ASP.NET Core Razor Pages Mastery: Advanced UI Patterns, Real-World Examples & Performance Optimization(Part-21 of 40)

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Introduction to Razor Pages Revolution
Razor Pages vs MVC: When to Use What
Page Model Deep Dive
Handler Methods Mastery
Advanced Data Binding Techniques
Real-World E-Commerce Application
Validation and Error Handling
Partial Views and View Components
Advanced Routing Scenarios
Performance Optimization
Security Best Practices
Testing Strategies
Deployment and DevOps

1. Introduction to Razor Pages Revolution

The Evolution of Web Development in ASP.NET Core

Razor Pages represents a significant paradigm shift in how we build web applications with ASP.NET Core. Unlike the traditional MVC pattern that separates concerns across multiple files, Razor Pages embraces a page-centric approach that brings related components together.

// Traditional MVC - Multiple Files// Controller: HomeController.cs// View: Views/Home/Index.cshtml// Model: Models/IndexViewModel.cs

// Razor Pages - Unified Approach// Pages/Index.cshtml// Pages/Index.cshtml.cs (Page Model)

Why Razor Pages Changed Everything

Real-Life Analogy: Think of building a house. With MVC, you have separate teams for foundation (Model), structure (Controller), and interior (View). With Razor Pages, you have dedicated teams for each room (Page) that handle everything related to that specific space.

// Before: MVC Controllers growing uncontrollablypublic class CustomerController : Controller{
    public IActionResult Index() { /* list customers */ }
    public IActionResult Create() { /* show form */ }
    public IActionResult Create(Customer customer) { /* process form */ }
    public IActionResult Edit(int id) { /* show edit form */ }
    public IActionResult Edit(Customer customer) { /* process edit */ }
    public IActionResult Delete(int id) { /* show delete confirmation */ }
    public IActionResult DeleteConfirmed(int id) { /* process delete */ }
    // ... and 20 more actions}

// After: Organized Razor Pages// Pages/Customers/Index.cshtml// Pages/Customers/Create.cshtml// Pages/Customers/Edit.cshtml// Pages/Customers/Delete.cshtml

Setting Up Your First Razor Pages Project

// Program.cs - Razor Pages Configurationvar builder = WebApplication.CreateBuilder(args);

// Add services to the container
builder.Services.AddRazorPages()
    .AddRazorPagesOptions(options =>
    {
        options.RootDirectory = "/Pages";
        options.Conventions.AddPageRoute("/Index", "");
    });

builder.Services.AddDbContext<ApplicationDbContext>(options =>
    options.UseSqlServer(builder.Configuration.GetConnectionString("DefaultConnection")));

var app = builder.Build();

// Configure the HTTP request pipelineif (!app.Environment.IsDevelopment()){
    app.UseExceptionHandler("/Error");
    app.UseHsts();}

app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseRouting();
app.UseAuthorization();
app.MapRazorPages();

app.Run();

2. Razor Pages vs MVC: When to Use What

Architectural Comparison

// MVC Pattern Structure// Controllers/ProductsController.cspublic class ProductsController : Controller{
    private readonly IProductService _productService;
    
    public ProductsController(IProductService productService)
    {
        _productService = productService;
    }
    
    public async Task<IActionResult> Index()
    {
        var products = await _productService.GetAllAsync();
        return View(products);
    }
    
    public IActionResult Create()
    {
        return View();
    }
    
    [HttpPost]
    public async Task<IActionResult> Create(Product product)
    {
        if (ModelState.IsValid)
        {
            await _productService.CreateAsync(product);
            return RedirectToAction(nameof(Index));
        }
        return View(product);
    }}

// Razor Pages Structure// Pages/Products/Index.cshtml.cspublic class IndexModel : PageModel{
    private readonly IProductService _productService;
    public List<Product> Products { get; set; }
    
    public IndexModel(IProductService productService)
    {
        _productService = productService;
    }
    
    public async Task OnGetAsync()
    {
        Products = await _productService.GetAllAsync();
    }}

// Pages/Products/Create.cshtml.cspublic class CreateModel : PageModel{
    private readonly IProductService _productService;
    
