Refactor to introduce nullable types and reduce compiler warnings (#801)

* Introduce nullable types to Core, Infrastructure and PublicApi projects

* Refactor unit tests

* Fixed failing tests

* Introduce Parameter object for CatalogItem update method

* Refactor CataloItemDetails param object

* Refactor following PR comments

src/ApplicationCore/ApplicationCore.csproj

@@ -3,10 +3,12 @@
 	<PropertyGroup>
 		<TargetFramework>net6.0</TargetFramework>
 		<RootNamespace>Microsoft.eShopWeb.ApplicationCore</RootNamespace>
+		<Nullable>enable</Nullable>
 	</PropertyGroup>
 
 	<ItemGroup>
 		<PackageReference Include="Ardalis.GuardClauses" Version="4.0.1" />
+		<PackageReference Include="Ardalis.Result" Version="4.1.0" />
 		<PackageReference Include="Ardalis.Specification" Version="6.1.0" />
 		<PackageReference Include="MediatR" Version="10.0.1" />
 		<PackageReference Include="System.Security.Claims" Version="4.3.0" />

src/ApplicationCore/CatalogSettings.cs

@@ -2,5 +2,5 @@
 
 public class CatalogSettings
 {
-    public string CatalogBaseUrl { get; set; }
+    public string? CatalogBaseUrl { get; set; }
 }

src/ApplicationCore/Entities/BasketAggregate/Basket.cs

@@ -1,5 +1,6 @@
 using System.Collections.Generic;
 using System.Linq;
+using Ardalis.GuardClauses;
 using Microsoft.eShopWeb.ApplicationCore.Interfaces;
 
 namespace Microsoft.eShopWeb.ApplicationCore.Entities.BasketAggregate;
@@ -25,7 +26,7 @@ public class Basket : BaseEntity, IAggregateRoot
             _items.Add(new BasketItem(catalogItemId, quantity, unitPrice));
             return;
         }
-        var existingItem = Items.FirstOrDefault(i => i.CatalogItemId == catalogItemId);
+        var existingItem = Items.First(i => i.CatalogItemId == catalogItemId);
         existingItem.AddQuantity(quantity);
     }
 

src/ApplicationCore/Entities/BuyerAggregate/Buyer.cs

@@ -12,10 +12,8 @@ public class Buyer : BaseEntity, IAggregateRoot
 
     public IEnumerable<PaymentMethod> PaymentMethods => _paymentMethods.AsReadOnly();
 
-    private Buyer()
-    {
-        // required by EF
-    }
+    #pragma warning disable CS8618 // Required by Entity Framework
+    private Buyer() { }
 
     public Buyer(string identity) : this()
     {

src/ApplicationCore/Entities/BuyerAggregate/PaymentMethod.cs

@@ -2,7 +2,7 @@
 
 public class PaymentMethod : BaseEntity
 {
-    public string Alias { get; private set; }
-    public string CardId { get; private set; } // actual card data must be stored in a PCI compliant system, like Stripe
-    public string Last4 { get; private set; }
+    public string? Alias { get; private set; }
+    public string? CardId { get; private set; } // actual card data must be stored in a PCI compliant system, like Stripe
+    public string? Last4 { get; private set; }
 }

src/ApplicationCore/Entities/CatalogItem.cs

@@ -11,9 +11,9 @@ public class CatalogItem : BaseEntity, IAggregateRoot
     public decimal Price { get; private set; }
     public string PictureUri { get; private set; }
     public int CatalogTypeId { get; private set; }
-    public CatalogType CatalogType { get; private set; }
+    public CatalogType? CatalogType { get; private set; }
     public int CatalogBrandId { get; private set; }
-    public CatalogBrand CatalogBrand { get; private set; }
+    public CatalogBrand? CatalogBrand { get; private set; }
 
     public CatalogItem(int catalogTypeId,
         int catalogBrandId,
@@ -30,15 +30,15 @@ public class CatalogItem : BaseEntity, IAggregateRoot
         PictureUri = pictureUri;
     }
 
-    public void UpdateDetails(string name, string description, decimal price)
+    public void UpdateDetails(CatalogItemDetails details)
     {
-        Guard.Against.NullOrEmpty(name, nameof(name));
-        Guard.Against.NullOrEmpty(description, nameof(description));
-        Guard.Against.NegativeOrZero(price, nameof(price));
+        Guard.Against.NullOrEmpty(details.Name, nameof(details.Name));
+        Guard.Against.NullOrEmpty(details.Description, nameof(details.Description));
+        Guard.Against.NegativeOrZero(details.Price, nameof(details.Price));
 
-        Name = name;
-        Description = description;
-        Price = price;
+        Name = details.Name;
+        Description = details.Description;
+        Price = details.Price;
     }
 
     public void UpdateBrand(int catalogBrandId)
@@ -62,4 +62,18 @@ public class CatalogItem : BaseEntity, IAggregateRoot
         }
         PictureUri = $"images\\products\\{pictureName}?{new DateTime().Ticks}";
     }
+
+    public readonly record struct CatalogItemDetails
+    {
+        public string? Name { get; }
+        public string? Description { get; }
+        public decimal Price { get; }
+
+        public CatalogItemDetails(string? name, string? description, decimal price)
+        {
+            Name = name;
+            Description = description;
+            Price = price;
+        }
+    }
 }

src/ApplicationCore/Entities/OrderAggregate/Address.cs

@@ -12,6 +12,7 @@ public class Address // ValueObject
 
     public string ZipCode { get; private set; }
 
+    #pragma warning disable CS8618 // Required by Entity Framework
     private Address() { }
 
     public Address(string street, string city, string state, string country, string zipcode)

src/ApplicationCore/Entities/OrderAggregate/CatalogItemOrdered.cs

@@ -19,10 +19,8 @@ public class CatalogItemOrdered // ValueObject
         PictureUri = pictureUri;
     }
 
-    private CatalogItemOrdered()
-    {
-        // required by EF
-    }
+    #pragma warning disable CS8618 // Required by Entity Framework
+    private CatalogItemOrdered() {}
 
     public int CatalogItemId { get; private set; }
     public string ProductName { get; private set; }

src/ApplicationCore/Entities/OrderAggregate/Order.cs

@@ -7,16 +7,12 @@ namespace Microsoft.eShopWeb.ApplicationCore.Entities.OrderAggregate;
 
 public class Order : BaseEntity, IAggregateRoot
 {
-    private Order()
-    {
-        // required by EF
-    }
+    #pragma warning disable CS8618 // Required by Entity Framework
+    private Order() {}
 
     public Order(string buyerId, Address shipToAddress, List<OrderItem> items)
     {
         Guard.Against.NullOrEmpty(buyerId, nameof(buyerId));
-        Guard.Against.Null(shipToAddress, nameof(shipToAddress));
-        Guard.Against.Null(items, nameof(items));
 
