Standalone code for Excel - vba

Question

Can VBA code be written to perform actions on any Excel file?
When I create a project in Visual Studio, it asks for an Excel file to be linked to it. All the code that I write is in ThisWorkbook.vb and hence acts only on the Excel file linked to the project.
Ideally, I want a script that:
When the user double-clicks, he/she should be allowed to select an Excel file of choice for the actions to be performed on that file.
If the above is not possible, I'd at least like to invoke the VBA script from within an Excel file.
Basically, the VBA code should be independent of any Excel file.
I looked at VBScript, but the syntax for it seems to differ slightly.

Answer 1

You've mentioned Visual Studio, VBA, and VBScript. The solution I'm outlining works directly with VBA rather than Visual Studio or VBScript. (However, you might adapt Visual Studio (C# or VB) along the lines of what I'm outlining below.) Hope it's helpful, so here goes:
Here's what I do, and, it ultimately results in an .xlam Excel AddIn as #chris above has commented.
Overview
I start with ordinary .xslx workbook, to which I add a VBA project, making it an .xlsm. Then create some VBA Subs (macros). Then create some Excel QAT (Quick Access Toolbar) buttons for the workbook, which are bound to (i.e. they invoke) the VBA subs/macros. Then convert the workbook (with VBA in it) to an .xlam, which is an Excel AddIn. When you're all done, the buttons are accessible from any workbook (and the VBA code has access to any user workbooks as well as those originally in your .xlsm). Further, the workbook associated with the .xlam is invisible. So it just looks like you've added some buttons to the QAT that appear on all users .xlsx windows. The .xlam is pretty easy for users to install (though I provide a buttons to uninstall/reinstall/check version). You can upgrade an .xlam independently of users' workbooks; users' workbooks can thus be data only (.xlsx, no VBA).
Details
Write some Excel Subs you want to use later
You need to be aware that the buttons can only invoke macros (VBA Subs) without parameters, so the macros will have to check things like ActiveSheet and ActiveWorkbook and Selection to figure on what sheet the button was pressed, hence what user data to really operate on. (If you need to refer to your workbook with the VBA code in it, use “ThisWorkbook”). You should be aware that there can be naming conflicts, so try to name the parameterless subs with rather long names, such as MySomewhatUniqueProjectName_button1, etc…
Add Buttons to your .xlsm
Using Excel 2010 (I think this works with 2007 or later), I put workbook-specific buttons on the QAT part of the ribbon, which connect to macros (VBA subs) in the VBA code.
To do this, you from the Quick Access Toolbar customization drop down (the tiny down arrow at the very top row of the Excel window, the last icon from left to right) choose "More Commands…". When the “Customize Quick Access Toolbar” dialog box comes up, from the (2nd) "Customize Quick Access Toolbar:" heading (top to the right), choose "For XYZ.xlsm" from the dropdown instead of the "For all documents (default)". And from under "Choose Commands From:", use "Macros" (instead of “Popular Commands”) from the dropdown. Once you have those two things selected, you can move VBA subs from the left box to the right box using “Add >>”. Each so moved will become buttons visible in your QAT. As you’re doing this you can also edit the icon and text for the buttons, add a separator as needed (I always end with a separator in case other .xlam’s use the QAT). (Now is a good time to save this .xlsm.)
Convert the .xlsm into a .xlam
Then I convert the .xlsm to an Excel add-in, by merely saving it as an .xlam file. This will end up (1) hiding the workbook associated with the code you have (though it is still accessible to itself.). Further, now, (2) the (invisible, as now it's an .xlam) workbook will load whenever Excel is loaded. (To keep this fast for when users use Excel but don’t run my VBA code, I don't run any code when the .xlam is loaded, I only run code when a button is pushed.)
You can manage the AddIn using Excel’s AddIn manager. To update the AddIn, you have to use some trickery. While you can copy over it when Excel is not running, on the other hand, you cannot directly uninstall the AddIn, you can only disable it from Excel. But once disabled, you can delete the .xlam, and relaunch Excel, go to the AddIn manager to try to work with the (now gone) AddIn and you’ll get Excel saying it can’t find it, so do you want to delete it. Say yes, and it will be uninstalled.
FYI, Notes
I keep the .xlsm to edit later, but you can actually debug and edit the .xlam and later convert it back to an .xlsm with a minor bit of trickery: find its "ThisWorkbook" entry in VBA, and then the "IsAddIn" property, set to false, its workbook will suddenly appear and you can save as .xlsm, or edit its workbook and set the property back to true to resave as .xlam directly.)

Answer 2

Answer 1
You can do that in VB.Net too. Just make a regular project. (comment by #SLaks)
This worked for me very well and was exactly what I was looking for.
Answer 2
The very descriptive answer posted by #ErikEidt
I haven't tried this, but it seems like a good and alternative way of getting macros to work.