create pdf thumbnail image c#: Edit pdf form application software tool html windows azure online AccessTutorial8-part967

Fig

3A VBA window and debugger

Project Explorer

Object Browser

List of controls

Events in

the control

Project explorer:

One class per form

Code module

if you have created the event procedure while looking

at the code in debug mode (see page 82).

The cure may be to select the event procedure from the

property box and compile the module (Debug ->

Compile), then close and open the form.

9. Create the missing event procedures for txtName as

shown on the Figure. Use the combo boxes to

create them. If you have followed the previous steps

closely, only the Enter procedure should be miss-

ing.

10. Type the debug statements shown in the proce-

dures, for instance:

Debug.Print "After", Me.txtName,

Me.txtName.Text

When executed, this statement prints something in

the Immediate window. We will explain more on

the Debug statement below.

Note that VBA automatically capitalizes the words that

it recognizes. If it doesn't, it may be because you have

spelled the word incorrectly, but it may also be because

the word comes after the bang operator (!). VBA can-

not recognize things after the bang at edit time, but it

will check all the words at execution time. At that time

it will give you an error message if it cannot recognize

the word.

Deleting an event procedure. If you want to delete an

event procedure, simply delete all the lines of the pro-

cedure. Don't try to delete it on the Event tab.

Closing the VBA window. You can close the VBA

window at any time. It only hides the window. Saving

the code doesn't happen until you close the form it be-

longs to. Sometimes you may want to save the code

explicitly. Use File -> Save (or Ctrl+S).

Debug command and event logging

The statement Debug.Print prints its parameters in the

Immediate window (also called the debug window). As

an example, when the AfterUpdate event occurs, the

event procedure will print the text "After" followed by

the current Value and Text. The event procedure for

Change behaves similarly, and as a result, the Immedi-

ate window will show a log of what happened. Try it:

11. Open the Immediate window with Ctrl+G. Adjust

the sizes so that you can see the Immediate window

as well as frmFindStay.

12. Type something in txtName. The Immediate win-

dow should log what happens.

This is the hard way to find out exactly which events

occur and what the situation is at these points. During

such experiments, you may want to temporarily disable

some statements. For instance, we might want to dis-

able one of the debug statements to avoid too many

lines in the Immediate window.

13. Comment-away statements that you don't want for

the moment. Set an apostrophe (a "ping") at the be-

ginning of the line. When you move the cursor, the

line turns green to show that this is a comment for

humans only.

5. Access through Visual Basic

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Breakpoints and debug

We are able to stop the program in the middle of an

event procedure. To do this, we set a breakpoint at the

code line where we want the program to stop. Figure

5.3A shows a breakpoint in the last event procedure -

marked with a big dot in the left margin. Try it:

14. Click at the left margin of the Debug.Print line. The

dot should appear and the line be high-lighted as on

the figure. You have set a breakpoint. (Clicking

again will remove the breakpoint. Don't worry

about the line Msgbox. We will add it later.)

15. Click in some other field on FindStay, then click in

txtName. This generates an Enter event in txtName.

As a result, the program stops just before executing

Debug.Print.

16. Current value. Try moving the cursor to an ex-

pression, for instance the Debug parameter

Me.txtName. After a moment, VBA shows the cur-

rent value of this expression. You can see the value

of expressions in the procedure where the execution

stopped, but not values in other procedures because

they are not active at present.

17. You can use the Immediate window to try out vari-

ous statements. The statements are executed as if

they were written where the breakpoint is. Try for

instance

Me.txtName.Text = "abc"

(should change the text in

the form)

? Me.subStayList.Form!name

(should print the

name of the first guest in the list)

In section 5.1 we used the Immediate window with

statements such as

Forms!frmFindStay!txtName = . . .

Now you can use Me. The reason is that now the Im-

mediate window runs in the context of the event proce-

dure. It can address the form object in the same way as

the program.

Correcting bugs at breakpoints. Using the Immediate

window in connection with a breakpoint is an impor-

tant way to find out what the program does. If you find

an error in the program, you may usually correct it

while at the breakpoint. However, sometimes VBA

cannot do the correction, for instance if you delete an

event procedure. It asks whether you want to "reset the

project". This question sounds threatening, but simply

means "stop the program execution and restart it from

the beginning". Nothing to worry about - nothing is

lost.

