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Fig 5.

Modules and menu functions

Form module

Form

Simple module

basCommon

Catch error when no

stay window is selected

Get stayID in the

selected Stay window

Change state for the

selected Stay window

Ask user to

select a stay window

5. Access through Visual Basic

111

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5.7.2 Define the menu item

We have now written the menu function. It is time to

connect it to the menu.

4. Close VBA, right-click the toolbar area and select

Customize.

5. Roll down the Stays menu, right-click CancelStay,

and select Properties (Figure 5.7B). Set the

OnAction property to:

=mniCancelStay( )

6. Close the customize boxes and try out the menu:

Open a stay through FindGuest, select the stay,

and use the menu point CancelStay. Unless you are

very, very lucky and careful, there will be errors in

your mni-procedure. Don't worry - it is normal.

Find the errors and repair them.

You may later set the stay back to booked or checked-

in with the buttons in the stay window. Also check that

the program behaves correctly when you use Cancel-

Stay without having a stay window in focus.

Menu procedures in the form module

Above, we put the menu procedure in a simple module.

The advantage is that we can call the procedure inde-

pendently of which forms are open or in focus. The

disadvantage is that we have to check that the right

form is in focus.

In some cases it is more convenient to have the menu

procedure in the form module. If you like, you can

make an experiment with how to do it.

•

Add another menu item to the Stays menu. Call it

CancelLocal as on Figure 5.7B. Set its OnAction

property to

=mniCancelLocal( )

•

Open the VBA module for frmStay and insert a

function that looks like mniCancelStay. However,

it should have the name mniCancelLocal. Further,

it should not use Screen.ActiveForm but Me in-

stead. The reason is that this function will be

called in the context of the form module, meaning

that the controls are always available. As a result,

you don't need all the error handling stuff.

•

Try out the new menu item. It should work cor-

rectly when used from a stay window, but gives

Access-language error messages when used from

other forms. Actually, Access cannot even find the

function if the form is not in focus.

This way of calling a menu function is particularly

suited when the menu is in a toolbar that is specific for

the form. You may remember (section 3.5.3) that tool-

bars may be attached to a form in such a way that the

toolbar is only shown when this form is in focus. In

this way the menu functions on the toolbar are always

available when the user can click on the menu item.

5.7.3 Managing modules and class modules

You can create a module through the database

window's Module tab, but the usual way is to do it

through the VBA editor. However, things work in a

strange way here. Figure 5.7C shows how to manage.

•

To create a module, right click an item in the

Project Explorer window. Select Insert and either

Module or Class Module.

You can now edit the module in the code window.

•

To name or rename a module, select it and use the

property icon on the tool bar. (You cannot right

click to change it.) Edit the name in the property

window.

•

To delete a module, select it, right click and use

Remove . . .

Class module

A class module corresponds to a class in other object-

oriented languages. It has procedures and declares

variables. You can create multiple objects based on the

class, each with their own variables. The only

difference between form modules and class modules is

that the latter are not visible to the user and have no

controls.

In order to create an object of class claGuest, declare a

reference to it and create it with New. Address public

variables and procedures in the object with the dot

notation:

Dim guest As claGuest

' References to a Guest object

. . .

Set guest = New claGuest

guest.address = . . .

guest.SendLetter(" . . . ")

This is similar to creating multiple open forms (section

5.5.12). Beware: the claGuest objects are just for

illustration. They exist only in memory. They are not

stored in the database and they have nothing to do with

the guest records in the database.

Module (simple)

A simple module is similar to a class but there is only

one object based on the module. The system creates

this object automatically. In order to address a public

procedure or variable in the module basCommon, use

the dot notation:

basCommon.simDate = . . .

d = basCommon.getDate()

112

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Properties for

Cancel stay menu item

Call mniCancelStay

Properties for

Cancel stay menu item

Fig 5.

