Making the application bulletproof

If you think about it, it's fairly easy to destroy a spreadsheet. Erasing one critical formula or value can cause errors throughout the entire worksheet - and perhaps even other dependent worksheets. Even worse, if the damaged workbook is saved, it replaces the good copy on disk. Unless a backup procedure is in place, the user of your application could be in trouble, and you'll probably be blamed for it.

Obviously, it's easy to see why you need to add some protection when users - especially novices - will be using your worksheets. Excel provides several techniques for protecting worksheets and parts of worksheets:

■ Lock specific cells: You can lock specific cells (by using the Protection tab in the Format Cells dialog box) so that they cannot be changed. This takes effect only when the document is protected with the

Review Changes Protect Sheet command. The Protect Sheet dialog box has options that allow you to specify which actions can be performed on a protected sheet. See Figure 6-5.

Figure 6-5: Using the Protect Sheet dialog box to specify what users can and cannot do.

■ Hide the formulas in specific cells: You can hide the formulas in specific cells (by using the Protection tab in the Format Cells dialog box) so that others can't see them. Again, this takes effect only when the document is protected by choosing the Review Changes' - Protect Sheet command.

■ Protect an entire workbook: You can protect an entire workbook - the structure of the workbook, the window position and size, or both. Use the Review

Changes Protect Workbook command for this purpose.

■ Lock objects on the worksheet: Use the Properties tab in the Size and Properties dialog box to lock objects (such as shapes) and prevent them from being moved or changed. To access the Size and Properties dialog box, select the object and then click the dialog box launcher in the Drawing Tools

Format

Size group. Locking objects takes effect only when the document is protected via the Review

Changes Protect Sheet command. By default, all objects are locked.

■ Hide rows, columns, sheets, and documents: You can hide rows, columns, sheets, and entire workbooks. This helps prevent the worksheet from looking cluttered and also provides some protection against prying eyes.

How Secure Are Excel's Passwords?

As far as I know, Microsoft has never advertised Excel as a secure program. And for good reason: It's actually quite easy to circumvent Excel's password system. Several commercial programs are available that can break passwords. Excel 2002 and later versions seem to have stronger security than previous versions, but they can still be cracked by a determined user. Bottom line? Don't think of password protection as foolproof. Sure, it will be effective for the casual user. But if someone really wants to break your password, he can probably do so._

Designate an Excel workbook as read-only recommended: You can designate an Excel workbook as read-only recommended (and use a password) to ensure that the file cannot be overwritten with any changes. You do this in the General Options dialog box. Display this dialog box by choosing File Save As. In the Save As dialog box, click the Tools button and choose General Options.

■ Assign a password: You can assign a password to prevent unauthorized users from opening your file. You do this in the General Options dialog box. Display this dialog box by choosing File

Save

As. In the Save As dialog box, click the Tools button and choose General Options.

■ Use a password-protected add-in: You can use a password-protected add-in, which doesn't allow the user to change anything on its worksheets.

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