Continue after breakpoint. When you have made

your experiments at the breakpoint, you can resume

ordinary program execution. There are several ways to

resume execution (Figure 5.3A):

•

Run (F5). The program continues in the normal

fashion.

•

Step Into (F8). The program executes the next

statement, then stops again in breakpoint mode.

With repeated use of F8, you can execute the pro-

gram step by step. If the program calls another

procedure, you will step into it statement by state-

ment.

•

Step Over (Shift+F8). As F8, but if the program

calls another procedure, it executes all of the pro-

cedure without stopping, then stops at the return

from the procedure.

Stop an endless loop. If the program gets into an end-

less loop, you can stop it with Ctrl+Break.

Pop-up help

You have probably noticed that as you type a state-

ment, Access often shows a list of what you can type at

this point. You may bring up several kinds of lists:

•

Ctrl+J brings up the property list. It shows you

the possible properties, procedures, and controls at

this point of typing. You may choose one with the

Tab-key.

•

Ctrl+Shift+J brings up the Constant list. It shows

you the possible named constants at this point of

typing. (Ctrl+J may be used too, but it brings up

the full list of named constants.)

•

Ctrl+I brings up the Quick Info list. It shows the

data type you are dealing with, or the list of pa-

rameters to the procedure you call, or the value of

a named constant.

Try the pop-up help

18. Start entering this statement in the Change proce-

dure (Figure 5.3B):

MsgBox "test", vbYesNo

When you have typed MsgBox, use Ctrl+I to bring

up the list of parameters. Notice that most of the pa-

rameters are optional (enclosed in brackets). When

you have typed the comma, bring up the list of pos-

sible constants with Ctrl+Shift+J. Select the right

constant with the Tab-key.

When executed, the MsgBox statement will show a

message to the user and ask for a Yes or No. (In section

3.6, we used MsgBox to print messages for a tool-

based mockup.)

19. Put the cursor on RecordSource and use Ctrl+J to

bring up a list of possible properties and procedures

at this point.

20. Put the cursor on txtName and use Ctrl+J to bring

up a list of possible controls at this point.

5. Access through Visual Basic

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Quick Info

Ctrl+I

Constant list

Ctrl+Shift+J

Fig 5.3B Breakpoint, pop-up help and object browser

Object Browser

Classes

(objects)

Properties and

procedures for

the class

Sometimes a

good explanation.

Else use F1.

Debug actions:

Run

Step Into

Step Over

Shift F8

Break

Ctrl+Break

Open Immediate window

Ctrl+G

Breakpoint

Property list

Ctrl+J

Object Browser (F2) and Help (F1)

The Object Browser gives an overview of the classes in

Access, Visual Basic, and your own database. Open the

browser with F2 (Figure 5.3B). To the left you see a

list of the classes, to the right all properties and proce-

dures of the selected class (called "members" in the

window). In the figure we have selected the SubForm

class. The right-hand side shows the properties and

event procedures.

For some properties and procedures, VBA shows a

good explanation at the bottom of the window, for oth-

ers you get a good explanation with F1 (Help). The

Object Browser doesn't show all the classes available.

For instance you will look in vain for the Debug class,

which we have used several times already.

Another way to get help about classes, properties, and

language structures such as if-then, is to position the

cursor on the word in the VBA code, then use F1. Usu-

ally you get good help. Try it:

21. Put the cursor on the word MsgBox and use F1.

You get an excellent explanation of the procedure

and its parameters.

22. Put the cursor on the word RecordSource in the

code and use F1. You get a reasonable explanation.

23. The quality of the help information varies. Put the

cursor on the word Debug in the code and use F1.

You may get the message Can't find project or li-

brary. Or you get an explanation, but not an excit-

ing one. You may try F1 with the cursor on the

word Print. You get a lot of information, although it

is hard to find out what it means.

5. Access through Visual Basic

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5.4 Command buttons

When the user clicks a command button, it receives a

Click event. The event procedure must perform what

the button is planned to do. We will look at a few typi-

cal examples.

