#3 Добавил валидацию.

CafeCritic/Controllers/ValidationController.cs

+using CafeCritic.Context;
+using Microsoft.AspNetCore.Mvc;
+
+namespace CafeCritic.Controllers;
+
+public class ValidationController : Controller
+{
+    private readonly CafeDbContext _db;
+
+    public ValidationController(CafeDbContext db)
+    {
+        _db = db;
+    }
+
+    public bool CheckEmail(string email) => !_db.Users.Any(u => u.Email == email);
+    
+    public bool CheckCafeTitle(string cafeTitle) => !_db.Cafes.Any(u => u.Title == cafeTitle);
+    
+}
\ No newline at end of file

CafeCritic/ViewModels/AddCafeViewModel.cs

@@ -6,6 +6,7 @@ namespace CafeCritic.ViewModels;
 public class AddCafeViewModel
 {
     [Required(ErrorMessage = "Поле обязательно")]
+    [Remote("CheckCafeTitle", "Validation", ErrorMessage = "Заведение с такмим названием уже существуе")]
     public required string Title { get; set; }
     
     [Required(ErrorMessage = "Поле обязательно")]

CafeCritic/ViewModels/IndexViewModel.cs

-using CafeCritic.Models;
-
-namespace CafeCritic.ViewModels;
-
-public class IndexViewModel
-{
-    public IEnumerable<Cafe> Cafes { get; set; }
-    public PageViewModel PageViewModel { get; set; }
-    public int Id { get; set; }
-    public string Title { get; set; }
-    public string Image { get; set; }
-    public string? Description { get; set; }
-}
\ No newline at end of file

CafeCritic/ViewModels/RegisterViewModel.cs

@@ -7,6 +7,7 @@ public class RegisterViewModel
 {
     [Required(ErrorMessage = "Для регистрации введите Email")]
     [DataType(DataType.EmailAddress)]
+    [Remote("CheckEmail", "Validation", ErrorMessage = "Пользователь с таким эл. адресом уже существует")]
     public string Email { get; set; }
     
     [Required(ErrorMessage = "Введите пароль")]

CafeCritic/Views/Account/Register.cshtml

@@ -12,7 +12,7 @@
                 <form asp-action="Register" method="post" asp-antiforgery="true">
                     <div class="validation" asp-validation-summary="All"></div>
                     <div class="mb-3">
-                        <input asp-for="Email" class="form-control" placeholder="Email">
+                        <input id="email" asp-for="Email" class="form-control" placeholder="Email">
                         <span asp-validation-for="Email" class="text-danger"></span>
                     </div>
                     <div class="mb-3">

CafeCritic/Views/Cafe/AddCafe.cshtml

@@ -12,7 +12,7 @@
                 <div asp-validation-summary="ModelOnly"></div>
                 <div>
                     <span asp-validation-for="Title" class="text-danger"></span>
-                    <input id="restaurantTitle" asp-for="Title" class="form-control my-2 text-muted" placeholder="Название заведения">
+                    <input id="cafeTitle" asp-for="Title" class="form-control my-2 text-muted" placeholder="Название заведения">
                 </div>
                 <div>
                     <span asp-validation-for="Image" class="text-danger"></span>
@@ -26,6 +26,7 @@
         </div>
     </div>
 </div>
+
 @section Scripts
 {
     @{