         BuyerId = buyerId;
         ShipToAddress = shipToAddress;

src/ApplicationCore/Entities/OrderAggregate/OrderItem.cs

@@ -6,10 +6,8 @@ public class OrderItem : BaseEntity
     public decimal UnitPrice { get; private set; }
     public int Units { get; private set; }
 
-    private OrderItem()
-    {
-        // required by EF
-    }
+    #pragma warning disable CS8618 // Required by Entity Framework
+    private OrderItem() {}
 
     public OrderItem(CatalogItemOrdered itemOrdered, decimal unitPrice, int units)
     {

src/ApplicationCore/Extensions/GuardExtensions.cs

@@ -7,12 +7,6 @@ namespace Ardalis.GuardClauses;
 
 public static class BasketGuards
 {
-    public static void NullBasket(this IGuardClause guardClause, int basketId, Basket basket)
-    {
-        if (basket == null)
-            throw new BasketNotFoundException(basketId);
-    }
-
     public static void EmptyBasketOnCheckout(this IGuardClause guardClause, IReadOnlyCollection<BasketItem> basketItems)
     {
         if (!basketItems.Any())

src/ApplicationCore/Extensions/JsonExtensions.cs

@@ -9,7 +9,7 @@ public static class JsonExtensions
         PropertyNameCaseInsensitive = true
     };
 
-    public static T FromJson<T>(this string json) =>
+    public static T? FromJson<T>(this string json) =>
         JsonSerializer.Deserialize<T>(json, _jsonOptions);
 
     public static string ToJson<T>(this T obj) =>

src/ApplicationCore/Interfaces/IBasketService.cs

@@ -1,5 +1,6 @@
 using System.Collections.Generic;
 using System.Threading.Tasks;
+using Ardalis.Result;
 using Microsoft.eShopWeb.ApplicationCore.Entities.BasketAggregate;
 
 namespace Microsoft.eShopWeb.ApplicationCore.Interfaces;
@@ -8,6 +9,6 @@ public interface IBasketService
 {
     Task TransferBasketAsync(string anonymousId, string userName);
     Task<Basket> AddItemToBasket(string username, int catalogItemId, decimal price, int quantity = 1);
-    Task<Basket> SetQuantities(int basketId, Dictionary<string, int> quantities);
+    Task<Result<Basket>> SetQuantities(int basketId, Dictionary<string, int> quantities);
     Task DeleteBasketAsync(int basketId);
 }

src/ApplicationCore/Services/BasketService.cs

@@ -1,6 +1,7 @@
 using System.Collections.Generic;
 using System.Threading.Tasks;
 using Ardalis.GuardClauses;
+using Ardalis.Result;
 using Microsoft.eShopWeb.ApplicationCore.Entities.BasketAggregate;
 using Microsoft.eShopWeb.ApplicationCore.Interfaces;
 using Microsoft.eShopWeb.ApplicationCore.Specifications;
@@ -22,7 +23,7 @@ public class BasketService : IBasketService
     public async Task<Basket> AddItemToBasket(string username, int catalogItemId, decimal price, int quantity = 1)
     {
         var basketSpec = new BasketWithItemsSpecification(username);
-        var basket = await _basketRepository.GetBySpecAsync(basketSpec);
+        var basket = await _basketRepository.FirstOrDefaultAsync(basketSpec);
 
         if (basket == null)
         {
@@ -39,15 +40,15 @@ public class BasketService : IBasketService
     public async Task DeleteBasketAsync(int basketId)
     {
         var basket = await _basketRepository.GetByIdAsync(basketId);
+        Guard.Against.Null(basket, nameof(basket));
         await _basketRepository.DeleteAsync(basket);
     }
 
-    public async Task<Basket> SetQuantities(int basketId, Dictionary<string, int> quantities)
+    public async Task<Result<Basket>> SetQuantities(int basketId, Dictionary<string, int> quantities)
     {
-        Guard.Against.Null(quantities, nameof(quantities));
         var basketSpec = new BasketWithItemsSpecification(basketId);
-        var basket = await _basketRepository.GetBySpecAsync(basketSpec);
-        Guard.Against.NullBasket(basketId, basket);
+        var basket = await _basketRepository.FirstOrDefaultAsync(basketSpec);
+        if (basket == null) return Result<Basket>.NotFound();
 
         foreach (var item in basket.Items)
         {
@@ -64,13 +65,11 @@ public class BasketService : IBasketService
 
     public async Task TransferBasketAsync(string anonymousId, string userName)
     {
-        Guard.Against.NullOrEmpty(anonymousId, nameof(anonymousId));
-        Guard.Against.NullOrEmpty(userName, nameof(userName));
         var anonymousBasketSpec = new BasketWithItemsSpecification(anonymousId);
-        var anonymousBasket = await _basketRepository.GetBySpecAsync(anonymousBasketSpec);
+        var anonymousBasket = await _basketRepository.FirstOrDefaultAsync(anonymousBasketSpec);
         if (anonymousBasket == null) return;
         var userBasketSpec = new BasketWithItemsSpecification(userName);
-        var userBasket = await _basketRepository.GetBySpecAsync(userBasketSpec);
+        var userBasket = await _basketRepository.FirstOrDefaultAsync(userBasketSpec);
         if (userBasket == null)
         {
             userBasket = new Basket(userName);

src/ApplicationCore/Services/OrderService.cs

@@ -30,9 +30,9 @@ public class OrderService : IOrderService
     public async Task CreateOrderAsync(int basketId, Address shippingAddress)
     {
         var basketSpec = new BasketWithItemsSpecification(basketId);
-        var basket = await _basketRepository.GetBySpecAsync(basketSpec);
+        var basket = await _basketRepository.FirstOrDefaultAsync(basketSpec);
 