Make a button open another form

We will first let the New guest button open a stay form

in the simplest way (Figure 5.4).

1. On frmFindStay give the NewGuest button the

name cmdNewGuest. (The usual prefix for a com-

mand button is cmd.)

2. Define the event procedure for the OnClick event.

The procedure body should be:

DoCmd.OpenForm "frmStay"

The object DoCmd can do various things. Here we use

it to open a form. You can use PgDown to step through

all the stays, and even add a new stay at the end.

Open a form to show only one record

You may use OpenForm to open a specific stay:

3. Try to change the procedure body to

DoCmd.OpenForm "frmStay", , , "stayid=2"

If you try it in user mode, you see that stay 2 is visible

and the user may edit it. Parameter 4 did the trick. It is

a filter - a text that automatically enters an SQL-

WHERE clause and restricts the visible stays. In prac-

tice, the program must compute the filter text so that 2

in the example becomes the stayID the user has chosen.

Section 5.5.2 explains more about opening a form for

various purposes. Section 5.5.3 explains the many

parameters for OpenForm.

Make a button reset the search criteria

The real FindGuest window has a button for resetting

the search criteria. Try to add it:

4. In design mode, add a button to frmFindStay. Give

it the label Reset criteria and the name cmdReset.

5. Define the event procedure for the OnClick event.

The procedure body should be:

Me.subStayList.Form.RecordSource = _

"select * from qryStayList; "

Me.txtName = ""

This procedure sets the record source to the full list of

stays, and it sets the search criterion to an empty text. If

there are more search criteria, for instance also the

phone number, they should be set too, of course.

Other button events

The click event happens if the user clicks the button.

What happens if the user tabs to the button and then

presses Enter? Or if the user uses a shortcut key? The

button doesn't notice. It receives a click event in all

these cases.

A command button also receives the same events as a

text box, for instance Enter, GotFocus, MouseDown,

KeyPress. There is rarely a need to do something for

these events. A button doesn't receive Change, Before-

Update and AfterUpdate. These events deal with stor-

ing some data, and a command button doesn't store

anything.

Default button and Cancel button

A form may have a default button. If the focus is

somewhere on the form or its subforms, and the user

presses Enter, the default button gets a click event.

(The exception is when the focus is on another button.

Then this button gets the click, of course.)

You may define any button as the default. In the prop-

erty box for the button, select the Other tab and set

Default = Yes

Access will automatically set Default = No for the

previous default button on the form.

A form may also have a cancel button. If the focus is

somewhere on the form or its subforms, and the user

presses Escape, the cancel button gets a click event.

You may select any button as the cancel button. In the

property box, select the Other tab and set

Cancel = Yes

You will typically let the cancel button close the form

without saving anything. This can be done with these

statements in the event procedure:

Me.Undo

DoCmd.Close

The Undo procedure sets all fields on the form to their

OldValue. As a result, Access will not save them at

close.

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Fig 5.4 Command buttons, default and cancel buttons

Private SubcmdNewGuest_Click()

DoCmd.OpenForm"frmStay"

End Sub

Or open only one stay:

DoCmd.OpenForm"frmStay", , , "stayid=2"

Default button -responds at Enter:

Other tab -> Default = Yes

Private Sub cmdReset_Click()

Me.subStayList.Form.RecordSource= "select * from qryStayList; "

Me.txtName= ""

End Sub

Cancel button -responds at Esc:

Other tab -> Cancel = Yes

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5.5 Forms

Forms are very complex. They handle many events and

have many properties. Below we explain the more

important ones.

5.5.1 Open, close, and events

In Visual Basic you can open a form in this way:

DoCmd.OpenForm "name of the form"

OpenForm can have many parameters, as explained in

section 5.5.3. You can close the form that is in focus

with

DoCmd.Close

As an example, a button on the form could close the

form in this way. When the user clicks the button, the

form is in focus.

If you want to close another form than the one cur-

rently in focus, you can specify the form through two

parameters, for instance:

DoCmd.Close acForm, "frmStay"

The parameter acForm says it is a form. The last pa-

rameter is the form name.

