SPFieldUserValue and SPPrincipal

Question

I have a complex application that have several field of type UserMulti. These fields can accept either SPUser or SPGroup or a combination of both kind.

I have in my code the ID and the name of a SPPrincipal object. How can I get the corresponding SPPrincipal object?

I have this code :

    public static SPPrincipal GetPrincipal(SPWeb web, int principalID, string principalName)
    {
        return web.AllUsers.Cast<SPPrincipal>().SingleOrDefault(u => u.ID == principalID && u.Name == principalName) ??
            web.SiteGroups.Cast<SPPrincipal>().SingleOrDefault(g => g.ID == principalID && g.Name == principalName);
    }

But that looks a bit weird to my eyes.

PS: I have ids and names because my value are coming from SPMetal, where fields of type UserMulti are created as one IList<int?> userIds and IList<string> userImnName

[Edit] I just realized that both SPGroup and SPUser are stored in the UserInformationList. So I can rely on Ids, I don't have to test the name.

My code is then slightly simplified :

    public static SPPrincipal GetPrincipal(SPWeb web, int principalID, string principalName)
    {
        return web.AllUsers.Cast<SPPrincipal>().SingleOrDefault(u => u.ID == principalID) ??
            web.SiteGroups.Cast<SPPrincipal>().SingleOrDefault(g => g.ID == principalID);
    }

Answer 1

I've done this by querying against the web.SiteGroups collection and the web.AllUsers collection, just as you have done, however I didn't cast. I instantiated an SPPrincipal to null and then set it equal to the SPGroup or SPUser before returning it.

Here's an example using iteration rather than Linq:

    private SPPrincipal GetPrincipal(SPSite spsSite, string strName)
    {
        SPPrincipal sppGroup = null;

        // Get user, if user is valid
        if (SPUtility.IsLoginValid(spsSite, strName))
        {
            sppGroup = spsSite.RootWeb.EnsureUser(strName); // <== Here I did not cast
        }
        else // Otherwise look for a group
        {
            foreach (SPGroup spgGroup in spsSite.RootWeb.SiteGroups)
            {
                if (spgGroup.Name.ToUpper(CultureInfo.InvariantCulture) ==
                    strName.ToUpper(CultureInfo.InvariantCulture))
                {
                    sppGroup = spgGroup; // <== and here I did not cast
                                         //  which is what I was talking about
                    break;
                }
            }
        }
        return sppGroup; // <== May be null here
    }

Answer 2

It looks like there is not simpler solution that this one :

public static SPPrincipal GetPrincipal(SPWeb web, int principalID, string principalName)
{
    return web.AllUsers.Cast<SPPrincipal>().SingleOrDefault(u => u.ID == principalID) ??
        web.SiteGroups.Cast<SPPrincipal>().SingleOrDefault(g => g.ID == principalID);
}

Feel free to suggest another approach if there is one.