Добавьте файлы проекта.

ContactsApp.csproj

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+<Project Sdk="Microsoft.NET.Sdk.Web">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.AspNetCore.OpenApi" Version="10.0.0" />
+    <PackageReference Include="Swashbuckle.AspNetCore" Version="10.0.1" />
+  </ItemGroup>
+
+</Project>

ContactsApp.http

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+@ContactsApp_HostAddress = http://localhost:5246
+
+GET {{ContactsApp_HostAddress}}/weatherforecast/
+Accept: application/json
+
+###

ContactsApp.slnx

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+<Solution>
+  <Project Path="ContactsApp.csproj" />
+</Solution>

Controllers/ContactController.cs

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+using Microsoft.AspNetCore.Mvc;
+using Microsoft.Extensions.Diagnostics.HealthChecks;
+using System.Text.Json;
+
+namespace ContactsApp
+{
+    [ApiController]
+    [Route("api/contacts")]
+    public class ContactController(IContactRepository repository) : ControllerBase
+    {
+        [HttpGet]
+        public ActionResult<IEnumerable<Contact>> GetAll()
+        {
+            var contacts = repository.GetAll().ToList();
+            return Ok(contacts);
+        }
+
+        [HttpPost]
+        public ActionResult<Contact> Create([FromBody] Contact contact)
+        {
+            repository.AddContact(contact);
+            return Ok(contact);
+        }
+
+        [HttpGet("{id:Guid}")]
+        public ActionResult<Contact> GetById(Guid id)
+        {
+            var contact = repository.GetContactById(id);
+            if (contact == null)
+                return NotFound();
+
+            return Ok(contact);
+        }
+        [HttpPost("{id:Guid}")]
+        public ActionResult Update(Guid id, [FromBody] Contact contact)
+        {
+            repository.UpdateContact(id, contact);
+            return Ok();
+        }
+
+        [HttpDelete("{id:Guid}")]
+        public ActionResult Delete(Guid id)
+        {
+            var contact = repository.GetContactById(id);
+            if (contact == null)
+                return NotFound();
+
+            repository.DeleteContact(id);
+            return Ok();
+        }
+
+        [HttpPost("upload")]
+        public async Task<IActionResult> Upload(IFormFile file)
+        {
+            if (file == null || file.Length == 0)
+                return BadRequest("<22><><EFBFBD><EFBFBD> <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD>.");
+
+            using var stream = new MemoryStream();
+            await file.CopyToAsync(stream);
+            string json = System.Text.Encoding.UTF8.GetString(stream.ToArray());
+
+            try
+            {
+                repository.LoadJSON(json); // <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> <20><><EFBFBD> <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD> JSON <20> <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
+            }
+            catch (Exception ex)
+            {
+                return BadRequest("<22><><EFBFBD><EFBFBD><EFBFBD><EFBFBD> <20><><EFBFBD> <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> JSON: " + ex.Message);
+            }
+
+            return Ok("<22><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>.");
+        }
+
+        [HttpGet("download")]
+        public IActionResult Download()
+        {
+            string json = repository.SaveJSON(); // <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> JSON <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
+            byte[] bytes = System.Text.Encoding.UTF8.GetBytes(json);
+
+            return File(
+                bytes,
+                "application/json",
+                "contacts.json"
+            );
+        }
+
+    }
+}
+
+

IContactRepository.cs

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+using System;
+using System.Threading.Tasks;
+
+namespace ContactsApp
+{
+    public interface IContactRepository
+    {
+        public void AddContact(Contact contact);
+        public void UpdateContact(Guid id, Contact contact);
+        public void DeleteContact(Guid id);
+        public Contact GetContactById(Guid id);
+        public IEnumerable<Contact> GetAll();
+        public void LoadJSON(string file);
+        public string SaveJSON();
+    }
+}

Models/Contact.cs

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+using System.ComponentModel.DataAnnotations;
+using System.Net.Mail;
+
+namespace ContactsApp
+{
+    public class Contact
+    {
+        public Guid Id { get; }
+        public string Name { get; set; }
+        public string? Email { get; set; }
+        public string Phone { get; set; }
+
+        public Contact(string name, string? email, string phone)
+        {
+            Id = Guid.NewGuid();
+            Name = name;
+            
+            try
+            {
+                var addr = new MailAddress(email);
+                Email = email;
+            }
+            catch (Exception ex)
+            {
+                throw new Exception("Invalid email");
+            }
+
+            Phone = phone;
+        }
+    }
+}
+
+

Program.cs

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+using Microsoft.AspNetCore.Mvc;
+using Swashbuckle.AspNetCore.Swagger;
+
+
+namespace ContactsApp;
+
+public class Program
+{
+    public static void Main(string[] args)
+    {
+        var builder = WebApplication.CreateBuilder(args);
+
+        // Add services to the container.
+        builder.Services.AddControllers();
+        builder.Services.AddAuthorization();
+        builder.Services.AddSwaggerGen();
+
+        builder.Services.AddSingleton<IContactRepository, ContactRepository>();
+
+        var app = builder.Build();
+
+        // Configure the HTTP request pipeline.
+        // <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD> -> <20><><EFBFBD><EFBFBD><EFBFBD> -> <20><> <20><><EFBFBD><EFBFBD><EFBFBD> -> <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> -> Kestrel -> (HttpContext) Middlewares -> Controller (mapping) -> AfterMethod -> Method -> BeforeMethod
+        app.UseAuthorization();
+        app.UseSwagger();
+        app.UseSwaggerUI();
+        
+
+        app.MapControllers();
+        app.Run();
+    }
+}

Properties/launchSettings.json

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+{
+  "$schema": "http://json.schemastore.org/launchsettings.json",
+  "profiles": {
+    "http": {
+      "commandName": "Project",
+      "dotnetRunMessages": true,
+      "launchBrowser": false,
+      "launchUrl": "swagger",
+      "applicationUrl": "http://localhost:5043",
+      "environmentVariables": {
+        "ASPNETCORE_ENVIRONMENT": "Development"
+      }
+    }
+  }
+}

Repositories/ContactRepository.cs

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+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");
+        }
+    }
+}

appsettings.Development.json

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+{
+  "Logging": {
+    "LogLevel": {
+      "Default": "Information",
+      "Microsoft.AspNetCore": "Warning"
+    }
+  }
+}

appsettings.json

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+{
+  "Logging": {
+    "LogLevel": {
+      "Default": "Information",
+      "Microsoft.AspNetCore": "Warning"
+    }
+  },
+  "AllowedHosts": "*"
+}