-        Guard.Against.NullBasket(basketId, basket);
+        Guard.Against.Null(basket, nameof(basket));
         Guard.Against.EmptyBasketOnCheckout(basket.Items);
 
         var catalogItemsSpecification = new CatalogItemsSpecification(basket.Items.Select(item => item.CatalogItemId).ToArray());

src/Infrastructure/Data/CatalogContext.cs

@@ -8,9 +8,8 @@ namespace Microsoft.eShopWeb.Infrastructure.Data;
 
 public class CatalogContext : DbContext
 {
-    public CatalogContext(DbContextOptions<CatalogContext> options) : base(options)
-    {
-    }
+    #pragma warning disable CS8618 // Required by Entity Framework
+    public CatalogContext(DbContextOptions<CatalogContext> options) : base(options) {}
 
     public DbSet<Basket> Baskets { get; set; }
     public DbSet<CatalogItem> CatalogItems { get; set; }

src/Infrastructure/Data/Config/BasketConfiguration.cs

@@ -9,7 +9,7 @@ public class BasketConfiguration : IEntityTypeConfiguration<Basket>
     public void Configure(EntityTypeBuilder<Basket> builder)
     {
         var navigation = builder.Metadata.FindNavigation(nameof(Basket.Items));
-        navigation.SetPropertyAccessMode(PropertyAccessMode.Field);
+        navigation?.SetPropertyAccessMode(PropertyAccessMode.Field);
 
         builder.Property(b => b.BuyerId)
             .IsRequired()

src/Infrastructure/Data/Config/OrderConfiguration.cs

@@ -10,7 +10,7 @@ public class OrderConfiguration : IEntityTypeConfiguration<Order>
     {
         var navigation = builder.Metadata.FindNavigation(nameof(Order.OrderItems));
 
-        navigation.SetPropertyAccessMode(PropertyAccessMode.Field);
+        navigation?.SetPropertyAccessMode(PropertyAccessMode.Field);
 
         builder.Property(b => b.BuyerId)
             .IsRequired()

src/Infrastructure/Data/FileItem.cs

@@ -2,10 +2,10 @@
 
 public class FileItem
 {
-    public string FileName { get; set; }
-    public string Url { get; set; }
+    public string? FileName { get; set; }
+    public string? Url { get; set; }
     public long Size { get; set; }
-    public string Ext { get; set; }
-    public string Type { get; set; }
-    public string DataBase64 { get; set; }
+    public string? Ext { get; set; }
+    public string? Type { get; set; }
+    public string? DataBase64 { get; set; }
 }

src/Infrastructure/Infrastructure.csproj

@@ -3,6 +3,7 @@
 	<PropertyGroup>
 		<TargetFramework>net6.0</TargetFramework>
 		<RootNamespace>Microsoft.eShopWeb.Infrastructure</RootNamespace>
+		<Nullable>enable</Nullable>
 	</PropertyGroup>
 
 	<ItemGroup>

src/PublicApi/CatalogItemEndpoints/UpdateCatalogItemEndpoint.cs

@@ -42,7 +42,8 @@ public class UpdateCatalogItemEndpoint : IEndpoint<IResult, UpdateCatalogItemReq
 
         var existingItem = await _itemRepository.GetByIdAsync(request.Id);
         
-        existingItem.UpdateDetails(request.Name, request.Description, request.Price);
+        CatalogItem.CatalogItemDetails details = new(request.Name, request.Description, request.Price);
+        existingItem.UpdateDetails(details);
         existingItem.UpdateBrand(request.CatalogBrandId);
         existingItem.UpdateType(request.CatalogTypeId);
 

src/PublicApi/PublicApi.csproj

@@ -6,6 +6,7 @@
     <UserSecretsId>5b662463-1efd-4bae-bde4-befe0be3e8ff</UserSecretsId>
     <DockerDefaultTargetOS>Linux</DockerDefaultTargetOS>
     <DockerfileContext>..\..</DockerfileContext>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>

src/Web/Areas/Identity/Pages/Account/Login.cshtml.cs

@@ -1,4 +1,5 @@
 using System.ComponentModel.DataAnnotations;
+using Ardalis.GuardClauses;
 using Microsoft.AspNetCore.Authentication;
 using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Identity;
@@ -73,17 +74,17 @@ public class LoginModel : PageModel
             // This doesn't count login failures towards account lockout
             // To enable password failures to trigger account lockout, set lockoutOnFailure: true
             //var result = await _signInManager.PasswordSignInAsync(Input.Email, Input.Password, Input.RememberMe, lockoutOnFailure: true);
-            var result = await _signInManager.PasswordSignInAsync(Input.Email, Input.Password, false, true);
+            var result = await _signInManager.PasswordSignInAsync(Input?.Email, Input?.Password, false, true);
 
             if (result.Succeeded)
             {
                 _logger.LogInformation("User logged in.");
-                await TransferAnonymousBasketToUserAsync(Input.Email);
+                await TransferAnonymousBasketToUserAsync(Input?.Email);
                 return LocalRedirect(returnUrl);
             }
             if (result.RequiresTwoFactor)
             {
-                return RedirectToPage("./LoginWith2fa", new { ReturnUrl = returnUrl, RememberMe = Input.RememberMe });
+                return RedirectToPage("./LoginWith2fa", new { ReturnUrl = returnUrl, RememberMe = Input?.RememberMe });
             }
             if (result.IsLockedOut)
             {
@@ -108,6 +109,7 @@ public class LoginModel : PageModel
             var anonymousId = Request.Cookies[Constants.BASKET_COOKIENAME];
             if (Guid.TryParse(anonymousId, out var _))
             {
+                Guard.Against.NullOrEmpty(userName, nameof(userName));
                 await _basketService.TransferBasketAsync(anonymousId, userName);
             }
             Response.Cookies.Delete(Constants.BASKET_COOKIENAME);

src/Web/Areas/Identity/Pages/Account/Register.cshtml.cs

@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.ComponentModel.DataAnnotations;
 using System.Text.Encodings.Web;
 using System.Threading.Tasks;
+using Ardalis.GuardClauses;
 using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Identity;
 using Microsoft.AspNetCore.Identity.UI.Services;
@@ -67,8 +68,8 @@ public class RegisterModel : PageModel
         returnUrl = returnUrl ?? Url.Content("~/");
         if (ModelState.IsValid)
         {
-            var user = new ApplicationUser { UserName = Input.Email, Email = Input.Email };
-            var result = await _userManager.CreateAsync(user, Input.Password);
+            var user = new ApplicationUser { UserName = Input?.Email, Email = Input?.Email };
+            var result = await _userManager.CreateAsync(user, Input?.Password);
             if (result.Succeeded)
             {
                 _logger.LogInformation("User created a new account with password.");
@@ -80,7 +81,8 @@ public class RegisterModel : PageModel
                     values: new { userId = user.Id, code = code },
                     protocol: Request.Scheme);
 
