c# Generic methods for phonebook project

Question

I'm trying to make Phonebook project, where i write/read data from bin file, I have two class in Domain class library, User and Contacts, now I want to make private generic functions in FileManager class, add/edit/delete and Get which will find/work for both, Contact and User,

How do I know which type is given in private T Get<T>(int id) where T : class function? for make it work with both variety

How do I correctly finish these functions?

namespace Phonebook.Data
{
  public class FileManager
  {
    protected string DataFilePath => @"D:\Data.bin";

    protected IList<User> Users { get; set; }
    protected IList<Contact> Contacts { get; set; }

    public IEnumerable<Contact> SearchContacts(Func<Contact, bool> predicate)
    {
      foreach ( var contact in Contacts )
        if ( predicate(contact) )
          yield return contact;
    }

    public IEnumerable<Contact> AllContacts()
    {
      return SearchContacts(x => true);
    }

    public Contact GetContact(int contactID)
    {
      return Get<Contact>(contactID);
    }

    private T Get<T>(int id) where T : class
    {
      return null;
    }

    private int GenerateContactID()
    {
      int id = 0;
      foreach ( var contact in AllContacts() )
        if ( contact.ContactID > id )
          id = contact.ContactID;
      return id + 1;
    }

    public void AddContact(Contact contact)
    {
      contact.ContactID = GenerateContactID();
      Contacts.Add(contact);
    }

    public void EditContact(Contact contact)
    {
      Edit<Contact>(contact);
    }

    public void DeleteContact(int contactID)
    {
      Delete<Contact>(contactID);
    }

    //add edit da delete generic for both, user / contacts

    private void Add<T>(T entry)
    {
    }

    private void Edit<T>(T entry)
    {
    }

    private void Delete<T>(int id)
    {
    }

    #region Data Load/Save Methods

    public IEnumerable<Contact> LoadData()
    {
      using ( FileStream fileStream = new FileStream(DataFilePath, FileMode.Open) )
      using ( BinaryReader reader = new BinaryReader(fileStream) )
      {
        List<Contact> contacts = new List<Contact>();
        reader.BaseStream.Position = 0;
        while ( reader.PeekChar() != -1 )
        {
          Contact contact = new Contact();
          contact.ContactID = reader.ReadInt32();
          contact.FirstName = reader.ReadString();
          contact.LastName = reader.ReadString();
          contact.Phone = reader.ReadString();
          contact.EMail = reader.ReadString();
          contact.UserID = reader.ReadInt32();
          contacts.Add(contact);
        }
        return contacts;
      }
    }

    public void SaveData(IEnumerable<Contact> contact)
    {
      using ( FileStream fileStream = new FileStream(DataFilePath, FileMode.OpenOrCreate) )
      using ( BinaryWriter writer = new BinaryWriter(fileStream) )
        foreach ( var item in contact )
        {
          writer.Write(item.ContactID);
          writer.Write(item.FirstName);
          writer.Write(item.LastName);
          writer.Write(item.Phone);
          writer.Write(item.EMail);
          writer.Write(item.UserID);
        }
    }

    #endregion
  }
}

Answer 1

I think you should do a generic interface and its implementations for the User and Contact classes separately. If a new class appears, for example Employee - you will do a new implementation of this interface, without any changes in User and Contact classes. And if the source is not a binary file, but a database - then separate implementations of this interface.

As below:

interface IManager<TEntity> where TEntity : class
    {
        IList<TEntity> GetAll();
        TEntity GetById(int id);
        void Add(TEntity entity);
        void Update(TEntity entity);
        void Remove(int id);
        int GenerateContactId();
        IList<TEntity> Search(Func<TEntity, bool> p);
    }

    class BinaryContactManager : IManager<Contact>
    {
        public void Add(Contact entity)
        {
            throw new NotImplementedException();
        }

        public int GenerateContactId()
        {
            throw new NotImplementedException();
        }

        public IList<Contact> GetAll()
        {
            throw new NotImplementedException();
        }

        public Contact GetById(int id)
        {
            throw new NotImplementedException();
        }

        public void Remove(int id)
        {
            throw new NotImplementedException();
        }

        public IList<Contact> Search(Func<Contact, bool> p)
        {
            throw new NotImplementedException();
        }

        public void Update(Contact entity)
        {
            throw new NotImplementedException();
        }
    }

    class BinaryUserManager : IManager<User>
    {
        public void Add(User entity)
        {
            throw new NotImplementedException();
        }

        public int GenerateContactId()
        {
            throw new NotImplementedException();
        }

        public IList<User> GetAll()
        {
            throw new NotImplementedException();
        }

        public User GetById(int id)
        {
            throw new NotImplementedException();
        }

        public void Remove(int id)
        {
            throw new NotImplementedException();
        }

        public IList<User> Search(Func<User, bool> p)
        {
            throw new NotImplementedException();
        }

        public void Update(User entity)
        {
            throw new NotImplementedException();
        }
    }