An alternative is to set the form in focus, and then

close it, for instance like this:

Forms!frmStay.SetFocus

DoCmd.Close

Event sequence

During open and close, the form receives many events.

Figure 5.5A gives an overview.

Open(Cancel). This is the first event that the form re-

ceives. At this point in time, the various data struc-

tures in the form have been created, including all

controls. The subforms have been opened and have

received their first Current event. Most controls

also have the right value, but some have not. The

form is invisible to the user at this point. The event

procedure may set the parameter Cancel=True,

thereby refusing to open the form.

Load event( ). Next the form receives a Load event.

The form is still invisible, but all unbound controls

have a value. Controls bound to a database record

may still not have the right value. Accessing data in

the database proceeds in parallel with opening the

form, and bound controls gradually get the right

value.

Resize( ). This event occurs during opening of the

form, and when the user resizes the form by drag-

ging its borders. The event procedure may adjust

the controls on the form to better utilize the avail-

able space. As an example, a subform area may ex-

pand and contract in step with the main form. See

section 5.5.13.

Activate( ). This event signals that the form, or one of

its controls, will get the focus (the title bar becomes

blue). The event doesn't occur when the user clicks

from another application. Subforms never receive

an Activate event. They are active when their main

form is active.

GotFocus( ). This event occurs only when the form

has no controls that can get the focus.

Current().This event occurs at open no matter whether

the form is bound to the database or not. For bound

forms, it also occurs when the user moves to

another record, for instance with PageDown. This

event is the place to update controls that depend on

the current record, for instance detail windows.

When Current is called during open, the form is

still invisible, but the controls have the right value.

When Current returns, the form becomes visible.

Dirty(Cancel). This event occurs the first time the user

edits some bound data in the current record. The

event occurs right before the Change event of the

control changed by the user. At return the Form

property Dirty becomes True.

BeforeUpdate(Cancel). This event occurs when the

form has data that is about to be saved in the data-

base. The event is an opportunity to check consis-

tency between pieces of data before they are saved.

The event procedure may set Cancel=True to stop

saving and let the user change the data. It may also

use Me.Undo to restore the old values and in that

way skip the saving.

BeforeUpdate will occur when the form is to be closed

or when focus moves to another record on the form.

Canceling the update means that the form will not be

closed or focus not moved.

BeforeUpdate will not occur when focus moves to an-

other Access form. However, it will occur when focus

moves to a subform on the same form, or when the user

switches to a different tab sheet on the same form. This

makes it difficult to maintain consistency between data

in the main form and data in its subforms.

AfterUpdate( ). This event occurs when values have

been saved in the database. Also the OldValue

properties have been set to the saved values. The

event is an opportunity to act on the new data.

Unload(Cancel). This is the first event when a form is

closed. The event procedure may check that every-

thing is okay. If it returns Cancel=True, the form

will remain open.

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Fig 5.5A Event sequence, Form open and close

User action Events

Property changes before call

Open

Open(Cancel)

The Form and its controls are created, but not

visible. Some controls are not initialized. Subforms

are open. The procedure may refuse to open the

form (Cancel=True).

Load

All controls are initialized, except bound ones.

The Form is still invisible.

Resize

(Opportunity to reposition controls)

Activate

(Signals that the form will get focus)

(GotFocus)

(Only for Forms where no control can get the

focus)

Current

A current record has been selected. Bound controls

have a value. At return, the form becomes visible.

Edit

Dirty

First time the user edits some data in the record.

PageDown, BeforeUpdate(Cancel) Some bound fields to be saved.

etc.

The procedure may refuse to save.

AfterUpdate

Bound fields saved. All values = OldValue.

Current

Form is invisible. A new record is current.

At return, the Form becomes visible.

Click in

BeforeUpdate(Cancel) As for PageDown.

subform

AfterUpdate

As for PageDown.

Unload(Cancel)

Form to be closed. The procedure may refuse and

the form stays open.

Deactivate

(Signals that the form will lose focus)

At return, the form becomes invisible and the

subforms are closed.Memory is released.