Action for menu items

Fig 5.7C Managing modules and class modules

Select properties

to change module name

Right click to

insert Module or

Class Module

Delete module

Property

window

Project

explorer

5. Access through Visual Basic

113

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5.7.4 Global variables

The only persistent data in our application is the data in

the database. Until now the only dialog data has been

some data in the forms, for instance the value of the

unbound controls. This data disappears when the form

closes, so we need some way to handle dialog data that

lives across the entire dialog with the user. Modules is

the solution to the problem because a module is open

and holds data as long as the application is running.

As a simple example, we will see how to handle a

simulated today's date. In the real system, we need to

show today's date in many places, for instance as the

default search criterion for arrival date. It is easy to do

by means of the built-in function Date( ) which always

gives us today's date (or rather the current date setting

in the computer).

However, when testing the system, our test data is

planned for some specific dates. We cannot change the

test data every day we need it. We might instead

change the current date setting of the system to the

simulated date, but this is not recommended because it

has side effects on other things, for instance the date of

files created or changed, which again may create havoc

in automatic backup procedures.

So let us create a simulated date and keep it in the bas-

Common module. Figure 5.7D shows the solution.

1. Open the basCommon module and enter this line at

the top

Public simDate As Date

Also create the public function getDate( ) as

shown.

The variable simDate is a variable in the simple mod-

ule and lives as long as the application is open. The

function getDate simply retrieves the simulated date

and returns it as its own value.

2. Open the module for frmFindStay and create the

load procedure as shown.

This is the place where the program will set the simu-

lated day to be used. The first thing we will do in the

hotel system is to open frmFindStay, and at that mo-

ment the Load procedure will be executed. It will set

the simulated date to the 23rd October 2002, the date

used in several pictures in the User Interface Design

book. Note how a date is written inside VBA. The for-

mat is a US date, independent of any regional settings

in Microsoft Windows. Note also how we address

simDate through basCommon. We might omit bas-

Common, but if we have many modules, we resolve

any name ambiguity in the way written. It also allows

the VBA editor to guide us in selecting the right ele-

ment in basCommon.

3. Look at the property box for the arrival date. De-

fine the default value as this expression:

=getData( )

4. Open frmFindStay and check that the default value

for the arrival date is correct. The user can of

course change the arrival date to what he likes.

The arrival date is a combo box and the real system

provides a list of dates around today for the user to

choose. The default day is still important, however.

You may wonder why we write basCommon.simDate

in the Load-procedure and =getDate() in the property.

The answer is that in properties we can only use public

functions, not public variables. Furthermore we cannot

use the basCommon prefix. The same rule applied

when we specified the OnAction property of the menu

item.

Once our system is tested and ready for use, how do we

let it use the real date? One simple way is to keep eve-

rything and just change the getDate function to

Public Function getDate( ) As Date

getDate = Date( )

End Function

The system will work exactly as before except that it

gets the real today instead of the simulated one.

114

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Fig 5.7D Global variables

Simple module

basCommon

Global variable

Form module

frmFindStay

Property box

cboArrival

Reference to

global variable

Access function

5. Access through Visual Basic

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6. Visual Basic reference

In this chapter we give an overview of VBA, the Visual

Basic language for Applications. We assume that you

know something about programming already. We also

assume that you have looked at some of the Visual

Basic examples in the booklet so that you have seen

program pieces written in this language.

Our discussion is mainly based on the examples shown

on the figures. Most of these figures are also available

as the VBA Reference Card. It may be downloaded

from www.itu.dk/people/slauesen

If you want additional explanation, you have to use the

on-line help or experiment on your own. Be prepared

that the official documentation (on-line help) is often

incomplete or outright wrong, in the sense that the

system does something different than described. The

examples we show in the figures are based on testing

what the system actually does.