-                await _emailSender.SendEmailAsync(Input.Email, "Confirm your email",
+                Guard.Against.Null(callbackUrl, nameof(callbackUrl));
+                await _emailSender.SendEmailAsync(Input?.Email, "Confirm your email",
                     $"Please confirm your account by <a href='{HtmlEncoder.Default.Encode(callbackUrl)}'>clicking here</a>.");
 
                 await _signInManager.SignInAsync(user, isPersistent: false);

src/Web/Configuration/RevokeAuthenticationEvents.cs

@@ -22,12 +22,12 @@ public class RevokeAuthenticationEvents : CookieAuthenticationEvents
 
     public override async Task ValidatePrincipal(CookieValidatePrincipalContext context)
     {
-        var userId = context.Principal.Claims.First(c => c.Type == ClaimTypes.Name);
+        var userId = context.Principal?.Claims.First(c => c.Type == ClaimTypes.Name);
         var identityKey = context.Request.Cookies[ConfigureCookieSettings.IdentifierCookieName];
 
-        if (_cache.TryGetValue($"{userId.Value}:{identityKey}", out var revokeKeys))
+        if (_cache.TryGetValue($"{userId?.Value}:{identityKey}", out var revokeKeys))
         {
-            _logger.LogDebug($"Access has been revoked for: {userId.Value}.");
+            _logger.LogDebug($"Access has been revoked for: {userId?.Value}.");
             context.RejectPrincipal();
             await context.HttpContext.SignOutAsync(CookieAuthenticationDefaults.AuthenticationScheme);
         }

src/Web/Controllers/ManageController.cs

@@ -1,5 +1,6 @@
 using System.Text;
 using System.Text.Encodings.Web;
+using Ardalis.GuardClauses;
 using Microsoft.AspNetCore.Authentication;
 using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Identity;
@@ -119,6 +120,7 @@ public class ManageController : Controller
 
         var code = await _userManager.GenerateEmailConfirmationTokenAsync(user);
         var callbackUrl = Url.EmailConfirmationLink(user.Id, code, Request.Scheme);
+        Guard.Against.Null(callbackUrl, nameof(callbackUrl));
         var email = user.Email;
         await _emailSender.SendEmailConfirmationAsync(email, callbackUrl);
 
@@ -405,7 +407,7 @@ public class ManageController : Controller
         }
 
         // Strip spaces and hypens
-        var verificationCode = model.Code.Replace(" ", string.Empty).Replace("-", string.Empty);
+        var verificationCode = model.Code?.Replace(" ", string.Empty).Replace("-", string.Empty);
 
         var is2faTokenValid = await _userManager.VerifyTwoFactorTokenAsync(
             user, _userManager.Options.Tokens.AuthenticatorTokenProvider, verificationCode);

src/Web/Controllers/OrderController.cs

@@ -1,4 +1,5 @@
-using MediatR;
+using Ardalis.GuardClauses;
+using MediatR;
 using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.eShopWeb.Web.Features.MyOrders;
@@ -21,6 +22,7 @@ public class OrderController : Controller
     [HttpGet]
     public async Task<IActionResult> MyOrders()
     {   
+        Guard.Against.Null(User?.Identity?.Name, nameof(User.Identity.Name));
         var viewModel = await _mediator.Send(new GetMyOrders(User.Identity.Name));
 
         return View(viewModel);
@@ -29,6 +31,7 @@ public class OrderController : Controller
     [HttpGet("{orderId}")]
     public async Task<IActionResult> Detail(int orderId)
     {
+        Guard.Against.Null(User?.Identity?.Name, nameof(User.Identity.Name));
         var viewModel = await _mediator.Send(new GetOrderDetails(User.Identity.Name, orderId));
 
         if (viewModel == null)

src/Web/Controllers/UserController.cs

@@ -28,7 +28,7 @@ public class UserController : ControllerBase
 
     private async Task<UserInfo> CreateUserInfo(ClaimsPrincipal claimsPrincipal)
     {
-        if (!claimsPrincipal.Identity.IsAuthenticated)
+        if (claimsPrincipal.Identity == null || claimsPrincipal.Identity.Name == null || !claimsPrincipal.Identity.IsAuthenticated)
         {
             return UserInfo.Anonymous;
         }

src/Web/Extensions/UrlHelperExtensions.cs

@@ -2,7 +2,7 @@
 
 public static class UrlHelperExtensions
 {
-    public static string EmailConfirmationLink(this IUrlHelper urlHelper, string userId, string code, string scheme)
+    public static string? EmailConfirmationLink(this IUrlHelper urlHelper, string userId, string code, string scheme)
     {
         return urlHelper.Action(
             action: "GET",

src/Web/Features/MyOrders/GetMyOrdersHandler.cs

@@ -28,7 +28,7 @@ public class GetMyOrdersHandler : IRequestHandler<GetMyOrders, IEnumerable<Order
         return orders.Select(o => new OrderViewModel
         {
             OrderDate = o.OrderDate,
-            OrderItems = o.OrderItems?.Select(oi => new OrderItemViewModel()
+            OrderItems = o.OrderItems.Select(oi => new OrderItemViewModel()
             {
                 PictureUrl = oi.ItemOrdered.PictureUri,
                 ProductId = oi.ItemOrdered.CatalogItemId,

src/Web/Features/OrderDetails/GetOrderDetailsHandler.cs

@@ -9,7 +9,7 @@ using Microsoft.eShopWeb.Web.ViewModels;
 
 namespace Microsoft.eShopWeb.Web.Features.OrderDetails;
 
-public class GetOrderDetailsHandler : IRequestHandler<GetOrderDetails, OrderViewModel>
+public class GetOrderDetailsHandler : IRequestHandler<GetOrderDetails, OrderViewModel?>
 {
     private readonly IReadRepository<Order> _orderRepository;
 
@@ -18,11 +18,11 @@ public class GetOrderDetailsHandler : IRequestHandler<GetOrderDetails, OrderView
         _orderRepository = orderRepository;
     }
 