Deactivate( ). This event signals that the form will lose

focus. It occurs during close and when the user

clicks in another main form. It doesn't occur when

the user clicks in another application. Subforms

never receive a Deactivate event.

Close( ). This is the last event the form receives. The

form is still visible and all subforms are still open.

When Close returns, the form becomes invisible, all

subforms are closed, and data structures are deleted.

5.5.2 CRUD control in Forms

When you create a form in the default way and open it

with DoCmd.OpenForm, the user is allowed to step

through all records, and add several new records. De-

pending on circumstances, we may want a more re-

stricted behavior. We may for instance want the user to

see and edit only one single stay. Or we may want the

user to create only one new record.

CRUD and filter properties

The way to control this is through the following prop-

erties, called CRUD properties because they specify

Create, Read, Update, and Delete of records (Figure

5.5B gives an overview). You can see their initial val-

ues on the form's data tab. You can change them at run

time through Visual Basic:

AllowEdits. If True, the user can edit fields in existing

records. Default=True.

AllowDeletions. If True, the user can delete a record

with the Del key. (Requires that the record selector

is displayed in the form.) Default=True.

AllowAdditions. If True, there will be an empty record

at the end of the list for adding new records.

Default = True.

Filter. A text that works as a WHERE clause in the

query behind the form. For instance, you may set

filter to "stayID=740". The result will be that the

user only sees the stay with ID=740. (Don't write

the word WHERE itself.)

FilterOn. If True, the Filter property works and selects

records. If False, the filter has no effect. FilterOn is

a property that you cannot see in the form's prop-

erty box, but you can set it through the program or

5. Access through Visual Basic

the Immediate window. The user can toggle Filter-

On with the filter icon on the toolbar.

AllowFilters. If True, the user is allowed to set filters

on and off through icons on the toolbar. The pro-

gram can always set filters independently of Al-

lowFilters.

DataEntry. If True, the user will not be able to see old

records. If AllowAdditions is also True, the user is

allowed to add new records. If the program sets

AllowAdditions to False while the user is editing a

new record, the user is allowed to finish editing the

record, but cannot add further records. Default =

False.

Setting DataEntry to False has an undocumented

side effect: It also sets FilterOn to False, in that

way blocking filters set up by the program. In this

case, let your program set FilterOn after setting

DataEntry.

All of these properties are dialog variables that don't

survive closing of the form. What you have specified in

the form's property box are simply the default value for

these variables. You may try out the properties in this

way:

1. Open frmStay in Datasheet mode. You should see

all records from the stay query. At the end of the

datasheet you should see an empty line for entering

a new record.

2. Open the Immediate window with Ctrl+G. Adjust

the sizes so that you see the datasheet and the Im-

mediate window at the same time. Try to change

the settings with statements such as:

Forms!frmStay.AllowAdditions = False

Forms!frmStay.Filter = "stayid=740"

Forms!frmStay.FilterOn = True

The datasheet should change accordingly, removing

the empty line, showing only one record, etc.

See and edit a single record

In order to see and edit a single record, set these prop-

erties in the form's property box (see overview in Fig-

ure 5.5B):

AllowEdits = True

AllowDeletions, AllowAdditions, DataEntry = False

Then open the form with a filter, for instance:

DoCmd.OpenForm "frmStay", , , "StayID=2"

In practice, the program must compute the filter text so

that 2 in the example becomes the stayID the user has

chosen.

Update the record. When the user enters something, it

is not stored in the database until he closes the form (or

enters a subform). Sometimes storing is needed earlier.

The program can do it with this statement:

Me.Recordset.Move(0)

This trick assumes that Access is configured for DAO

3.6 (see section 5.6). The statement works on the open

recordset bound to the form. It moves current record

back or forth a number of records, and as part of this it

saves the current record. In our case we move zero

records away - to the same record. But we get the

update anyway. (See more in section 5.6.3.)