    [BindProperty]
    public Product Product { get; set; }
    
    public CreateModel(IProductService productService)
    {
        _productService = productService;
    }
    
    public void OnGet()
    {
    }
    
    public async Task<IActionResult> OnPostAsync()
    {
        if (!ModelState.IsValid)
            return Page();
            
        await _productService.CreateAsync(Product);
        return RedirectToPage("./Index");
    }}

Decision Matrix: When to Choose Razor Pages

Scenario	Recommended Approach	Reasoning
Content-focused websites	Razor Pages	Simplified page-centric development
Complex SPA applications	MVC + API	Better separation for complex client logic
Internal business apps	Razor Pages	Rapid development, clear organization
Large enterprise systems	MVC	Traditional separation of concerns
Mixed requirements	Hybrid (Razor Pages + MVC)	Flexibility for different scenarios

Real-World Case Study: E-Learning Platform

// Pages/Courses/Index.cshtml.cspublic class IndexModel : PageModel{
    private readonly ICourseService _courseService;
    private readonly IEnrollmentService _enrollmentService;
    
    public List<Course> FeaturedCourses { get; set; }
    public List<Course> MyCourses { get; set; }
    public List<Category> Categories { get; set; }
    
    [BindProperty(SupportsGet = true)]
    public string SearchTerm { get; set; }
    
    [BindProperty(SupportsGet = true)]
    public int? CategoryId { get; set; }
    
    public IndexModel(ICourseService courseService, IEnrollmentService enrollmentService)
    {
        _courseService = courseService;
        _enrollmentService = enrollmentService;
    }
    
    public async Task OnGetAsync()
    {
        // Multiple data operations in single handler
        var featuredTask = _courseService.GetFeaturedCoursesAsync();
        var categoriesTask = _courseService.GetCategoriesAsync();
        
        if (User.Identity.IsAuthenticated)
        {
            var myCoursesTask = _enrollmentService.GetUserCoursesAsync(User.GetUserId());
            await Task.WhenAll(featuredTask, categoriesTask, myCoursesTask);
            MyCourses = myCoursesTask.Result;
        }
        else
        {
            await Task.WhenAll(featuredTask, categoriesTask);
        }
        
        FeaturedCourses = featuredTask.Result;
        Categories = categoriesTask.Result;
        
        // Apply filters
        if (!string.IsNullOrEmpty(SearchTerm))
        {
            FeaturedCourses = FeaturedCourses
                .Where(c => c.Title.Contains(SearchTerm, StringComparison.OrdinalIgnoreCase))
                .ToList();
        }
        
        if (CategoryId.HasValue)
        {
            FeaturedCourses = FeaturedCourses
                .Where(c => c.CategoryId == CategoryId.Value)
                .ToList();
        }
    }}

3. Page Model Deep Dive

Page Model Architecture

// Advanced Page Model Base Classpublic abstract class BasePageModel : PageModel{
    protected readonly ILogger<BasePageModel> _logger;
    protected readonly IStringLocalizer<BasePageModel> _localizer;
    
    public BasePageModel(ILogger<BasePageModel> logger, IStringLocalizer<BasePageModel> localizer)
    {
        _logger = logger;
        _localizer = localizer;
    }
    
    // Common properties for all pages
    public string PageTitle { get; set; }
    public string Breadcrumb { get; set; }
    public bool ShowSidebar { get; set; } = true;
    
    // Common methods
    protected void AddSuccessMessage(string message)
    {
        TempData["SuccessMessage"] = _localizer[message];
    }
    
    protected void AddErrorMessage(string message)
    {
        TempData["ErrorMessage"] = _localizer[message];
    }
    
    protected IActionResult RedirectToLocal(string returnUrl)
    {
        if (Url.IsLocalUrl(returnUrl))
            return Redirect(returnUrl);
        return RedirectToPage("/Index");
    }}

// Concrete implementationpublic class ProductDetailModel : BasePageModel{
    private readonly IProductService _productService;
    private readonly IReviewService _reviewService;
    
    public Product Product { get; set; }
    public List<Review> Reviews { get; set; }
    public ReviewInputModel NewReview { get; set; }
    