-    public async Task<OrderViewModel> Handle(GetOrderDetails request,
+    public async Task<OrderViewModel?> Handle(GetOrderDetails request,
         CancellationToken cancellationToken)
     {
         var spec = new OrderWithItemsByIdSpec(request.OrderId);
-        var order = await _orderRepository.GetBySpecAsync(spec, cancellationToken);
+        var order = await _orderRepository.FirstOrDefaultAsync(spec, cancellationToken);
 
         if (order == null)
         {

src/Web/HealthChecks/HomePageHealthCheck.cs

@@ -19,8 +19,8 @@ public class HomePageHealthCheck : IHealthCheck
         HealthCheckContext context,
         CancellationToken cancellationToken = default(CancellationToken))
     {
-        var request = _httpContextAccessor.HttpContext.Request;
-        string myUrl = request.Scheme + "://" + request.Host.ToString();
+        var request = _httpContextAccessor.HttpContext?.Request;
+        string myUrl = request?.Scheme + "://" + request?.Host.ToString();
 
         var client = new HttpClient();
         var response = await client.GetAsync(myUrl);

src/Web/Pages/Basket/Checkout.cshtml.cs

@@ -1,4 +1,5 @@
-using Microsoft.AspNetCore.Authorization;
+using Ardalis.GuardClauses;
+using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Identity;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.RazorPages;
@@ -68,6 +69,7 @@ public class CheckoutModel : PageModel
 
     private async Task SetBasketModelAsync()
     {
+        Guard.Against.Null(User?.Identity?.Name, nameof(User.Identity.Name));
         if (_signInManager.IsSignedIn(HttpContext.User))
         {
             BasketModel = await _basketViewModelService.GetOrCreateBasketForUser(User.Identity.Name);
@@ -75,7 +77,7 @@ public class CheckoutModel : PageModel
         else
         {
             GetOrSetBasketCookieAndUserName();
-            BasketModel = await _basketViewModelService.GetOrCreateBasketForUser(_username);
+            BasketModel = await _basketViewModelService.GetOrCreateBasketForUser(_username!);
         }
     }
 

src/Web/Pages/Basket/Index.cshtml.cs

@@ -1,4 +1,5 @@
-using Microsoft.AspNetCore.Mvc;
+using Ardalis.GuardClauses;
+using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.RazorPages;
 using Microsoft.eShopWeb.ApplicationCore.Entities;
 using Microsoft.eShopWeb.ApplicationCore.Interfaces;
@@ -66,11 +67,13 @@ public class IndexModel : PageModel
 
     private string GetOrSetBasketCookieAndUserName()
     {
+        Guard.Against.Null(Request.HttpContext.User.Identity, nameof(Request.HttpContext.User.Identity));
         string? userName = null;
 
         if (Request.HttpContext.User.Identity.IsAuthenticated)
         {
-            return Request.HttpContext.User.Identity.Name;
+            Guard.Against.Null(Request.HttpContext.User.Identity.Name, nameof(Request.HttpContext.User.Identity.Name));
+            return Request.HttpContext.User.Identity.Name!;
         }
 
         if (Request.Cookies.ContainsKey(Constants.BASKET_COOKIENAME))

src/Web/Pages/Shared/Components/BasketComponent/Basket.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Threading.Tasks;
+using Ardalis.GuardClauses;
 using Microsoft.AspNetCore.Identity;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.eShopWeb.Infrastructure.Identity;
@@ -33,17 +34,18 @@ public class Basket : ViewComponent
     {
         if (_signInManager.IsSignedIn(HttpContext.User))
         {
+            Guard.Against.Null(User?.Identity?.Name, nameof(User.Identity.Name));
             return await _basketService.CountTotalBasketItems(User.Identity.Name);
         }
 
-        string anonymousId = GetAnnonymousIdFromCookie();
+        string? anonymousId = GetAnnonymousIdFromCookie();
         if (anonymousId == null)
             return 0;
 
         return await _basketService.CountTotalBasketItems(anonymousId);
     }
 
-    private string GetAnnonymousIdFromCookie()
+    private string? GetAnnonymousIdFromCookie()
     {
         if (Request.Cookies.ContainsKey(Constants.BASKET_COOKIENAME))
         {

src/Web/Services/BasketViewModelService.cs

@@ -28,7 +28,7 @@ public class BasketViewModelService : IBasketViewModelService
     public async Task<BasketViewModel> GetOrCreateBasketForUser(string userName)
     {
         var basketSpec = new BasketWithItemsSpecification(userName);
-        var basket = (await _basketRepository.GetBySpecAsync(basketSpec));
+        var basket = (await _basketRepository.FirstOrDefaultAsync(basketSpec));
 
         if (basket == null)
         {

src/Web/Services/CatalogItemViewModelService.cs

@@ -1,4 +1,4 @@
-using System.Threading.Tasks;
+using Ardalis.GuardClauses;
 using Microsoft.eShopWeb.ApplicationCore.Entities;
 using Microsoft.eShopWeb.ApplicationCore.Interfaces;
 using Microsoft.eShopWeb.Web.Interfaces;
@@ -18,7 +18,11 @@ public class CatalogItemViewModelService : ICatalogItemViewModelService
     public async Task UpdateCatalogItem(CatalogItemViewModel viewModel)
     {
         var existingCatalogItem = await _catalogItemRepository.GetByIdAsync(viewModel.Id);
-        existingCatalogItem.UpdateDetails(viewModel.Name, existingCatalogItem.Description, viewModel.Price);
+
+        Guard.Against.Null(existingCatalogItem, nameof(existingCatalogItem));
+
+        CatalogItem.CatalogItemDetails details = new(viewModel.Name, existingCatalogItem.Description, viewModel.Price);
+        existingCatalogItem.UpdateDetails(details);
         await _catalogItemRepository.UpdateAsync(existingCatalogItem);
     }
 }

src/Web/SlugifyParameterTransformer.cs

@@ -5,11 +5,13 @@ namespace Microsoft.eShopWeb.Web;
 
 public class SlugifyParameterTransformer : IOutboundParameterTransformer
 {
-    public string? TransformOutbound(object value)
+    public string? TransformOutbound(object? value)
     {
         if (value == null) { return null; }
+        string? str = value.ToString();
+        if (string.IsNullOrEmpty(str)) { return null; }
 
         // Slugify value
-        return Regex.Replace(value.ToString(), "([a-z])([A-Z])", "$1-$2").ToLower();
+        return Regex.Replace(str, "([a-z])([A-Z])", "$1-$2").ToLower();
     }
 }

tests/FunctionalTests/Web/Pages/HomePageOnGet.cs

@@ -1,6 +1,4 @@
-using System.Net.Http;
-using System.Threading.Tasks;
-using Microsoft.eShopWeb.FunctionalTests.Web;
+using Microsoft.eShopWeb.FunctionalTests.Web;
 using Xunit;
 
 namespace Microsoft.eShopWeb.FunctionalTests.WebRazorPages;

tests/IntegrationTests/Repositories/OrderRepositoryTests/GetByIdWithItemsAsync.cs

@@ -49,7 +49,7 @@ public class GetByIdWithItemsAsync
 
         //Act
         var spec = new OrderWithItemsByIdSpec(secondOrderId);
-        var orderFromRepo = await _orderRepository.GetBySpecAsync(spec);
+        var orderFromRepo = await _orderRepository.FirstOrDefaultAsync(spec);
 
