public static string ImportUsers(InstagraphContext context, string jsonString)
        {
            UserDTO[]     UsersFromJSON = JsonConvert.DeserializeObject <UserDTO[]>(jsonString);
            StringBuilder sb            = new StringBuilder();

            HashSet <string> usedUsernames = new HashSet <string>();

            Dictionary <string, Picture> pictures = context.Pictures.ToDictionary(k => k.Path);

            foreach (UserDTO userDTO in UsersFromJSON)
            {
                bool usernameIsNullOrEmpty     = string.IsNullOrWhiteSpace(userDTO.Username);
                bool passwordIsNullOrEmpty     = string.IsNullOrWhiteSpace(userDTO.Password);
                bool profilePictureIsNullEmpty = string.IsNullOrWhiteSpace(userDTO.ProfilePicture);

                bool usernameIsNotUnique = true;
                if (!usernameIsNullOrEmpty)
                {
                    usernameIsNotUnique = usedUsernames.Contains(userDTO.Username);
                }

                bool noSuchProfilePicture = true;
                if (!profilePictureIsNullEmpty)
                {
                    noSuchProfilePicture = !pictures.ContainsKey(userDTO.ProfilePicture);
                }

                if (!usernameIsNullOrEmpty && !passwordIsNullOrEmpty && !profilePictureIsNullEmpty && !usernameIsNotUnique && !noSuchProfilePicture)
                {
                    User newUser = new User()
                    {
                        Username         = userDTO.Username,
                        Password         = userDTO.Password,
                        ProfilePictureId = pictures[userDTO.ProfilePicture].Id,
                        ProfilePicture   = pictures[userDTO.ProfilePicture]
                    };

                    context.Add(newUser);
                    sb.AppendLine($"Successfully imported User {newUser.Username}.");
                    usedUsernames.Add(newUser.Username);
                }
                else
                {
                    sb.AppendLine("Error: Invalid data.");
                }
            }

            context.SaveChanges();

            string result = sb.ToString();

            return(result);
        }