DynamicParameters works with DbString but not with IEnumerable<DbString>

If the value of a DynamicParameters is a ICustomQueryParameter that implementation's AddParameter(IDbCommand, string) implementation will be used. This means that DynamicParameters transparently wraps DbString; this works:

var id = new DbString() { IsAnsi = true, Value = "foo" };
var p1 = new DynamicParameters();
pn.Add("@id", id, DbType.AnsiString);
conn.QueryAsync<int>("select 1 from tbl where id = @id", p1).

This does not scale to values of type IEnumerable<ICustomQueryParameter>; this does not work:

var ids = new DbString[] { id };
var pn = new DynamicParameters();
pn.Add("@ids", ids, DbType.AnsiString);
conn.QueryAsync<int>("select 1 from tbl where id in @ids", pn).

This relates to

• the documented in operator support, whose combination with string types is non-obvious, difficult to troubleshoot, and dangerous
• #789, which asserts that the way to resolve the above is with an anonymous type whose value is some IEnumerable<DbString>
• #1672, which generalizes the built-in support for IEnumerable<DbString> to IEnumerable<ICustomQueryParameter>.

As a consequence, I can find no way to combine the in operator with either DynamicParameters or, in some other way, DbType. I would like to be able to do so because DbString is much more cumbersome to work with than the core DbType (or SqlDbType) and does not generalize to other types, and because it is far too easy to write perfectly sensible, non-functional code as is.

Up until such a time this gap is closed, the documentation should feature much more prominent examples of how to use the in operator with varchar (and char) columns and still avoid https://www.brentozar.com/blitz/implicit-conversion/.

Start by reproducing the two DynamicParameters examples, then trace the IEnumerable and DbString handling described in the issue. Read the README sections on list support and ANSI strings/varchar, along with issues #789 and #1672; done should cover the requested list-parameter behavior or clearly document the varchar/char workaround.