ContactsApp/Repositories/ContactRepository.cs

using Microsoft.Win32;
using System;
using System.Runtime;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace ContactsApp
{
    public class ContactRepository : IContactRepository
    {
        List<Contact> Contacts = new ();


        public void AddContact(Contact contact)
        {
            CheckUnique(contact);
            Contacts.Add(contact);
        }

        public void UpdateContact(Guid id, Contact contact)
        {
            CheckUnique(contact);
            Contact previous = GetContactById(id);
            if (previous == null)
            {
                Contacts.Add(contact);
                return;
            }
            
            previous.Name = contact.Name;
            previous.Email = contact.Email;
            previous.Phone = contact.Phone;
        }

        public void DeleteContact(Guid id)
        {
            var contact = GetContactById(id);
            if (contact == null)
                return;
            Contacts.Remove(contact);
        }

        public Contact GetContactById(Guid id)
        {
            return Contacts.Find(x => x.Id == id);
        }
        public void LoadJSON(string json)
        {
            var list = JsonSerializer.Deserialize<List<Contact>>(json);
            Contacts.AddRange(list);
        }
        public string SaveJSON()
        {
            return JsonSerializer.Serialize(Contacts, new JsonSerializerOptions { WriteIndented = true });
        }

        public IEnumerable<Contact> GetAll() => Contacts;
        private void CheckUnique(Contact contact)
        {
            if (Contacts.FirstOrDefault(x => x.Equals(contact)) != null)
                throw new Exception("Такой контакт уже есть");
        }
    }
}