The following 0 count means the array never gets enumerated anyway

I see call sites for some of these virtuals checking for and allowing null. Is non-nullable correct for the return value for all of these?

The default implementation coalesces to string.Empty, and it's supposed to be a string you can always concatenate before/after the string. It's true that it's virtual so anything's possible, though I'm only seeing IsNullOrEmpty in the Odbc and OleDb providers, and SqlClient just calls base.QuotePrefix.

Where are you seeing null checks, anything persuasive? I can make it nullable if you think that's safer.

Where are you seeing null checks

The IsNullOrEmpty checks you mentioned. But it's not just in Odbc and OleDb, e.g. DBCommandBuilder allows null in both private void BuildInformation(DataTable schemaTable) and private string QuotedColumn(string column).

OK, yeah - I see them though they're also IsNullOrEmpty. I think the point is to check for length 0 rather than for null (the coalescing behavior of the property getters is a good indication), but if you think we should annotate these as nullable I can do that. We could also do a more thorough search outside the BCL to see who's overriding DbCommandBuilder and what they're doing, let me know.