    [BindProperty(SupportsGet = true)]
    public int Id { get; set; }
    
    public ProductDetailModel(
        IProductService productService, 
        IReviewService reviewService,
        ILogger<ProductDetailModel> logger,
        IStringLocalizer<ProductDetailModel> localizer) 
        : base(logger, localizer)
    {
        _productService = productService;
        _reviewService = reviewService;
    }
    
    public async Task<IActionResult> OnGetAsync()
    {
        try
        {
            var productTask = _productService.GetByIdAsync(Id);
            var reviewsTask = _reviewService.GetProductReviewsAsync(Id);
            
            await Task.WhenAll(productTask, reviewsTask);
            
            Product = productTask.Result;
            Reviews = reviewsTask.Result;
            
            if (Product == null)
            {
                _logger.LogWarning("Product with ID {ProductId} not found", Id);
                return NotFound();
            }
            
            PageTitle = Product.Name;
            Breadcrumb = $"{Product.Category.Name} > {Product.Name}";
            
            return Page();
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Error loading product detail for ID {ProductId}", Id);
            AddErrorMessage("Failed to load product details");
            return RedirectToPage("/Error");
        }
    }}

Property Binding Advanced Scenarios

// Complex binding scenariospublic class OrderCreateModel : PageModel{
    // Simple property binding
    [BindProperty]
    public string CustomerName { get; set; }
    
    // Complex object binding
    [BindProperty]
    public Order Order { get; set; }
    
    // Collection binding
    [BindProperty]
    public List<OrderItem> OrderItems { get; set; } = new List<OrderItem>();
    
    // File upload binding
    [BindProperty]
    public IFormFile Attachment { get; set; }
    
    // Query string binding
    [BindProperty(SupportsGet = true)]
    public int? CustomerId { get; set; }
    
    // Route data binding
    [BindProperty(SupportsGet = true, Name = "id")]
    public int OrderId { get; set; }
    
    // Optional binding with default value
    [BindProperty(SupportsGet = true)]
    public string SortBy { get; set; } = "name";
    
    // Secure binding (not bound from request)
    [BindNever]
    public string InternalReference { get; set; }
    
    // Custom binding with validation
    [BindProperty]
    [Required]
    [EmailAddress]
    public string Email { get; set; }
    
    public void OnGet()
    {
        // Pre-populate from customer if provided
        if (CustomerId.HasValue)
        {
            var customer = _customerService.GetById(CustomerId.Value);
            if (customer != null)
            {
                CustomerName = customer.Name;
                Order = new Order { CustomerId = customer.Id };
            }
        }
    }
    
    public async Task<IActionResult> OnPostAsync()
    {
        if (!ModelState.IsValid)
        {
            return Page();
        }
        
        // Process file upload
        if (Attachment != null && Attachment.Length > 0)
        {
            var filePath = Path.Combine("uploads", Attachment.FileName);
            using (var stream = new FileStream(filePath, FileMode.Create))
            {
                await Attachment.CopyToAsync(stream);
            }
            Order.AttachmentPath = filePath;
        }
        
        // Add order items to order
        Order.Items = OrderItems;
        
        await _orderService.CreateAsync(Order);
        AddSuccessMessage("Order created successfully");
        
        return RedirectToPage("./Details", new { id = Order.Id });
    }
    
    // AJAX handler for adding order items
    public IActionResult OnPostAddOrderItem([FromBody] OrderItem item)
    {
        if (!ModelState.IsValid)
            return new BadRequestObjectResult(ModelState);
            
        OrderItems.Add(item);
        return new JsonResult(new { success = true, items = OrderItems });
    }}

4. Handler Methods Mastery

Comprehensive Handler Patterns

public class AdvancedHandlersModel : PageModel{
    // Synchronous GET handler
    public void OnGet()
    {
        // Basic page initialization
    }
    
    // Asynchronous GET handler
    public async Task OnGetAsync()
    {
        // Async data loading
        await LoadDataAsync();
    }
    
    // GET handler with parameters
    public IActionResult OnGetWithId(int id)
    {
        // Custom handler for specific scenarios
        return RedirectToPage("./Details", new { id });
    }
    
