- добавил библиотеку для валидации.

PhoneStore/Controllers/FeedbackController.cs

 using System.Linq;
+using System.Threading.Tasks;
 using Microsoft.AspNetCore.Mvc;
 using PhoneStore.Models;
 using PhoneStore.Services.Interfaces;
+using PhoneStore.Validations;
 using PhoneStore.ViewModels;
 using PhoneStore.ViewModels.Feedback;
 
@@ -11,17 +13,27 @@ namespace PhoneStore.Controllers
     {
         private readonly IFeedbackService _feedbackService;
         private readonly MobileContext _db;
+        private readonly FeedbackValidation _feedbackValidation;
 
-        public FeedbackController(IFeedbackService feedbackService, MobileContext db)
+        public FeedbackController(
+            IFeedbackService feedbackService, 
+            MobileContext db, 
+            FeedbackValidation feedbackValidation)
         {
             _feedbackService = feedbackService;
             _db = db;
+            _feedbackValidation = feedbackValidation;
         }
 
         [HttpPost]
-        public IActionResult Create(FeedbackCreateViewModel model)
+        public async Task<IActionResult> Create(FeedbackCreateViewModel model)
         {
-            //TODO: валидация.
+            var validationResult = await _feedbackValidation.ValidateAsync(model);
+            if (!validationResult.IsValid)
+            {
+                //TODO: сформировать сообщение 
+                return NotFound();
+            }
             var feedbackViewModel = _feedbackService.Create(model, User);
             return PartialView("PartialViews/FeedbackPartialView", feedbackViewModel);
         }

PhoneStore/Helpers/ValidationsConnector.cs

+using Microsoft.Extensions.DependencyInjection;
+using PhoneStore.Validations;
+
+namespace PhoneStore.Helpers
+{
+    public static class ValidationsConnector
+    {
+        public static void AddValidationServices(this IServiceCollection services)
+        {
+            services.AddTransient<FeedbackValidation>();
+        }
+    }
+}
\ No newline at end of file

PhoneStore/PhoneStore.csproj

@@ -5,6 +5,7 @@
     </PropertyGroup>
 
     <ItemGroup>
+        <PackageReference Include="FluentValidation" Version="11.2.0" />
         <PackageReference Include="Microsoft.AspNetCore.Identity.EntityFrameworkCore" Version="5.0.17" />
         <PackageReference Include="Microsoft.EntityFrameworkCore.Design" Version="5.0.17">
             <PrivateAssets>all</PrivateAssets>

PhoneStore/Startup.cs

@@ -29,6 +29,7 @@ namespace PhoneStore
                 .AddEntityFrameworkStores<MobileContext>();
             services.AddControllersWithViews();
             services.AddApplicationServices(Configuration);
+            services.AddValidationServices();
         }
 
         // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.

PhoneStore/Validations/FeedbackValidation.cs

+using FluentValidation;
+using PhoneStore.ViewModels.Feedback;
+
+namespace PhoneStore.Validations
+{
+    public class FeedbackValidation : AbstractValidator<FeedbackCreateViewModel>
+    {
+        public FeedbackValidation()
+        {
+            RuleFor(f => f.Text).NotEmpty().MinimumLength(10);
+            RuleFor(f => f.PhoneId).NotEmpty().NotEqual(0);
+        }
+    }
+}
\ No newline at end of file