Using VBA functions outside VBA. Although we de-

scribe VBA below, most of the built-in functions and

operators are available also in SQL-statements and in

some control properties (e.g. ControlSource). In these

places you may for instance use the functions

IIF(a, b, c)

and

DMin("roomId", "tblRooms", "roomType=2")

However, the regional settings may influence the syn-

tax. As an example, with Central European settings,

you have to separate parameters with semicolon when

working outside VBA. (See more in section 6.6.)

Also notice that when you use the functions from

VBA, you get excellent help and excellent error mes-

sages, but when using them in SQL or ControlSource,

you get little help and very confusing error reactions.

6.1 Statements

Line continuation. A simple VBA statement consists

of a line of text. If the statement is too long for a line,

you can split it into two or more lines. To do this, you

write a space followed by an underscore at the end of

the line (Figure 6.1A). You cannot break the line in the

middle of a text string. You have to compose a long

text from shorter texts joined with the &-operator.

Comment. You can write a comment at the end of the

line. It starts with an apostrophe ( ' ). The compiler then

ignores the rest of the line. You can only have com-

ments on the last of the continued lines.

Assignment statement. An assignment statement

computes a value and stores it in the variable to the left

of the =. The Set statement is a special version of as-

signment. It doesn't store a computed value, but a ref-

erence to some object. The figure shows how it stores a

reference to the first open Form, how it creates a new

open form object and stores a reference to it, and how

it can set the reference to point at nothing.

Whenever you set a reference, VBA checks whether

this overwrites an earlier reference. If so, VBA also

checks whether this is the last reference to the object,

and if so it deletes the object since nobody can refer to

it any more (this is called garbage collection).

Conditional statements

Conditional statements are executed when some condi-

tion is met. They are examples of compound state-

ments, which may consist of more than one simple

statement. As Figure 6.1A shows, there are several

kinds of conditional statements.

Simple If-Then. The simplest version consists of an If-

Then clause followed by a single statement, which is

executed when the condition is True. It must all be on

one line, possibly broken with line continuations.

If-Then-Else. The more general version consists of an

If-Then clause followed by one or more statements,

which may be compound themselves. These statements

are executed when the condition is True. If the condi-

tion is False, the program continues with any ElseIf-

Then clauses, each testing their own condition, and

passing the control on if the condition is False. If all

these conditions are False, the program continues with

the statements after any Else clause. The net result is

that the statements after at most one of the clauses are

executed.

Select-Case is often a more elegant way to choose

between statements. In the example, we test the vari-

able zip. If zip is 4000 the program executes the state-

ments after Case 4000. If zip is 4001 or between 5000

and 5999, it executes the statements after this clause.

And if none of this is True, the program executes any

Case-Else statements. Again, the net result is that the

statements after at most one of the clauses are exe-

cuted.

116

6. Visual Basic reference

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Fig 6.1A Visual Basic Statements

Conditional statements

If a=1 Then c=d+2

‘Single statement

If a=1 Then

c=d+2 . . .

‘Multiple statements

ElseIf a=2 Then

c=d/2 . . .

Else

c=0 . . .

End If

Select Case zip

Case 4000

type = a . . .

Case 4001, 5000 To 5999

type = b . . .

Case Else

type = c . . .

End Select

On Error Resume Next ‘Ignore error

. . . IfErr> 0 Then. . . ' Test for error

On Error GoTo fail

‘Enable error handler

. . .

fail: MsgBox( . . . )

‘Continue here at error

On Error GoTo 0

‘Let VBA handle errors

Optional

Line continuation, comments, assignment

i = i+2 ‘Comment

s = “long text A”& _

“long text B” ‘Comment in last line only

Set f = Forms(0)

Store a reference

Set f = New Form_frmG Create object, store ref

Set f = Nothing

Delete object if last ref

Error handling

On-Error statements switch error trapping on and off

inside the current procedure. In order to allow the pro-

gram to handle errors at all, you have to set an option

in VBA:

Access 2000 and 2003:

Tools -> Options -> General ->

Break on Unhandled Errors

Access 97:

Tools -> Options -> Advanced ->

Break on Unhandled Errors

After On Error Resume Next, any program error or

other unexpected situation just skips the statement

where the error occurred. For instance, if the error

occurred during an assignment to x, nothing will be

assigned to x, so x is left unchanged.

