关于 c#:UserPrincipal SamAccountName throws DirectoryServicesCOMException unhandled “A local error has occurred”

public static string GetName(string uName)
    {
        StringBuilder builder = new StringBuilder();
        using (PrincipalContext context = new PrincipalContext(ContextType.Domain,"serverName"))
        {
            UserPrincipal user = new UserPrincipal(context);

            user.SamAccountName = uName;

            PrincipalSearcher srch = new PrincipalSearcher(user);
            srch.QueryFilter = user;
            PrincipalSearchResult<Principal> res = srch.FindAll();
            foreach (UserPrincipal u in res)
            {
                builder.Append(u.GivenName);
                builder.Append("");
                builder.Append(u.Surname);
            }
            return builder.ToString();
        }
    }

user.SamAccountName = uName;