public async Task AddStudentsToClassAsync(Class class1, string[] studentsid) { if (class1 == null) { throw new ArgumentNullException("class1"); } if (studentsid == null) { throw new ArgumentNullException("studentsid"); } var list = new List <User>(); foreach (var student in studentsid) { var user = await UserStore.FindByIdAsync(student); if (user != null) { list.Add(user); } } await UserStore.AddStudentsToClassAsync(class1, list.ToArray()); }