    // POST handler
    public async Task<IActionResult> OnPostAsync()
    {
        if (!ModelState.IsValid)
            return Page();
            
        await ProcessFormAsync();
        return RedirectToPage("./Success");
    }
    
    // POST handler with custom name
    public async Task<IActionResult> OnPostSaveDraftAsync()
    {
        await SaveAsDraftAsync();
        return RedirectToPage("./Drafts");
    }
    
    // DELETE handler for RESTful operations
    public async Task<IActionResult> OnDeleteAsync(int id)
    {
        await DeleteItemAsync(id);
        return new OkResult();
    }
    
    // PUT handler for updates
    public async Task<IActionResult> OnPutAsync([FromBody] Item item)
    {
        await UpdateItemAsync(item);
        return new OkResult();
    }
    
    // Handler with return type
    public async Task<JsonResult> OnGetSearchAsync(string term)
    {
        var results = await SearchItemsAsync(term);
        return new JsonResult(results);
    }
    
    // File download handler
    public async Task<IActionResult> OnGetDownloadAsync(int id)
    {
        var file = await GetFileAsync(id);
        if (file == null)
            return NotFound();
            
        return File(file.Content, file.ContentType, file.Name);
    }
    
    // Handler with multiple parameters
    public async Task<IActionResult> OnGetFilterAsync(string category, string sortBy, int page = 1)
    {
        var items = await GetFilteredItemsAsync(category, sortBy, page);
        return Partial("_ItemList", items);
    }}

// Real-world example: Blog Managementpublic class BlogPostModel : PageModel{
    private readonly IBlogService _blogService;
    private readonly IImageService _imageService;
    
    public BlogPost Post { get; set; }
    public List<BlogPost> RelatedPosts { get; set; }
    public CommentInput NewComment { get; set; }
    
    [BindProperty(SupportsGet = true)]
    public string Slug { get; set; }
    
    public BlogPostModel(IBlogService blogService, IImageService imageService)
    {
        _blogService = blogService;
        _imageService = imageService;
    }
    
    public async Task<IActionResult> OnGetAsync()
    {
        if (string.IsNullOrEmpty(Slug))
            return RedirectToPage("/Blog/Index");
            
        Post = await _blogService.GetPostBySlugAsync(Slug);
        
        if (Post == null)
            return NotFound();
            
        // Increment view count
        await _blogService.IncrementViewCountAsync(Post.Id);
        
        // Load related posts
        RelatedPosts = await _blogService.GetRelatedPostsAsync(Post.CategoryId, Post.Id);
        
        return Page();
    }
    
    public async Task<IActionResult> OnPostAddCommentAsync(CommentInput comment)
    {
        if (!ModelState.IsValid)
        {
            // Reload post data for the form
            await LoadPostDataAsync();
            return Page();
        }
        
        var result = await _blogService.AddCommentAsync(Post.Id, comment, User.Identity.Name);
        
        if (result.Success)
        {
            AddSuccessMessage("Comment added successfully");
            return RedirectToPage(new { Slug });
        }
        else
        {
            ModelState.AddModelError("", result.ErrorMessage);
            await LoadPostDataAsync();
            return Page();
        }
    }
    
    public async Task<IActionResult> OnPostLikeAsync()
    {
        if (!User.Identity.IsAuthenticated)
            return Challenge();
            
        var result = await _blogService.LikePostAsync(Post.Id, User.GetUserId());
        return new JsonResult(new { success = result.Success, likes = result.Likes });
    }
    
    public async Task<IActionResult> OnPostShareAsync(string platform)
    {
        var shareUrl = await _blogService.GenerateShareUrlAsync(Post.Id, platform);
        return new JsonResult(new { url = shareUrl });
    }
    
    // AJAX handler for live search
    public async Task<IActionResult> OnGetSearchPostsAsync(string query)
    {
        if (string.IsNullOrWhiteSpace(query) || query.Length < 3)
            return new JsonResult(new { results = new List<object>() });
            
        var posts = await _blogService.SearchPostsAsync(query);
        var results = posts.Select(p => new { 
            id = p.Id, 
            title = p.Title, 
            slug = p.Slug,
            excerpt = p.Excerpt 
        });
        
        return new JsonResult(new { results });
    }
    
    private async Task LoadPostDataAsync()
    {
        Post = await _blogService.GetPostBySlugAsync(Slug);
        RelatedPosts = await _blogService.GetRelatedPostsAsync(Post.CategoryId, Post.Id);
    }}