After On Error GoTo L, any unexpected situation

causes the program to continue at label L. Should fur-

ther errors occur here, they cause the procedure to re-

turn with an error condition to be treated by the calling

procedure.

After On Error GoTo 0 (zero), VBA will handle all

unexpected situations, halting the program if necessary.

When the procedure returns, the calling procedure will

handle all errors according to its own On-Error set-

tings.

When Access detects a program error, it sets an Err

object with information about the error. Err has several

properties, for instance

Err.Number (or just Err): The error ID. Err = 0

means no error.

Err.Source: The program that generated the error.

Err.Description: A text describing the error (or giving

the error message).

Notice that Access doesn't clear the Err object until the

procedure returns. This can be confusing in program

patterns where the program tries various things to

succeed:

On Error Resume Next

. . . Do something that may cause an error

If Err > 0 Then

. . . Try something else

If Err > 0 Then . . . Give up

If Try something else actually succeeds, Err is still > 0

and the program gives up by mistake. The right pattern

is to use

Err.Clear

Err = 0

just before Try something else.

The main problem when using the Err object is to find

out what the error ID means. I have not seen a list of

the codes. The idea is that each subsystem defines its

own error ID's, but this makes it even harder to know

the ID's.

6. Visual Basic reference

117

Loop statements

Loop statements repeat one or more statements until

some condition is met or until an explicit Exit from the

loop (Figure 6.1B). The repeated statements may be

compound themselves.

While-Wend repeats the statements as long as the

While-condition is True. If the condition is False from

the beginning, none of the statements in the loop will

be executed. It is not possible to break the loop with an

Exit-statement.

Do-While-Loop is similar to While-Wend, the only

difference being that it is possible to break out of the

loop with an Exit Do.

Do-Loop-While is also similar, but the condition is

tested at the end of the loop, meaning that the state-

ments will be executed at least once.

For-To-Next updates the loop variable (i in the exam-

ple) for each trip around the loop. In the example, i was

one for the first round through the loop. Then i was

increased by 2 before the next trip, and so on. When i

>last, the loop terminates. If i >last from the beginning,

the loop is not executed at all. Statements inside the

loop may break out of the loop with an Exit For. In this

case the value of i is defined. However, if the loop ter-

minates due to i >last, it is not defined what the value

of i will be. So don't even rely on it being >last.

For-Each-Next scans through all objects in a collec-

tion. The example shows how to scan through all

Forms in the collection of open Forms. The reference

variable f will in turn point to each of the objects in the

collection. It is possible to break out of the loop with

an Exit For.

118

6. Visual Basic reference

Fig 6.1B Loop statements

Loops

While a<10

‘Maybe empty loop

c=c*2

. . .

‘Exit not allowed

Wend

Do While a<10

‘Maybe empty loop

c=c*2

. . . Exit Do

‘Exit optional

. . .

Loop

‘Loop at least once

c=c*2

. . . Exit Do

‘Exit optional

. . .

Loop While a<10

For i=1 To last Step 2 ‘Step optional

c=c*2

‘Maybe empty loop

. . . Exit For

‘Exit optional

. . .

Next i

‘Don’t trust value of i when loop ends without Exit

For Each f In Forms

‘Scan collection

call print(f.name . . . )

. . . Exit For

‘Exit optional

. . .

6. Visual Basic reference

119

6.2 Declarations

The ancestor of Visual Basic, the programming lan-

guage Basic, was designed for teaching programming

with a minimum of formalities about the writing. This

meant that there was no need to declare variables at all.

