* VB-CODE (2)
Tip 150: Determining How Many Items Can Be Displayed in the List Box
         Control

September 5, 1995

Abstract
The List Box control in Microsoft? Visual Basic? allows you to display
a list of items to the user of your program. When you design a List Box
control in a Visual Basic program, you can use the Microsoft Windows?
application programming interface (API) SendMessage function to determine
the size of individual items in the control. This article explains how
you can calculate the number of items that can be displayed in the
control, based on the size of each entry.

Determining How Many Entries Can Be Stored in a List Box
The List Box control allows you to display a list of items to your user.
For example, you can store a list of vendors in a List Box control. When
your Microsoft? Visual Basic? program is running, you can add or remove
entries from the vendor list.
When you design your application, you can set the initial size of the
List Box control. Usually you would make the List Box large enough to
display several entries. At run time, you can programmatically determine
how many entries you can display at one time in the control. To do this,
you need to determine the height of an entry in the List Box control by
sending an LB_GETITEMHEIGHT message to the control.

Because you want to find the height of an entry in the List Box control,
you use the Microsoft Windows? application programming interface (API)
SendMessage function to send an LB_GETITEMHEIGHT message to that control.
After the program calls the function, the entry's height is returned as a
long value. Then, to calculate how many entries can be displayed in the
List Box control, you must divide the height of the List Box by the height
of the entry. When making this calculation, you first need to account for
the height (which is one pixel) of the top and bottom borders of the List
Box. Therefore, you subtract two from the height of the List Box control.
Next, you divide the height of the List Box by the height of the entry.
The result of this calculation gives you the number of items you can
display in the List Box control.

Example Program
This program shows how to calculate the number of items that can be
displayed in a List Box control.

 1. Create a new project in Visual Basic. Form1 is created by default.
 2. Add the following Constant and Declare statements to the General
    Declarations section of Form1 (note that the Declare statement must
    be typed as a single line of code):

Private Declare Function SendMessage Lib "user32" Alias "SendMessageA"
   (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Integer,
   ByVal lParam As Long) As Long
Const LB_GETITEMHEIGHT = &H1A1

 3. Add the following code to the Form_Load event for Form1 (note that
    the Items = line must be typed as a single line of code):

Private Sub Form_Load()
    Dim ItemHeight As Long
    Dim Items As Integer
 
    ItemHeight = SendMessage(List1.hwnd, LB_GETITEMHEIGHT, 0, 0&)

    Items = (List1.Height - 2 * Screen.TwipsPerPixelY)
        / (Screen.TwipsPerPixelY * ItemHeight)
    Text1.Text = "Maximum # entries : " & Str(Items)
 
    List1.AddItem "Item #1"
    List1.AddItem "Item #2"
    List1.AddItem "Item #3"
End Sub

 4. Add a Text Box control to Form1. Text1 is created by default.
 5. Add a List Box control to Form1. List1 is created by default.
 6. Add a Command Button control to Form1. Command1 is created by default.
 7. Add the following code to the Click event for Command1 (note that the
    Items = line must be typed as a single line of code):

Private Sub Command1_Click()
    Dim ItemHeight As Long
    Dim Items As Integer
    Dim X As Integer
 
    ItemHeight = SendMessage(List1.hwnd, LB_GETITEMHEIGHT, 0, 0&)
    Items = (List1.Height - 2 * Screen.TwipsPerPixelY)
        / (Screen.TwipsPerPixelY * ItemHeight)
    X = List1.ListCount
    X = X + 1
    If X <= Items Then
        List1.AddItem "Item #" & Str(X)
    End If
End Sub

Run the example program by pressing F5. Three items are displayed in the
List Box control. In addition, the Text Box control shows the maximum
number of items that can be displayed in the List Box control at any
one time. Each time you click the command button, you add a new item
to the List Box control. However, if the number of items stored in the
List Box control equals the maximum number of items that can actually
be displayed, you can add no more new entries to the list.

Additional References
"LB_GETITEMHEIGHT." (Development Library, Product Documentation, SDKs,
   Win32 SDK, Win32 Programmer's Reference, Reference, Messages,
   EM_SETLIMITTEXT to LB_SETCARETINDEX)
"List Box Controls." (Development Library, Technical Articles,
   Windows Articles, User Interface Articles, Controls, List Box Controls)
"SendMessage." (Development Library, Product Documentation, SDKs,
   Win32 SDK, Win32 Programmer's Reference, Reference, Functions,
   RemoveProp to SetBrushOrg)