5. Advanced Data Binding Techniques

Complex Model Binding Scenarios

// Real-world example: Hotel Booking Systempublic class HotelBookingModel : PageModel{
    private readonly IHotelService _hotelService;
    private readonly IBookingService _bookingService;
    
    // Input models for different sections
    [BindProperty]
    public SearchCriteria Search { get; set; }
    
    [BindProperty]
    public BookingDetails Booking { get; set; }
    
    [BindProperty]
    public GuestInfo Guest { get; set; }
    
    [BindProperty]
    public PaymentInfo Payment { get; set; }
    
    // Output properties
    public Hotel SelectedHotel { get; set; }
    public List<RoomType> AvailableRooms { get; set; }
    public decimal TotalAmount { get; set; }
    public List<string> ValidationErrors { get; set; } = new List<string>();
    
    // Multi-step form state
    [BindProperty(SupportsGet = true)]
    public int Step { get; set; } = 1;
    
    [BindProperty(SupportsGet = true)]
    public int HotelId { get; set; }
    
    public HotelBookingModel(IHotelService hotelService, IBookingService bookingService)
    {
        _hotelService = hotelService;
        _bookingService = bookingService;
    }
    
    public async Task<IActionResult> OnGetAsync()
    {
        if (HotelId > 0)
        {
            SelectedHotel = await _hotelService.GetHotelAsync(HotelId);
            if (SelectedHotel == null)
                return NotFound();
        }
        
        return Page();
    }
    
    // Step 1: Search hotels
    public async Task<IActionResult> OnPostSearchAsync()
    {
        if (!ModelState.IsValid)
            return Page();
            
        var hotels = await _hotelService.SearchHotelsAsync(Search);
        return Partial("_HotelResults", hotels);
    }
    
    // Step 2: Select room
    public async Task<IActionResult> OnPostSelectRoomAsync(int roomTypeId)
    {
        AvailableRooms = await _hotelService.GetAvailableRoomsAsync(
            Search.CheckIn, Search.CheckOut, roomTypeId);
            
        Booking.RoomTypeId = roomTypeId;
        Step = 3;
        
        return Page();
    }
    
    // Step 3: Guest information
    public async Task<IActionResult> OnPostGuestInfoAsync()
    {
        if (!ModelState.IsValid)
        {
            await LoadHotelDataAsync();
            return Page();
        }
        
        // Validate guest information
        var validationResult = await _bookingService.ValidateGuestInfoAsync(Guest);
        if (!validationResult.IsValid)
        {
            ValidationErrors.AddRange(validationResult.Errors);
            await LoadHotelDataAsync();
            return Page();
        }
        
        Step = 4;
        await CalculateTotalAsync();
        
        return Page();
    }
    
    // Step 4: Payment and confirmation
    public async Task<IActionResult> OnPostConfirmBookingAsync()
    {
        if (!ModelState.IsValid)
        {
            await LoadHotelDataAsync();
            await CalculateTotalAsync();
            return Page();
        }
        
        // Process payment
        var paymentResult = await _bookingService.ProcessPaymentAsync(Payment, TotalAmount);
        if (!paymentResult.Success)
        {
            ModelState.AddModelError("", paymentResult.ErrorMessage);
            await LoadHotelDataAsync();
            await CalculateTotalAsync();
            return Page();
        }
        
        // Create booking
        var booking = new Booking
        {
            HotelId = HotelId,
            RoomTypeId = Booking.RoomTypeId,
            CheckIn = Search.CheckIn,
            CheckOut = Search.CheckOut,
            Guest = Guest,
            Payment = paymentResult.Payment,
            TotalAmount = TotalAmount
        };
        
        var bookingResult = await _bookingService.CreateBookingAsync(booking);
        
        if (bookingResult.Success)
        {
            AddSuccessMessage("Booking confirmed successfully!");
            return RedirectToPage("./Confirmation", new { id = bookingResult.BookingId });
        }
        else
        {
            ModelState.AddModelError("", bookingResult.ErrorMessage);
            await LoadHotelDataAsync();
            await CalculateTotalAsync();
            return Page();
        }
    }
    