         //Assert
         Assert.Equal(secondOrderId, orderFromRepo.Id);

tests/UnitTests/ApplicationCore/Entities/CatalogItemTests/UpdateDetails.cs

@@ -1,55 +0,0 @@
-using System;
-using Microsoft.eShopWeb.ApplicationCore.Entities;
-using Xunit;
-
-namespace Microsoft.eShopWeb.UnitTests.ApplicationCore.Entities.CatalogItemTests;
-
-public class UpdateDetails
-{
-    private CatalogItem _testItem;
-    private int _validTypeId = 1;
-    private int _validBrandId = 2;
-    private string _validDescription = "test description";
-    private string _validName = "test name";
-    private decimal _validPrice = 1.23m;
-    private string _validUri = "/123";
-
-    public UpdateDetails()
-    {
-        _testItem = new CatalogItem(_validTypeId, _validBrandId, _validDescription, _validName, _validPrice, _validUri);
-    }
-
-    [Fact]
-    public void ThrowsArgumentExceptionGivenEmptyName()
-    {
-        string newValue = "";
-        Assert.Throws<ArgumentException>(() => _testItem.UpdateDetails(newValue, _validDescription, _validPrice));
-    }
-
-    [Fact]
-    public void ThrowsArgumentExceptionGivenEmptyDescription()
-    {
-        string newValue = "";
-        Assert.Throws<ArgumentException>(() => _testItem.UpdateDetails(_validName, newValue, _validPrice));
-    }
-
-    [Fact]
-    public void ThrowsArgumentNullExceptionGivenNullName()
-    {
-        Assert.Throws<ArgumentNullException>(() => _testItem.UpdateDetails(null, _validDescription, _validPrice));
-    }
-
-    [Fact]
-    public void ThrowsArgumentNullExceptionGivenNullDescription()
-    {
-        Assert.Throws<ArgumentNullException>(() => _testItem.UpdateDetails(_validName, null, _validPrice));
-    }
-
-    [Theory]
-    [InlineData(0)]
-    [InlineData(-1.23)]
-    public void ThrowsArgumentExceptionGivenNonPositivePrice(decimal newPrice)
-    {
-        Assert.Throws<ArgumentException>(() => _testItem.UpdateDetails(_validName, _validDescription, newPrice));
-    }
-}

tests/UnitTests/ApplicationCore/Extensions/TestParent.cs

@@ -1,7 +1,4 @@
-using System;
-using System.Collections.Generic;
-using System.Diagnostics.CodeAnalysis;
-using System.Linq;
+using System.Diagnostics.CodeAnalysis;
 
 namespace Microsoft.eShopWeb.UnitTests.ApplicationCore.Extensions;
 
@@ -9,12 +6,22 @@ public class TestParent : IEquatable<TestParent>
 {
     public int Id { get; set; }
 
-    public string Name { get; set; }
+    public string? Name { get; set; }
 
-    public IEnumerable<TestChild> Children { get; set; }
+    public IEnumerable<TestChild>? Children { get; set; }
 
-    public bool Equals([AllowNull] TestParent other) =>
-        other?.Id == Id && other?.Name == Name &&
-        (other?.Children is null && Children is null ||
-        (other?.Children?.Zip(Children)?.All(t => t.First?.Equals(t.Second) ?? false) ?? false));
+    public bool Equals([AllowNull] TestParent other) 
+    {
+        if (other?.Id == Id && other?.Name == Name)
+        {
+            if (Children is null)
+            {
+                return other?.Children is null;
+            }
+
+            return other?.Children?.Zip(Children).All(t => t.First?.Equals(t.Second) ?? false) ?? false;
+        }
+
+        return false;
+    }
 }

tests/UnitTests/ApplicationCore/Services/BasketServiceTests/AddItemToBasket.cs

@@ -12,19 +12,20 @@ public class AddItemToBasket
 {
     private readonly string _buyerId = "Test buyerId";
     private readonly Mock<IRepository<Basket>> _mockBasketRepo = new();
+    private readonly Mock<IAppLogger<BasketService>> _mockLogger = new();
 
     [Fact]
     public async Task InvokesBasketRepositoryGetBySpecAsyncOnce()
     {
         var basket = new Basket(_buyerId);
         basket.AddItem(1, It.IsAny<decimal>(), It.IsAny<int>());
-        _mockBasketRepo.Setup(x => x.GetBySpecAsync(It.IsAny<BasketWithItemsSpecification>(), default)).ReturnsAsync(basket);
+        _mockBasketRepo.Setup(x => x.FirstOrDefaultAsync(It.IsAny<BasketWithItemsSpecification>(), default)).ReturnsAsync(basket);
 
-        var basketService = new BasketService(_mockBasketRepo.Object, null);
+        var basketService = new BasketService(_mockBasketRepo.Object, _mockLogger.Object);
 
         await basketService.AddItemToBasket(basket.BuyerId, 1, 1.50m);
 
-        _mockBasketRepo.Verify(x => x.GetBySpecAsync(It.IsAny<BasketWithItemsSpecification>(), default), Times.Once);
+        _mockBasketRepo.Verify(x => x.FirstOrDefaultAsync(It.IsAny<BasketWithItemsSpecification>(), default), Times.Once);
     }
 
     [Fact]
@@ -32,9 +33,9 @@ public class AddItemToBasket
     {
         var basket = new Basket(_buyerId);
         basket.AddItem(1, It.IsAny<decimal>(), It.IsAny<int>());
-        _mockBasketRepo.Setup(x => x.GetBySpecAsync(It.IsAny<BasketWithItemsSpecification>(), default)).ReturnsAsync(basket);
+        _mockBasketRepo.Setup(x => x.FirstOrDefaultAsync(It.IsAny<BasketWithItemsSpecification>(), default)).ReturnsAsync(basket);
 
-        var basketService = new BasketService(_mockBasketRepo.Object, null);
+        var basketService = new BasketService(_mockBasketRepo.Object, _mockLogger.Object);
 
         await basketService.AddItemToBasket(basket.BuyerId, 1, 1.50m);
 

tests/UnitTests/ApplicationCore/Services/BasketServiceTests/DeleteBasket.cs

@@ -11,6 +11,7 @@ public class DeleteBasket
 {
     private readonly string _buyerId = "Test buyerId";
     private readonly Mock<IRepository<Basket>> _mockBasketRepo = new();
+    private readonly Mock<IAppLogger<BasketService>> _mockLogger = new();
 
