Добавьте файлы проекта.

Yuna.Website/Yuna.Website.Server/Services/UserService/IUserService.cs

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+using System.Collections.Generic;
+using Yuna.Website.Server.Model;
+
+namespace Yuna.Website.Server.Services.UserService
+{
+    public interface IUserService
+    {
+        public Task<User?> GetById(long id);
+        public Task<User?> GetByUsername(string username);
+        public Task<IReadOnlyList<User>> GetList();
+        public Task<User?> Create(User user);
+        public Task<User?> Update(User user);
+        public Task<User?> Delete(long id);
+
+    }
+}

Yuna.Website/Yuna.Website.Server/Services/UserService/UserService.cs

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+using Yuna.Website.Server.Model;
+using Yuna.Website.Server.Storage.Repositories.User;
+
+namespace Yuna.Website.Server.Services.UserService
+{
+    public class UserService : IUserService
+    {
+        private readonly IUserRepository _userRepository;
+        public UserService(IUserRepository userRepository)
+        {
+            _userRepository = userRepository;
+        }
+
+        public async Task<User?> Create(User user)
+        {
+            var existingUser = await _userRepository.GetByUsername(user.UserName);
+            if (existingUser is not null) return null;
+
+            var result = await _userRepository.Create(user);
+            return result;
+        }
+
+        public async Task<User?> Delete(long id)
+        {
+            var user = await _userRepository.GetById(id);
+            if (user is null) return null;
+
+            var result = await _userRepository.Delete(id);
+            return user;
+        }
+
+        public async Task<User?> GetById(long id)
+        {
+            var result = await _userRepository.GetById(id);
+            return result;
+        }
+
+        public async Task<User?> GetByUsername(string username)
+        {
+            var user = await _userRepository.GetByUsername(username);
+            return user;
+        }
+
+        public async Task<IReadOnlyList<User>> GetList()
+        {
+            var result = await _userRepository.GetList();
+            return result ?? [];
+        }
+
+        public async Task<User?> Update(User user)
+        {
+            var userFromDb = await _userRepository.GetById(user.Id);
+            if(userFromDb is null) return null;
+
+            return await _userRepository.Update(user);
+        }
+    }
+}