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Figure E-1. The References dialog box

Figure E-1. The References dialog box

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Dim XlApp As Excel.Application



which the Automation client will understand, because it can now check the server's object library.

Note that we need to qualify the object name, since other object models have an Application

object as well.

Next, we want to start the Excel Automation server, create an Excel Application object, and get a

reference to that object. This is done in the following line:

Set XLApp = New Excel.Application



At this point, we have complete access to Excel's object model. It is important to note, however,

that the previous line starts the Excel Automation server, but does not start Excel's graphical user

interface, so Excel will be running invisibly. To make Excel visible, we just set its Visible

property to True:

XLApp.Visible = True



We can now program as though we were within the Excel VBA IDE. For instance, the following

code creates a new workbook, adds a worksheet to it, puts a value in cell A1, and then saves the

workbook:

Sub MakeWorkbook()

Dim XlApp As Excel.Application

Dim wb As Excel.Workbook

Dim ws As Excel.Worksheet

Set XlApp = New Excel.Application

XlApp.Visible = True

Set wb = XlApp.Workbooks.Add

Set ws = wb.Worksheets.Add

ws.Name = "Sales"

ws.Range("A1").Value = 123

wb.SaveAs "d:\temp\SalesBook"

End Sub



Note that the Excel server will not terminate by itself, even if the XLApp variable is destroyed. If

we have made Excel visible, then we can close it programmatically, as well as from the user

interface in the usual way (choosing Exit from the File menu, for instance). But if the Excel server

is invisible, it must be closed using the Quit method:

XlApp.Quit



(If we fail to terminate the Excel server, it will remain running invisibly, taking up system

resources, until the PC is restarted.)



E.2.1 An Alternative Approach

The approach described for programming Excel from within another application is the preferred

approach, since it is the most efficient. However, there is an alternative approach that you may

encounter, so let us discuss it briefly. As before, we assume that a reference has been set to the

Excel object library.

E.2.1.1 The CreateObject function



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The CreateObject function can start an Automation server, create an object, and assign it to an

object variable. Thus, we can write:

Dim XLApp as Excel.Application

Set XLApp = CreateObject("Excel.Application")



This approach will execute more slowly than the previous approach using the New keyword, but it

is perfectly valid.

As before, we must remember to close Excel using the Quit method (or through normal means if

Excel is visible).

E.2.1.2 The GetObject function

If Excel is already running, the CreateObject function will start a second copy of the Excel

server. To use the currently running version, we can use the GetObject function to set a

reference to the Application object of a running copy of Excel. This is done as follows:

Set XLApp = GetObject(, "Excel.Application")



(The first parameter of GetObject is not used here.)

One of the problems with using GetObject is that it will produce an error if Excel is not running.

Thus, we need some code that will start Excel if it is not running or use the existing copy of Excel

if it is running.

The trick to this is to know that if GetObject fails to find a running copy of Excel, then it issues

error number 429 ("ActiveX component can't create object"). Thus, the following code does the

trick:

Dim XLApp As Excel.Application

On Error Resume Next

' Try to get reference to running Excel

Set XLApp = GetObject(, "Excel.Application")

If Err.Number = 429 Then

' If error 429, then create new object

Set XLApp = CreateObject("Excel.Application")

ElseIf Err.Number <> 0 Then

' If another type of error, report it

MsgBox "Error: " & Err.Description

Exit Sub

End If



E.2.1.3 No object library reference

We have been assuming that the client application has a reference to the server's object library.

However, it is still possible for a client application (an Automation client) to program the objects

of an Automation server (such as Excel) without such a reference. Under these circumstances, we

cannot refer to objects by name in code, since the client will not understand these names. Instead,

we must use the generic Object data type, as in the following code:

Dim XLApp As Object

Dim wb As Object



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Set XLApp = CreateObject("Excel.Application")

XLApp.Visible = True

Set wb = XLApp.Workbooks.Add

wb.SaveAs "d:\temp\SalesBook"



This code will run even more slowly than the previous code, which, in turn, is slower than the first

version.

Thus, we have three versions of Automation:









Using the New keyword syntax (requires an object library reference)

Using CreateObject and specific object variable declarations (requires an object

library reference)

Using CreateObject with generic As Object declarations (does not use an object

library reference)



These versions of automation are sometimes referred to by the names very early binding, early

binding, and late binding, respectively (although you may hear these terms used somewhat

differently).