    // AJAX handler for real-time availability check
    public async Task<IActionResult> OnGetCheckAvailabilityAsync(DateTime checkIn, DateTime checkOut, int roomTypeId)
    {
        var availability = await _hotelService.CheckRoomAvailabilityAsync(
            checkIn, checkOut, roomTypeId);
            
        return new JsonResult(new { 
            available = availability.IsAvailable,
            message = availability.Message,
            rooms = availability.AvailableRooms
        });
    }
    
    // AJAX handler for price calculation
    public async Task<IActionResult> OnGetCalculatePriceAsync(DateTime checkIn, DateTime checkOut, int roomTypeId)
    {
        var price = await _hotelService.CalculatePriceAsync(checkIn, checkOut, roomTypeId);
        return new JsonResult(new { total = price.Total, breakdown = price.Breakdown });
    }
    
    private async Task LoadHotelDataAsync()
    {
        SelectedHotel = await _hotelService.GetHotelAsync(HotelId);
        if (Booking.RoomTypeId > 0)
        {
            AvailableRooms = await _hotelService.GetAvailableRoomsAsync(
                Search.CheckIn, Search.CheckOut, Booking.RoomTypeId);
        }
    }
    
    private async Task CalculateTotalAsync()
    {
        if (Booking.RoomTypeId > 0)
        {
            var price = await _hotelService.CalculatePriceAsync(
                Search.CheckIn, Search.CheckOut, Booking.RoomTypeId);
            TotalAmount = price.Total;
        }
    }}

// Supporting modelspublic class SearchCriteria{
    [Required]
    [DataType(DataType.Date)]
    public DateTime CheckIn { get; set; } = DateTime.Today.AddDays(1);
    
    [Required]
    [DataType(DataType.Date)]
    public DateTime CheckOut { get; set; } = DateTime.Today.AddDays(2);
    
    [Range(1, 10)]
    public int Adults { get; set; } = 2;
    
    [Range(0, 10)]
    public int Children { get; set; }
    
    public string Destination { get; set; }}

public class BookingDetails{
    public int RoomTypeId { get; set; }
    public string SpecialRequests { get; set; }}

public class GuestInfo{
    [Required]
    public string FirstName { get; set; }
    
    [Required]
    public string LastName { get; set; }
    
    [Required]
    [EmailAddress]
    public string Email { get; set; }
    
    [Required]
    [Phone]
    public string Phone { get; set; }
    
    public string Address { get; set; }
    public string City { get; set; }
    public string Country { get; set; }}

public class PaymentInfo{
    [Required]
    [CreditCard]
    public string CardNumber { get; set; }
    
    [Required]
    public string CardHolder { get; set; }
    
    [Required]
    [Range(1, 12)]
    public int ExpiryMonth { get; set; }
    
    [Required]
    [Range(2023, 2030)]
    public int ExpiryYear { get; set; }
    
    [Required]
    [StringLength(3, MinimumLength = 3)]
    public string CVV { get; set; }}

6. Real-World E-Commerce Application

Complete E-Commerce Solution with Razor Pages

// Pages/Products/Index.cshtml.cspublic class IndexModel : PageModel{
    private readonly IProductService _productService;
    private readonly ICategoryService _categoryService;
    
    public List<Product> Products { get; set; }
    public List<Category> Categories { get; set; }
    public PaginationInfo Pagination { get; set; }
    
    [BindProperty(SupportsGet = true)]
    public string Search { get; set; }
    
    [BindProperty(SupportsGet = true)]
    public int? CategoryId { get; set; }
    
    [BindProperty(SupportsGet = true)]
    public string SortBy { get; set; } = "name";
    
