How best to convert a SharePoint multi-user field string into an array of SPUsers?

Question

I'm writing an ItemAdding handler for a SharePoint list, and the list contains multi-user fields. Since the SPItem is not actually available at this point, I think I'm relegated to using the string that comes back from the SPItemEventDataCollection. That string will look something like this when user1, user2, and user3 are present:

1;#MYDOMAIN\user1;#4;#MYDOMAIN\user2;#10;#MYDOMAIN\user3

I'd like to convert this into an array of SPUser objects in order to pass it into another existing method. Is there any SharePoint built-in way to handle these strings, or am I relegated to parsing this string?

Also, assuming I need to deal with this string, it looks like the integer tokens here always correspond to the domain\username that follows. Are there any cases where this will not be true and either the integer or the domain\username are missing or otherwise incorrect? Would it be safe to just use the numbers and use SPWeb's SiteUsers.GetByID(id) method? In a handful of tests I can't get that to fail, but it seems weird that both the numeric and string data would be included if they're totally redundant.

Thanks!

Answer 1

SPFieldUserValueCollection does exactly what you're looking for. Its constructor accepts an SPWeb and string of users. The SPFieldUserValue items in the collection provide a User property that returns the corresponding SPUser objects for the web.

private void ProcessUsers(SPListItem item, string fieldName)
{
  string fieldValue = item[fieldName] as string;
  SPFieldUserValueCollection users = new SPFieldUserValueCollection(item.Web, fieldValue);

  foreach(SPFieldUserValue uv in users)
  {
    SPUser user = uv.User;
    // Process user
  }
}

The SPFieldUser type inherits its behavior from the standard SPFieldLookup, which also stores both ID and Title and has a corresponding SPFieldLookupValueCollection. The ID supports (weak) referential integrity while the cached Value allows for quick queries.