     [Fact]
     public async Task ShouldInvokeBasketRepositoryDeleteAsyncOnce()
@@ -20,7 +21,7 @@ public class DeleteBasket
         basket.AddItem(2, It.IsAny<decimal>(), It.IsAny<int>());
         _mockBasketRepo.Setup(x => x.GetByIdAsync(It.IsAny<int>(), default))
             .ReturnsAsync(basket);
-        var basketService = new BasketService(_mockBasketRepo.Object, null);
+        var basketService = new BasketService(_mockBasketRepo.Object, _mockLogger.Object);
 
         await basketService.DeleteBasketAsync(It.IsAny<int>());
 

tests/UnitTests/ApplicationCore/Services/BasketServiceTests/SetQuantities.cs

@@ -1,34 +0,0 @@
-using System;
-using System.Threading.Tasks;
-using Microsoft.eShopWeb.ApplicationCore.Entities.BasketAggregate;
-using Microsoft.eShopWeb.ApplicationCore.Exceptions;
-using Microsoft.eShopWeb.ApplicationCore.Interfaces;
-using Microsoft.eShopWeb.ApplicationCore.Services;
-using Moq;
-using Xunit;
-
-namespace Microsoft.eShopWeb.UnitTests.ApplicationCore.Services.BasketServiceTests;
-
-public class SetQuantities
-{
-    private readonly int _invalidId = -1;
-    private readonly Mock<IRepository<Basket>> _mockBasketRepo = new();
-
-    [Fact]
-    public async Task ThrowsGivenInvalidBasketId()
-    {
-        var basketService = new BasketService(_mockBasketRepo.Object, null);
-
-        await Assert.ThrowsAsync<BasketNotFoundException>(async () =>
-            await basketService.SetQuantities(_invalidId, new System.Collections.Generic.Dictionary<string, int>()));
-    }
-
-    [Fact]
-    public async Task ThrowsGivenNullQuantities()
-    {
-        var basketService = new BasketService(null, null);
-
-        await Assert.ThrowsAsync<ArgumentNullException>(async () =>
-            await basketService.SetQuantities(123, null));
-    }
-}

tests/UnitTests/ApplicationCore/Services/BasketServiceTests/TransferBasket.cs

@@ -16,34 +16,19 @@ public class TransferBasket
     private readonly string _nonexistentUserBasketBuyerId = "newuser@microsoft.com";
     private readonly string _existentUserBasketBuyerId = "testuser@microsoft.com";
     private readonly Mock<IRepository<Basket>> _mockBasketRepo = new();
-
-    [Fact]
-    public async Task ThrowsGivenNullAnonymousId()
-    {
-        var basketService = new BasketService(null, null);
-
-        await Assert.ThrowsAsync<ArgumentNullException>(async () => await basketService.TransferBasketAsync(null, "steve"));
-    }
-
-    [Fact]
-    public async Task ThrowsGivenNullUserId()
-    {
-        var basketService = new BasketService(null, null);
-
-        await Assert.ThrowsAsync<ArgumentNullException>(async () => await basketService.TransferBasketAsync("abcdefg", null));
-    }
+    private readonly Mock<IAppLogger<BasketService>> _mockLogger = new();
 
     [Fact]
     public async Task InvokesBasketRepositoryFirstOrDefaultAsyncOnceIfAnonymousBasketNotExists()
     {
         var anonymousBasket = null as Basket;
         var userBasket = new Basket(_existentUserBasketBuyerId);
-        _mockBasketRepo.SetupSequence(x => x.GetBySpecAsync(It.IsAny<BasketWithItemsSpecification>(), default))
+        _mockBasketRepo.SetupSequence(x => x.FirstOrDefaultAsync(It.IsAny<BasketWithItemsSpecification>(), default))
             .ReturnsAsync(anonymousBasket)
             .ReturnsAsync(userBasket);
-        var basketService = new BasketService(_mockBasketRepo.Object, null);
+        var basketService = new BasketService(_mockBasketRepo.Object, _mockLogger.Object);
         await basketService.TransferBasketAsync(_nonexistentAnonymousBasketBuyerId, _existentUserBasketBuyerId);
-        _mockBasketRepo.Verify(x => x.GetBySpecAsync(It.IsAny<BasketWithItemsSpecification>(), default), Times.Once);
+        _mockBasketRepo.Verify(x => x.FirstOrDefaultAsync(It.IsAny<BasketWithItemsSpecification>(), default), Times.Once);
     }
 
     [Fact]
@@ -55,10 +40,10 @@ public class TransferBasket
         var userBasket = new Basket(_existentUserBasketBuyerId);
         userBasket.AddItem(1, 10, 4);
         userBasket.AddItem(2, 99, 3);
-        _mockBasketRepo.SetupSequence(x => x.GetBySpecAsync(It.IsAny<BasketWithItemsSpecification>(), default))
+        _mockBasketRepo.SetupSequence(x => x.FirstOrDefaultAsync(It.IsAny<BasketWithItemsSpecification>(), default))
             .ReturnsAsync(anonymousBasket)
             .ReturnsAsync(userBasket);
-        var basketService = new BasketService(_mockBasketRepo.Object, null);
+        var basketService = new BasketService(_mockBasketRepo.Object, _mockLogger.Object);
         await basketService.TransferBasketAsync(_nonexistentAnonymousBasketBuyerId, _existentUserBasketBuyerId);
         _mockBasketRepo.Verify(x => x.UpdateAsync(userBasket, default), Times.Once);
         Assert.Equal(3, userBasket.Items.Count);
@@ -72,10 +57,10 @@ public class TransferBasket
     {
         var anonymousBasket = new Basket(_existentAnonymousBasketBuyerId);
         var userBasket = new Basket(_existentUserBasketBuyerId);
-        _mockBasketRepo.SetupSequence(x => x.GetBySpecAsync(It.IsAny<BasketWithItemsSpecification>(), default))
+        _mockBasketRepo.SetupSequence(x => x.FirstOrDefaultAsync(It.IsAny<BasketWithItemsSpecification>(), default))
             .ReturnsAsync(anonymousBasket)
             .ReturnsAsync(userBasket);
-        var basketService = new BasketService(_mockBasketRepo.Object, null);
+        var basketService = new BasketService(_mockBasketRepo.Object, _mockLogger.Object);
         await basketService.TransferBasketAsync(_nonexistentAnonymousBasketBuyerId, _existentUserBasketBuyerId);
         _mockBasketRepo.Verify(x => x.UpdateAsync(userBasket, default), Times.Once);
         _mockBasketRepo.Verify(x => x.DeleteAsync(anonymousBasket, default), Times.Once);
@@ -86,10 +71,10 @@ public class TransferBasket
     {
         var anonymousBasket = new Basket(_existentAnonymousBasketBuyerId);
         var userBasket = null as Basket;
-        _mockBasketRepo.SetupSequence(x => x.GetBySpecAsync(It.IsAny<BasketWithItemsSpecification>(), default))
+        _mockBasketRepo.SetupSequence(x => x.FirstOrDefaultAsync(It.IsAny<BasketWithItemsSpecification>(), default))
             .ReturnsAsync(anonymousBasket)
             .ReturnsAsync(userBasket);
-        var basketService = new BasketService(_mockBasketRepo.Object, null);
+        var basketService = new BasketService(_mockBasketRepo.Object, _mockLogger.Object);
         await basketService.TransferBasketAsync(_existentAnonymousBasketBuyerId, _nonexistentUserBasketBuyerId);
         _mockBasketRepo.Verify(x => x.AddAsync(It.Is<Basket>(x => x.BuyerId == _nonexistentUserBasketBuyerId), default), Times.Once);
     }

tests/UnitTests/ApplicationCore/Specifications/CatalogFilterPaginatedSpecification.cs