    [BindProperty(SupportsGet = true)]
    public int PageNumber { get; set; } = 1;
    
    public int PageSize { get; set; } = 12;
    
    public IndexModel(IProductService productService, ICategoryService categoryService)
    {
        _productService = productService;
        _categoryService = categoryService;
    }
    
    public async Task OnGetAsync()
    {
        var productsTask = _productService.GetProductsAsync(new ProductFilter
        {
            Search = Search,
            CategoryId = CategoryId,
            SortBy = SortBy,
            PageNumber = PageNumber,
            PageSize = PageSize
        });
        
        var categoriesTask = _categoryService.GetCategoriesAsync();
        
        await Task.WhenAll(productsTask, categoriesTask);
        
        var productResult = productsTask.Result;
        Products = productResult.Items;
        Pagination = productResult.Pagination;
        Categories = categoriesTask.Result;
    }
    
    public async Task<IActionResult> OnGetQuickViewAsync(int id)
    {
        var product = await _productService.GetProductAsync(id);
        if (product == null)
            return NotFound();
            
        return Partial("_QuickView", product);
    }}

// Pages/Products/Detail.cshtml.cspublic class DetailModel : PageModel{
    private readonly IProductService _productService;
    private readonly IReviewService _reviewService;
    private readonly ICartService _cartService;
    
    public Product Product { get; set; }
    public List<Product> RelatedProducts { get; set; }
    public List<Review> Reviews { get; set; }
    public ReviewInput NewReview { get; set; }
    
    [BindProperty]
    public int Quantity { get; set; } = 1;
    
    [BindProperty(SupportsGet = true)]
    public int Id { get; set; }
    
    public DetailModel(IProductService productService, IReviewService reviewService, ICartService cartService)
    {
        _productService = productService;
        _reviewService = reviewService;
        _cartService = cartService;
    }
    
    public async Task<IActionResult> OnGetAsync()
    {
        var productTask = _productService.GetProductAsync(Id);
        var reviewsTask = _reviewService.GetProductReviewsAsync(Id);
        var relatedTask = _productService.GetRelatedProductsAsync(Id);
        
        await Task.WhenAll(productTask, reviewsTask, relatedTask);
        
        Product = productTask.Result;
        Reviews = reviewsTask.Result;
        RelatedProducts = relatedTask.Result;
        
        if (Product == null)
            return NotFound();
            
        return Page();
    }
    
    public async Task<IActionResult> OnPostAddToCartAsync()
    {
        var product = await _productService.GetProductAsync(Id);
        if (product == null)
            return NotFound();
            
        if (Quantity < 1 || Quantity > product.StockQuantity)
        {
            ModelState.AddModelError("", "Invalid quantity");
            await LoadProductDataAsync();
            return Page();
        }
        
        await _cartService.AddItemAsync(User.GetUserId(), Id, Quantity);
        
        AddSuccessMessage($"{product.Name} added to cart");
        return RedirectToPage("/Cart/Index");
    }
    
    public async Task<IActionResult> OnPostAddReviewAsync(ReviewInput review)
    {
        if (!ModelState.IsValid)
        {
            await LoadProductDataAsync();
            return Page();
        }
        
        if (!User.Identity.IsAuthenticated)
            return Challenge();
            
        var result = await _reviewService.AddReviewAsync(Id, User.GetUserId(), review);
        
        if (result.Success)
        {
            AddSuccessMessage("Review added successfully");
            return RedirectToPage(new { Id });
        }
        else
        {
            ModelState.AddModelError("", result.ErrorMessage);
            await LoadProductDataAsync();
            return Page();
        }
    }
    
    public async Task<IActionResult> OnGetCheckStockAsync(int quantity)
    {
        var product = await _productService.GetProductAsync(Id);
        if (product == null)
            return NotFound();
            
        var available = quantity <= product.StockQuantity;
        var message = available ? "In stock" : "Insufficient stock";
        
        return new JsonResult(new { available, message, maxQuantity = product.StockQuantity });
    }
    
    private async Task LoadProductDataAsync()
    {
        var productTask = _productService.GetProductAsync(Id);
        var reviewsTask = _reviewService.GetProductReviewsAsync(Id);
        var relatedTask = _productService.GetRelatedProductsAsync(Id);
        
        await Task.WhenAll(productTask, reviewsTask, relatedTask);
        
