Cannot get a list of valid users in TFS for AssignedTo field while creating new Workitems via API

Question

Hi I am trying to create new workitems via the TFS API and this is the method I have used below to get a list of valid users who can be assigned workitems. Somehow, it gives me a null reference exception on validUserSids line. Anyone know what's wrong here?

private string[] TFSUsers(string server)
    {
        // Get a Reference to Team Foundation Server.
        TeamFoundationServer tfs = tfsdata.GetTFS(server);
        // Get a reference to Group Security Service.
        IGroupSecurityService gss = (IGroupSecurityService)tfs.GetService(typeof(IGroupSecurityService));
        // Resolve to SIDs
        Identity validUserSids = gss.ReadIdentity(SearchFactor.AccountName, "TFS Valid Users", QueryMembership.Expanded);
        // Resolve to actual users
        Identity[] validUsers = gss.ReadIdentities(SearchFactor.Sid, validUserSids.Members, QueryMembership.None);
        List<string> Users = new List<string>();
        foreach (Identity user in validUsers)
        {
            Users.Add(user.DisplayName);
        }
        return Users.ToArray();
    }

Answer 1

Here's how you would get the list of users in TFS:

var tfs = TeamFoundationServerFactory.GetServer("http://vstspioneer:8080/tfs/VSTSDF");
var workItemStore = (WorkItemStore)tfs.GetService(typeof(WorkItemStore));
var allowedValues = workItemStore.FieldDefinitions[CoreField.AssignedTo].AllowedValues;

foreach (String value in allowedValues)
{
    Console.WriteLine(value);
}