These terms refer to the time at which VBA can associate (or bind ) the object, property, and

method names in our code to the actual addresses of these items. In very early binding, all

bindings are done at compile time by VBA—that is, before the program runs. In early binding,

some of the bindings are done at compile time and others are done at run time. In late binding, all

bindings are done at run time.

The issue is now evident. The more binding that needs to be done at run time, the more slowly the

program will run. Thus, very early binding is the most efficient, followed by early binding, and

then late binding.



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Appendix F. High-Level and Low-Level Languages

In this appendix, we'll examine the position of Visual Basic as a programming language by taking

a somewhat closer look at high-level and low-level languages, with some examples for

comparison.

A low-level language is characterized by its ability to manipulate the computer's operating system

and hardware more or less directly. For instance, a programmer who is using a low-level language

may be able to easily turn on the motor of a floppy drive, check the status bits of the printer

interface, or look at individual sectors on a disk, whereas these tasks may be difficult, if not

impossible, with a high-level language. Another benefit of low-level languages is that they tend to

perform tasks more quickly than high-level languages.

On the other hand, the power to manipulate the computer at a low level comes at a price. L owlevel languages are generally more cryptic—they tend to be farther removed from ordinary spoken

languages and are therefore harder to learn, remember, and use. High-level languages (and

application-level languages, which many people would refer to simply as high-level languages)

tend to be more user-friendly, but the price we pay for that friendliness is less control over the

computer and slower running programs.

To illustrate, consider the task of printing some text. A low-level language may only be able to

send individual characters to a printer. The process of printing with a low-level language might go

something like the following:

1.

2.

3.

4.

5.

6.



Check the status of the printer.

If the printer is free, initialize the printer.

Send a character to the printer.

Check to see if this character arrived safely.

If not, send the character again.

If so, start over with the next character.



The "lowest" level language that programmers use is called assembly language . Indeed, assembly

language has essentially complete control over the computer's hardware. To illustrate assembly

language code, the following program prints the message "Happy printing." Don't worry if these

instructions seem meaningless—you can just skim over them to get the feel. In fact, the very point

we want to make is that low-level languages are much more cryptic than high-level languages.

(Lines that begin with a semicolon are comments. We have left out error checking to save a little

space.)

; -------------------; Data for the program

; -------------------; message to print

Message

DB

'Happy printing', 0Dh, 0Ah

; length of message

Msg_Len

EQU

$-Message

; -------------------; Initialize printer 0

; -------------------mov ah,1

mov dx,0

int 17h



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; --------------------; Printing instructions

; --------------------; get number of characters to print

mov cx,Msg_Len

; get location of message

mov bx,offset Message

; get printer number (first printer is printer 0)

mov dx,0

Print_Loop:

; send character to printer 0

mov ah,0

mov al,[bx]

int 17h

; do next character

inc bx

loop Print_Loop



For comparison, let us see how this same task would be accomplished in the BASIC programming

language:

LPRINT "Happy printing"



The difference is pretty obvious.

As we have discussed, high-level languages are usually designed for a specific purpose. Generally,

this purpose is to write software applications of a specific type. For instance, Visual C++ and

Visual Basic are used primarily to write standalone Windows applications. Indeed, Microsoft

Excel itself is written in Visual C++. As another example, FORTRAN (which is a contraction of

Formula Translation) is designed to write scientific and computational applications for various

platforms (including Windows). COBOL is used to write business-related applications (generally

for mainframe computers).

At the highest level in the programming language hierarchy, we find programs such as Excel VBA,

whose primary purpose is not to manipulate the operating system or hardware, nor to write

standalone Windows applications, but rather to manipulate a high-level software application (in

this case Microsoft Excel).

Just for fun, let us take a brief look at a handful of the more common programming languages.



F.1 BASIC

The word BASIC is an acronym for Beginners All-Purpose Symbolic Instruction Code, the key

word here being Beginners. BASIC was developed at Dartmouth College in 1963, by two

mathematicians: John Kemeny and Thomas Kurtz. The intention was to design a programming

language for liberal arts students, who made up the vast majority of the student population at

Dartmouth. The goal was to create a language that would be friendly to the user and have a fast

turn-around time so it could be used effectively for homework assignments. (In those days, a

student would submit a program to a computer operator, who would place the program in a queue,



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Figure E-1. The References dialog box

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