Рефакторинг.

PhoneStore/Controllers/FeedbackController.cs

@@ -23,8 +23,7 @@ namespace PhoneStore.Controllers
         public ActionResult<FeedbackViewModel> Create(FeedbackCreateViewModel model)
         {
             //TODO: валидация.
-            var userId = int.Parse(_userManager.GetUserId(User));
-            var feedbackViewModel = _feedbackService.Create(model, userId);
+            var feedbackViewModel = _feedbackService.Create(model, User);
             
             return Json(feedbackViewModel);
         }

PhoneStore/Services/FeedbackService.cs

 using System;
 using System.Linq;
+using System.Security.Claims;
 using Microsoft.AspNetCore.Identity;
 using Microsoft.EntityFrameworkCore;
 using PhoneStore.Helpers;
@@ -22,14 +23,14 @@ namespace PhoneStore.Services
             _userManager = userManager;
         }
 
-        public FeedbackViewModel Create(FeedbackCreateViewModel model, int userId)
+        public FeedbackViewModel Create(FeedbackCreateViewModel model, ClaimsPrincipal user)
         {
             Feedback feedback = new Feedback
             {
                 CreationDateTime = DateTime.Now,
                 Text = model.Text,
                 PhoneId = model.PhoneId,
-                UserId = userId
+                UserId = int.Parse(_userManager.GetUserId(user))
             };
             _db.Feedbacks.Add(feedback);
             _db.SaveChanges();

PhoneStore/Services/Interfaces/IFeedbackService.cs

+using System.Security.Claims;
 using PhoneStore.ViewModels.Feedback;
 
 namespace PhoneStore.Services.Interfaces
 {
     public interface IFeedbackService
     {
-        FeedbackViewModel Create(FeedbackCreateViewModel model, int userId);
+        FeedbackViewModel Create(FeedbackCreateViewModel model, ClaimsPrincipal user);
     }
 }
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