Create a single record

In order to create a single record without seeing exist-

ing records, set these properties in the form's property

box (see overview in Figure 5.5B):

AllowAdditions = True

DataEntry = True

This will still allow the user to enter a record and then

move on to enter a new record. To avoid this, block

further record creation when the first record is updated:

Private Sub Form_AfterUpdate()

Me.AllowAdditions = False

End Sub

Open the form with

DoCmd.OpenForm "frmStay"

Prevent two blank lines in datasheet

The method above for creating a single record works

okay when the Form is shown in normal mode. In

Datasheet mode, however, the user may be confused at

what happens. The user sees the blank record at the end

of the datasheet, but as soon as he starts typing in it,

another blank line appears.

Access creates the second blank line right after calling

the Dirty-event procedure. However, it is impossible to

adjust things at this point because the user's character

hasn't yet ended where it should be.

The solution is to store something in the new blank

record before the user types anything. This initiates a

temporary record for further editing. Then AllowAddi-

tions is set to False to prevent further new records. This

can be done at the Current event:

Private Sub Form_Current()

If Not IsNull(Me.someField) Then Exit Sub

Me.someField = " "

Me.AllowAdditions = False

Me.someField = Null

End Sub

The first statement checks whether something has been

stored in the field to be initialized. In this case the user

has selected an existing record and nothing should be

done now. The next statement stores a space in the

field. This creates the temporary record. Next Allow-

Additions is set to False. This prevents further records

being created. Finally the field is set back to the initial

empty state.

The solution must be extended with means to add an-

other record, for instance a button or a response to

Enter. Care must also be taken not to leave empty

5. Access through Visual Basic

DoCmd.OpenForm“frmStay”, view, , “StayID=5”, CRUDproperties, windowMode, openArgs

acDesign

acFormDS

acNormal

acPreview

acDialog

acHidden

acIcon

acWindowNormal

Datasheet

view

preview

Available to the

Form procedures

Form properties: CRUD and Filter

AllowEdits: See and edit data

AllowDeletions: Del-key works

AllowAdditions: Empty record at end

DataEntry: Old data invisible

Filter:

e.g. StayID=740

FilterOn:

Use filter

AllowFilters:User-controlled filter

To see and edit a single record:

(Also set the filter)

True

False

To create a single record:

Initially

False

True

Form_AfterUpdate

False

True

acFormEdit

True

False

acFormAdd

True

acFormReadOnly

False

acFormPropertySettings (default)

Fig 5.5B Controlling Open, CRUD, and filter

Filter

Filter name

records in the database in case the user doesn't enter

something.

5.5.3 The OpenForm parameters

From VBA you normally open a form with

DoCmd.OpenForm. A lot of parameters determine

what goes on (see the details on Figure 5.5B).

Form name. The first parameter is the name of the

form, for instance "frmStay".

View. The second parameter specifies whether the

form is to be shown in design view, as a datasheet,

as a normal form (default), or as a preview of a

print.

Filter name. The name of a query, for instance

"qryStay". (I have not been able to figure out what

this parameter does.)

Filter. A WHERE condition for the records to show,

for instance "StayID=5".

CRUDproperties. A choice of CRUD combinations

that determine whether records can be edited,

added, etc. Figure 5.5B shows that acFormEdit sets

AllowEdits, AllowDeletions, AllowAdditions to

True and DataEntry to False. In other words, the

user can do anything to the records. At run time

these settings overrule the initial settings from the

Form's property box.

As another example, acFormPropertySettings (the

default), doesn't set any of the CRUD properties but

lets the initial settings rule.

WindowMode. Specifies whether the form is to be

shown as a dialog box, a hidden window, an icon,

or a normal window. A dialog box keeps the focus

until the user closes it. It has borders that cannot be

resized. A hidden window is like a normal window

except that it is invisible. The event procedures can

make it visible with me.Visible=True. The window

mode acIcon seems to have no effect.

OpenArgs. This parameter is stored in the property

OpenArgs in the Form. It can tell the event proce-

dures in the Form what to do. The parameter might

for instance specify whether the form is to be used

for creating a new record or viewing an existing

one, in this way allowing the Form to do it in its

own way. Below we show how this is used in the

hotel system to let the same form handle a new

guest, a new stay, or an existing stay.

5. Access through Visual Basic

5.5.4 Multi-purpose forms (hotel system)

In the hotel system we use frmStay for creating guest

and stay records, as well as editing existing records.

