption
|
<<
|
|
Command5
|
Name
|
Cmdquit
|
|
|
Caption
|
&Quit
Program
|
I have named command buttons to represent
the direction in which the button moves subjects.
For example, CMDMALR means, command button
to move All items from Left to Right. The same convention is followed for other
cases as well.
Populating List box at design time
You can place a set of predefined values
into a List box at design time using the following procedure.
1.
Select the List box and press F4 to invoke properties window.
2.
Select List property and click on down arrow.
3.
Enter first subject in the
given empty area and select CTRL+ENTER
to move to next line.
4.
Continue the above process
until all values are entered and the press ENTER
key.
Note: To populate List box at runtime use ADDITEM method. If you want to
initially load a predefined set of values at runtime, use ADDITEM method in
LOAD event of the form.
Writing code for sample application
Now let us write code for command buttons
as follows:
Moving all subjects
Write code for >> and <<
command buttons. Command button >> moves all available subjects to
selected subjects List box. Command button << moves all subjects from
selected List box to available List box.
Here is the code for both the command
buttons.
' move all items from left list box to right list box
Private Sub
cmdmalr_Click()
Dim i As Integer
With lstavailable
'move all subjects to
lstselected
For i = 0 To .ListCount - 1
lstselected.AddItem
.List(i)
Next
' clear subjects from
lstavailable
.Clear
End With
End Sub
' move all items from right list box to left list box
Private Sub
cmdmarl_Click()
Dim i As Integer
With lstselected
'move all subjects to
lstavailable
For i = 0 To .ListCount - 1
lstavailable.AddItem
.List(i)
Next
'clear subjects from
lstselected
.Clear
End With
End Sub
Listing
5.4: Code to move all items from one List box to
another.
Now let us write code for > and < buttons. This code is slightly complicated, because you have
to find out what are the items that are currently selected and move only those
items. After moving the items to other List box they are to be removed from the
List box.
Here is the code to move selected subjects
from available subjects to selected subjects List box. The code invokes a
procedure called DELETEITEMS, which takes a List box as parameter and deletes
all the currently selected subjects from the List box.
Private
Sub cmdmlr_Click()
With
lstavailable
'
move currently selected subjects to LSTSELECTED
For i = 0 To .ListCount - 1
If .Selected(i) Then
' move current item
lstselected.AddItem .List(i)
End If
Next
End
With
'
delete currently selected subjects from LSTAVAILABLE by calling
'
DELETEITEMS procedure that takes the List box as the parameter
deleteitems lstavailable
End Sub
Listing
5.5: Code to move selected items from left to
right.
Creating user-defined procedure (DELETEITEMS)
The following are the steps to create
user-defined procedure DELETEITEMS.
1.
Invoke Code window by pressing F7.
2.
Select Tools-> Add Procedure.
3.
Enter the name of the procedure
as DeleteItems.
4.
Click on Ok. Now Visual Basic displays skeleton for the named procedure.
5.
Enter code between Sub and End
Sub.
Private
Sub cmdMRL_Click ()
With
lstselected
'
move currently selected subject to LSTAVAILABLE
For i = 0 To .ListCount - 1
If .Selected(i) Then
' move current item
lstavailable.AddItem .List(i)
End If
Next
End
With
'
delete currently selected subjects from LSTSELECTED by calling
'
DELETEITEMS procedure that takes the List box as the parameter
deleteitems
lstselected
End Sub
Private
Sub cmdmlr_Click()
With
lstavailable
'
move currently selected subject to LSTSELECTED
For i = 0 To .ListCount - 1
If .Selected(i) Then
' move current item
lstselected.AddItem .List(i)
End If
Next
End
With
'
delete currently selected subjects from LSTAVAILABLE by calling
'
DELETEITEMS procedure that takes the List box as the parameter
deleteitems lstavailable
End Sub
Listing
5.6: Code for > and < command buttons.
The following is the code for CMDQUIT
button.
Private Sub cmdquit_Click()
Unload Me
End Sub
Listing
5.7: Code for quit button.
Public
Sub deleteitems(lb As List box)
Dim i
as integer
‘
set the loop to move from bottom to top.
For
i = lb.ListCount - 1 To 0 Step –1
‘
delete the item if the item is currently selected
If lb.Selected(i) Then
lb.RemoveItem i
End If
Next
End Sub
Listing
5.8: Code for procedure DELETEITEMS.
Test Run
Take the following steps to understand what
this application can do!
1.
Run the project using F5.
2.
Select Oracle and Visual Basic
and click on > to move those two
subjects.
3.
Now click on << to move all selected subjects
back to Available subjects List box.
4.
Click on >> to select all available subjects.
5.
Select C++ and Java and click
on < to move subjects from Selected List box to Available subjects List box.
The following is the procedure DELETEITEMS
that is used to delete all selected items in a multiselect List box.
Combo box control
A Combo box control provides the features
of a List box, selecting an item from the list of items, and a textbox, where
user can enter a new value if the value is not existing in the supplied list.
A Combo box may be used in the following
cases:
¨
Where user wants to key-in the
value if the required value is not existing in the given list
¨
Where you have less space but
you have to accommodate a list of values. As Combo box displays list only on
demand (when user click on down arrow) and display only the selected item, it
doesn’t occupy much space on the form.
Properties of Combo box
Though most of the properties of List box
also apply to Combo box, here are the properties that are specific to Combo
box.
|
Property
|
Meaning |
|
Locked
|
If it is set
to True, you can enter value in
Combo box. You can only select existing value.
|
|
Style
|
Determines
the appearance and functionality of the Combo box. The following are the
valid values:
0 (Default) Dropdown Combo – Allows selection as well as editing the text.
1 - Simple Combo –
Displays List box, which doesn’t drop down and a textbox.
2 – Dropdown List -
Allows selection only from the drop-down list.
|
|
Selstart, Sellength, and Seltext
|
They carry the same meaning that they have for Textbox. See
chapter 3
|
Table
5.4: Properties that are specific to Combo box.
Note: multiple selection doesn’t apply to a Combo box. So properties like
Selcount, Selected() are not available to Combo box.
Exercises
1.
What property is used to get
the count of items in a List box?
2. What is the difference between ListCount
property and SelCount property?
3.
What is the return value of
property Selected (0) when first
item in a multiselect List box is selected.
4.
Which property returns the item
that is currently selected?
5.
Which property returns the
index of the item that is currently selected in List box?
6.
Is Selected property available for a Combo box?
7.
How do you add item “Teddy” at
5th position of a List box?
8.
Which method is used to delete
all the items of a List Box?
9.
When an item is removed using RemoveItem method, does it change ListCount property immediately?
10.
How do you make a Combo box a
drop-down List box?