Basic created the necessary variables as they were

needed. This is still possible in Visual Basic, but soft-

ware developers agree that when developing larger

programs it is a huge advantage to declare all variables.

You can tell VBA to insist on declarations. Write this

specification at the beginning of the module

Option Explicit

Variant type. Even if you declare a variable, you don't

have to specify its type. Without an explicit type, the

variable is of type Variant. This means that its actual

type may change dynamically according to what the

program stores into it. It may thus hold a number at one

point in time, a text string at another point in time.

Apart from the value in the variable, VBA also stores a

tag telling what the type is at present.

Simple variables

Declarations of variables usually start with the word

Dim (for dimension) followed by a list of the variables.

Figure 6.2A shows such a list

Dim B, C As Byte

The result is that variable B is of type Variant and vari-

able C of type Byte. Unfortunately this is counterintui-

tive and cumbersome. Most people would believe that

B as well as C are of type Byte. You have to specify a

type for each of the variables to avoid them becoming

variants.

Simple variables may be of the types shown on the fig-

ure: Byte, Boolean, Integer, etc. We have met most of

them already. Byte, Integer and Long hold integers

with different range. Single and Double hold floating

point numbers with at least 6 significant digits (Single)

or 14 significant digits (Double). (See section 2.2 for

details.)

Currency. The Currency type is a funny in-between

intended for keeping track of monetary amounts. It is a

very long integer (64 bits) interpreted as this integer

divided by 10,000. This means that the integer 147,000

is the number 14.7 exactly. Why this rule? It is because

we can guarantee that amounts are rounded correctly as

a bookkeeper would do it. With floating point numbers

you have no control over the round-off.

Date. A date value is technically a Double. The integer

part is the number of days since 12/30-1899 0:00, the

fractional part is the time within the day. As an exam-

ple, the number 1 corresponds to 12/31-1899 at 0:00,

the number 1.75 to 12/31-1899 at 18:00 (6 PM).

Object and Form variables are references to objects,

not the objects themselves. You can set the references

by means of the Set-assignment and test them by

means of the Is-operator.

Variant. You can explicitly declare the variable as

Variant, but if you don't specify a type, the variable is

Variant anyway. Variants can not only hold the simple

values above, but also values such as Null or Empty.

Notice that VBA treats all fields in database records as

Variant.

Initial values. When a record field is empty, it has the

value Null. When a Variant is just created, it has the

value Empty, because no memory is allocated for the

value. When a String is just created, it holds a text of

length 0. When a numerical variable is just created, it

holds the value 0.

Strings come in two versions. Strings of variable

length and strings of fixed length. The former change

their length to match what the program stores in them.

The length may be very long, nominally 2 billion, in

practice limited by memory space. Strings of fixed

length always use space from the left, and fill up the

remaining characters with spaces (blanks).

The field types text and memo correspond to VBA

strings of variable length.

Arrays

Arrays can have one or more dimensions. In Figure

6.2A, array c has two dimensions. The first index

ranges from zero to 5 (zero is the default), the second

from 1 to 6. You may specify the type of the elements

of the array. If omitted, they are Variants.

The second array, d, is dynamic, meaning that its di-

mensions and index ranges can change over time. The

program can dynamically change the dimensions and

ranges by means of the Redim statement, but in general

the values stored in the array don't survive this opera-

tion. You can use Redim Preserve if you only change

the range of the last dimension. In that case, the values

survive.

You can release the memory occupied by a dynamic

array by means of the Erase statement.

Type declarations

You can declare types of your own (user-defined

types). They will typically take the form of a record

declaration as shown on the figure. Notice that each

field of the record must be specified on a line of its

own.

You can only declare types in simple modules, not in

Form modules. Once you have declared a type, you can

use it for declaring variables and arrays.

Procedures

There are two kinds of procedures: subroutines and

functions. The only difference is that a function returns

a value. For this reason you can store the result of a

120

6. Visual Basic reference

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