I am going berserk trying to do something that I would expect to be straightforward. I have searched for the last 3 days to get directions, but have run into numerous false leads and dead-ends.

I have a UDF (call it MyUDF) that I would like to use in any workbook I open on my computer (only) and I'd like to refer to it via only the function name without a qualifying workbook name (e.g. =MyUDF(args) instead of ='MyUDFWorkbook'!MyUDF(args)). I have had this working in the past (used in 50+ workbooks), though I must admit to not understanding/remembering how enough to be able to recreate it. I had programmed the UDF in a uniquely named workbook I saved as an Excel Add-In (i.e. with a .xlam extension). I'm not wedded to that approach, but am willing to use it again. I broke this several days ago when trying to overcome an Interop.Excel exception I was getting in a C# Winforms app I'm creating.

I have read about and tried programming the UDF in PERSONAL.XLSB instead of a uniquely named workbook, overtly putting that workbook or my uniquely named one in the Excel AddIns and/or XLSTART folders (under my Windows user name), setting the IsAddIn property somehow (I could find how to do that), using the Add-Ins manager (which is still showing the name of my UDF even though I've deleted it from both the AddIns and XLSTART folders), and other things. There are moments where things I've tried seem to work, but then they don't when I shut down Excel and reopen the workbooks that use the UDF.

I'd really appreciate guidance on how to clear out any residual junk I may have created and start over to get this to work. I'm using MS Office Home & Business 2010 on a Windows 8.1 machine. Thanks.