System.Web.Security.MembershipProvider.FindUsersByName C# (CSharp) Method

		public abstract MembershipUserCollection FindUsersByName (string nameToMatch, int pageIndex, int pageSize, out int totalRecords);
		public abstract MembershipUserCollection GetAllUsers (int pageIndex, int pageSize, out int totalRecords);

        internal bool ValidateUniqueLogin(MemberSave contentItem, MembershipProvider membershipProvider, HttpActionContext actionContext)
        {
            if (contentItem == null) throw new ArgumentNullException("contentItem");
            if (membershipProvider == null) throw new ArgumentNullException("membershipProvider");

            int totalRecs;
            var existingByName = membershipProvider.FindUsersByName(contentItem.Username.Trim(), 0, int.MaxValue, out totalRecs);
            switch (contentItem.Action)
            {
                case ContentSaveAction.Save:

                    //ok, we're updating the member, we need to check if they are changing their login and if so, does it exist already ?
                    if (contentItem.PersistedContent.Username.InvariantEquals(contentItem.Username.Trim()) == false)
                    {
                        //they are changing their login name
                        if (existingByName.Cast<MembershipUser>().Select(x => x.UserName)
                            .Any(x => x == contentItem.Username.Trim()))
                        {
                            //the user cannot use this login
                            return false;
                        }
                    }
                    break;
                case ContentSaveAction.SaveNew:
                    //check if the user's login already exists
                    if (existingByName.Cast<MembershipUser>().Select(x => x.UserName)
                        .Any(x => x == contentItem.Username.Trim()))
                    {
                        //the user cannot use this login
                        return false;
                    }
                    break;
                default:
                    //we don't support this for members
                    throw new HttpResponseException(HttpStatusCode.NotFound);
            }

            return true;
        }