Using the same Form is an advantage to the user (for

recognition) and for the developer (he needs to main-

tain only one Form).

Now, how does the Form know what it should do?

OpenArgs is intended for such things. In the hotel sys-

tem, we use OpenArgs in this way:

•

OpenArgs < 0: Open an existing stay. The

WhereCondition parameter in the OpenForm

statement selects the right one.

•

OpenArgs = 0: Create a new guest and a new

stay.

•

OpenArgs > 0: Create a new stay for an existing

guest. OpenArgs is the guestID for the existing

guest.

Figure 5.5C shows the full solution. It deals also with

error cases where the user for instance clicks the

ShowStay button, but hasn't selected a stay. The Find

Guest screen has three buttons for opening a stay. The

figure shows the three click-event procedures.

cmdNewGuest

This is the simplest of the buttons. Independent of what

the user has chosen, the button should create a new

guest and a stay for him. What is needed is to open the

form with OpenArgs=0.

cmdNewStay

Here we have two situations. (1) The user has selected

a line, and thus a guest. (2) The list is empty so the user

has not chosen anything. The property CurrentRecord

allows the program to distinguish the two situations.

CurrentRecord gives the sequential number of the se-

lected record. It is zero if nothing is selected. If the list

is empty, we might claim it is an error to make a new

stay for a non-existing guest, but it turns out that users

find it natural to create a new guest in this case. So this

is what the program does; it opens the form with Open-

Args = 0. If a line is selected, it opens the form with

OpenArgs = guestID.

Notice the If-Then-Else-EndIf construct that makes the

program choose alternative paths, either executing the

Then-statements or the Else-statements. The line-

breaks must be as shown, with Else and EndIf on lines

of their own.

Notice also the comment we have put after Then to

explain to the reader what situation the program deals

with after Then.

cmdShowStay

Here we have three situations. (1) A stay is selected.

(2) A guest without stay is selected. (3) The list is

empty. In the last two cases, the program cannot open a

stay. The current value of stayID allows the program to

distinguish. StayID is >0 in the first case, and Null in

the last two cases. Notice that stayID exists even if the

list is empty and CurrentRecord is zero. The value of

stayID is just Null.

When stayID>0, we open the form with OpenArgs=-1.

We also have to set the filter so that only this stay is

visible. The WhereCondition-parameter to OpenForm

allows us to do this in a simple manner. If the user for

instance has selected a line with stayID=740, the

WhereCondition has to be the text

stayID=740

In the program we compute this text by concatenating

the text "stayID=" with the current stayID using the &-

operator. StayID is a number, but Visual Basic converts

it to a text before the concatenation.

When stayID is Null (or a number <=0) we have an

error situation. The program may show a message

using MsgBox, but in our case we just make the pro-

gram beep. The DoCmd-object can do this too. (Us-

ability tests show that users understand what is wrong

without any message explaining about it.)

FrmStay, Load event

Inside frmStay an event procedure must take care of

setting the right CRUD properties. Access calls the

Load-event procedure during open, and this is the place

we set the CRUD properties. Figure 5.5C shows the

procedure. Let us follow the path through the code for

each of the three cases.

OpenArgs is < 0: We open an existing stay. The

WhereCondition parameter has set the filter, so we

don't have to do much. The procedure just executes the

last line, which sets AllowAdditions to False.

OpenArgs = 0: We must create a new guest record and

a new stay record. First we allow additions and data

entry. Data entry means that existing records are not

visible, so the current record is a new "blank line".

FrmStay is bound to a join of tblGuest and tblStay, so

the current record will now consist of Null data for the

guest as well as the stay. In order to create the records,

we have to store something in them.

First we set the guest name to a single space character.

This creates a new guest record, and Access gives it an

AutoNumber. Notice how we address name with a

bang ( ! ) to distinguish it from the built-in Name prop-

erty.

Next we set the foreign key of the stay record to this

guest. This creates the stay record and also links it

properly to the guest. Then we set the name back to a

Null so that the user doesn't see a name starting with a

space when he enters the guest name. Notice the

strange use of square parentheses. We explain more

about them below.

5. Access through Visual Basic

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