C# Regex.Matches and nullability issue

Question

I'm using C# 8 with nullable enabled and now I'm having a problem with my regex loop:

public static async Task<IEnumerable<WorkerDto>?> GetOwnersAsync(LampContext context, string? ownerString)
{
    if (string.IsNullOrWhiteSpace(ownerString))
        return null;

    var wwids = new List<int>();

    var matches = Regex.Matches(ownerString, @"\d+");
    foreach (Match match in matches)
        wwids.Add(int.Parse(match.Value));

It's saying I have a possible null reference assignment to the match iteration variable, and I'm not sure why it would say that, or how to get around it. The documentation for Matches says it'll return an empty collection, not null.

How am I supposed to write that code now?

Answer 1

The problem is that MatchCollection implements the non-generic IList interface in preference to the generic IList<Match> interface, for backwards compatibility, and IEnumerator.Current is defined as object?, so the foreach actually casts this nullable object to a non-nullable Match. It's as if you had written

foreach (object o? in matches) {
    Match match = (Match) o;
    ...
}

All this comes from a long forgotten time before we had C# 2, and now comes back to bite us in C# 8.

There are several effective workarounds; one of the simplest is to assign matches to a variable of type IList<Match> or IEnumerable<Match> (as it does implement the generic interface):

IList<Match> matches = Regex.Matches(ownerString, @"\d+");
foreach (Match match in matches)
    wwids.Add(int.Parse(match.Value));

Note that we do not need a cast here. Another is to use .AsEnumerable() to effectively cast it:

var matches = Regex.Matches(ownerString, @"\d+");
foreach (Match match in matches.AsEnumerable())
    wwids.Add(int.Parse(match.Value));

And last but not least, if all you're doing is .Adding these values to a new list, you may as well construct the list in one go with LINQ:

var wwids = matches.Select(m => int.Parse(m.Value)).ToList();