How Can You Tell if You Found a Valid List ID?

I’m currently working with several different custom SharePoint lists.  Some of the lists have references to other lists, via a “Lookup” column.  For example, a book might have a reference back to the library it came from.

Sometimes I only know the name of the library, and I want to be able to set a reference to it on the book’s list.  So I retrieve the library’s ID using the name of the library.  SharePoint Designer dutifully warns me that I’m not looking up the value using a unique ID, and that it will just give me the first one it finds.  I’m OK with that, since I control the names and I know that they’re all unique.

But I’m only human, and sometimes I don’t have a library name that exists in my library list.

In those cases, when I log the contents of the list ID variable into which I placed the data, it shows me “0”.  [That’s the number zero, not to be confused with a lower-case “o”.  I’m considering changing the font for this blog… -sj]

As it turns out, this is the way you can tell if you have a valid list ID.  Valid IDs start at 1, I presume, and invalid IDs are all zero.

[It’s easy to tell if you have a bad reference to another list when you retrieve string values, since they show up in log messages as “????”.]