A workbook that you convert to an add-in must have at least one worksheet.
For example,if your workbook contains only chart sheets or Excel 5/95 dia-
log sheets,the Microsoft Excel add-in (*.xla) option does not appear in the
Save As dialog box.
With previous versions of Excel (before Excel 97),to modify an add-in,you
had to open the original XLS file,make your changes,and then recreate the
add-in.For Excel 97 and later versions,this is no longer necessary.As long as
the add-in is not protected,you can make changes to the add-in in the VB
Editor,and then save your changes.If the add-in is protected,you must enter
the password to unprotect it.Therefore,with Excel 97 or later, keeping an
XLS version of your add-in is not necessary.
After you create your add-in, you can install it by using the standard procedure:
Select ToolsAdd-Ins, and click the Browse button in the Add-Ins dialog box.
Then, locate your *.xla file.
This chapter covered some essential details to help you develop effective custom
functions. I discussed the type of arguments that you can use, and I described how
to make your function appear in a specific category in the Paste Function dialog
box. This chapter also presented some techniques to help debug functions and
ended with instructions for creating an add-in to hold your functions.
The next chapter discusses VBA programming concepts.
Part VI: Developing Custom Worksheet Functions
A Few Words about Security
Microsoft has never promoted Excel as a product that creates applications with secure
source code. The password feature provided in Excel is sufficient to prevent casual
users from accessing parts of your application that you want to keep hidden. But, the
truth is, several password-cracking utilities are available. The security features in Excel
2002 are much better than those in previous versions, but it’s possible that these can
also be cracked. If you must absolutely be sure that no one ever sees your code or
formulas, Excel is not your best choice as a development platform.