@@ -1,6 +1,4 @@
-using System.Collections.Generic;
-using System.Linq;
-using Microsoft.eShopWeb.ApplicationCore.Entities;
+using Microsoft.eShopWeb.ApplicationCore.Entities;
 using Xunit;
 
 namespace Microsoft.eShopWeb.UnitTests.ApplicationCore.Specifications;
@@ -12,9 +10,7 @@ public class CatalogFilterPaginatedSpecification
     {
         var spec = new eShopWeb.ApplicationCore.Specifications.CatalogFilterPaginatedSpecification(0, 10, null, null);
 
-        var result = GetTestCollection()
-            .AsQueryable()
-            .Where(spec.WhereExpressions.FirstOrDefault().Filter);
+        var result = spec.Evaluate(GetTestCollection());
 
         Assert.NotNull(result);
         Assert.Equal(4, result.ToList().Count);
@@ -25,9 +21,7 @@ public class CatalogFilterPaginatedSpecification
     {
         var spec = new eShopWeb.ApplicationCore.Specifications.CatalogFilterPaginatedSpecification(0, 10, 1, 1);
 
-        var result = GetTestCollection()
-            .AsQueryable()
-            .Where(spec.WhereExpressions.FirstOrDefault().Filter);
+        var result = spec.Evaluate(GetTestCollection()).ToList();
 
         Assert.NotNull(result);
         Assert.Equal(2, result.ToList().Count);

tests/UnitTests/ApplicationCore/Specifications/CatalogFilterSpecification.cs

@@ -19,9 +19,7 @@ public class CatalogFilterSpecification
     {
         var spec = new eShopWeb.ApplicationCore.Specifications.CatalogFilterSpecification(brandId, typeId);
 
-        var result = GetTestItemCollection()
-            .AsQueryable()
-            .Where(spec.WhereExpressions.FirstOrDefault().Filter);
+        var result = spec.Evaluate(GetTestItemCollection()).ToList();
 
         Assert.Equal(expectedCount, result.Count());
     }

tests/UnitTests/ApplicationCore/Specifications/CatalogItemsSpecification.cs

@@ -14,9 +14,7 @@ public class CatalogItemsSpecification
         var catalogItemIds = new int[] { 1 };
         var spec = new eShopWeb.ApplicationCore.Specifications.CatalogItemsSpecification(catalogItemIds);
 
-        var result = GetTestCollection()
-            .AsQueryable()
-            .Where(spec.WhereExpressions.FirstOrDefault().Filter);
+        var result = spec.Evaluate(GetTestCollection()).ToList();
 
         Assert.NotNull(result);
         Assert.Single(result.ToList());
@@ -28,9 +26,7 @@ public class CatalogItemsSpecification
         var catalogItemIds = new int[] { 1, 3 };
         var spec = new eShopWeb.ApplicationCore.Specifications.CatalogItemsSpecification(catalogItemIds);
 
-        var result = GetTestCollection()
-            .AsQueryable()
-            .Where(spec.WhereExpressions.FirstOrDefault().Filter);
+        var result = spec.Evaluate(GetTestCollection()).ToList();
 
         Assert.NotNull(result);
         Assert.Equal(2, result.ToList().Count);

tests/UnitTests/ApplicationCore/Specifications/CustomerOrdersWithItemsSpecification.cs

@@ -15,14 +15,12 @@ public class CustomerOrdersWithItemsSpecification
     {
         var spec = new eShopWeb.ApplicationCore.Specifications.CustomerOrdersWithItemsSpecification(_buyerId);
 
-        var result = GetTestCollection()
-            .AsQueryable()
-            .FirstOrDefault(spec.WhereExpressions.FirstOrDefault().Filter);
+        var result = spec.Evaluate(GetTestCollection()).FirstOrDefault();
 
         Assert.NotNull(result);
         Assert.NotNull(result.OrderItems);
         Assert.Equal(1, result.OrderItems.Count);
-        Assert.NotNull(result.OrderItems.FirstOrDefault().ItemOrdered);
+        Assert.NotNull(result.OrderItems.FirstOrDefault()?.ItemOrdered);
     }
 
     [Fact]
@@ -30,15 +28,12 @@ public class CustomerOrdersWithItemsSpecification
     {
         var spec = new eShopWeb.ApplicationCore.Specifications.CustomerOrdersWithItemsSpecification(_buyerId);
 
-        var result = GetTestCollection()
-            .AsQueryable()
-            .Where(spec.WhereExpressions.FirstOrDefault().Filter)
-            .ToList();
+        var result = spec.Evaluate(GetTestCollection()).ToList();
 
         Assert.NotNull(result);
         Assert.Equal(2, result.Count);
         Assert.Equal(1, result[0].OrderItems.Count);
-        Assert.NotNull(result[0].OrderItems.FirstOrDefault().ItemOrdered);
+        Assert.NotNull(result[0].OrderItems.FirstOrDefault()?.ItemOrdered);
         Assert.Equal(2, result[1].OrderItems.Count);
         Assert.NotNull(result[1].OrderItems.ToList()[0].ItemOrdered);
         Assert.NotNull(result[1].OrderItems.ToList()[1].ItemOrdered);

tests/UnitTests/MediatorHandlers/OrdersTests/GetOrderDetails.cs

@@ -22,7 +22,7 @@ public class GetOrderDetails
         Order order = new Order("buyerId", address, new List<OrderItem> { item });
 
         _mockOrderRepository = new Mock<IReadRepository<Order>>();
-        _mockOrderRepository.Setup(x => x.GetBySpecAsync(It.IsAny<OrderWithItemsByIdSpec>(), default))
+        _mockOrderRepository.Setup(x => x.FirstOrDefaultAsync(It.IsAny<OrderWithItemsByIdSpec>(), default))
             .ReturnsAsync(order);
     }