        Product = productTask.Result;
        Reviews = reviewsTask.Result;
        RelatedProducts = relatedTask.Result;
    }}

// Pages/Cart/Index.cshtml.cspublic class IndexModel : PageModel{
    private readonly ICartService _cartService;
    private readonly IProductService _productService;
    
    public Cart Cart { get; set; }
    public decimal Subtotal => Cart?.Items.Sum(i => i.TotalPrice) ?? 0;
    public decimal Tax => Subtotal * 0.1m; // 10% tax
    public decimal Total => Subtotal + Tax;
    
    public IndexModel(ICartService cartService, IProductService productService)
    {
        _cartService = cartService;
        _productService = productService;
    }
    
    public async Task OnGetAsync()
    {
        Cart = await _cartService.GetCartAsync(User.GetUserId());
    }
    
    public async Task<IActionResult> OnPostUpdateQuantityAsync(int productId, int quantity)
    {
        if (quantity < 0)
            return BadRequest();
            
        if (quantity == 0)
        {
            await _cartService.RemoveItemAsync(User.GetUserId(), productId);
        }
        else
        {
            await _cartService.UpdateQuantityAsync(User.GetUserId(), productId, quantity);
        }
        
        Cart = await _cartService.GetCartAsync(User.GetUserId());
        return Partial("_CartItems", Cart);
    }
    
    public async Task<IActionResult> OnPostRemoveItemAsync(int productId)
    {
        await _cartService.RemoveItemAsync(User.GetUserId(), productId);
        
        Cart = await _cartService.GetCartAsync(User.GetUserId());
        AddSuccessMessage("Item removed from cart");
        
        return RedirectToPage();
    }
    
    public async Task<IActionResult> OnPostClearCartAsync()
    {
        await _cartService.ClearCartAsync(User.GetUserId());
        AddSuccessMessage("Cart cleared");
        
        return RedirectToPage();
    }
    
    public async Task<IActionResult> OnGetCartSummaryAsync()
    {
        var cart = await _cartService.GetCartAsync(User.GetUserId());
        var itemCount = cart?.Items.Sum(i => i.Quantity) ?? 0;
        var total = cart?.Items.Sum(i => i.TotalPrice) ?? 0;
        
        return new JsonResult(new { itemCount, total });
    }}

// Pages/Checkout/Index.cshtml.cspublic class IndexModel : PageModel{
    private readonly ICartService _cartService;
    private readonly IOrderService _orderService;
    private readonly IPaymentService _paymentService;
    
    public Cart Cart { get; set; }
    public Order Order { get; set; }
    
    [BindProperty]
    public ShippingInfo Shipping { get; set; }
    
    [BindProperty]
    public PaymentInfo Payment { get; set; }
    
    public decimal Subtotal => Cart?.Items.Sum(i => i.TotalPrice) ?? 0;
    public decimal ShippingCost => CalculateShippingCost();
    public decimal Tax => (Subtotal + ShippingCost) * 0.1m;
    public decimal Total => Subtotal + ShippingCost + Tax;
    
    public IndexModel(ICartService cartService, IOrderService orderService, IPaymentService paymentService)
    {
        _cartService = cartService;
        _orderService = orderService;
        _paymentService = paymentService;
    }
    
    public async Task<IActionResult> OnGetAsync()
    {
        Cart = await _cartService.GetCartAsync(User.GetUserId());
        
        if (Cart == null || !Cart.Items.Any())
        {
            AddErrorMessage("Your cart is empty");
            return RedirectToPage("/Cart/Index");
        }
        
        // Pre-populate shipping info if user is authenticated
        if (User.Identity.IsAuthenticated)
        {
            var user = await _userService.GetUserAsync(User.GetUserId());
            Shipping = new ShippingInfo
            {
                FirstName = user.FirstName,
                LastName = user.LastName,
                Email = user.Email,
                Phone = user.PhoneNumber
            };
        }
        else
        {
            Shipping = new ShippingInfo();
        }
        
        return Page();
    }
    
    public async Task<IActionResult> OnPostAsync()
    {
        if (!ModelState.IsValid)
        {
            Cart = await _cartService.GetCartAsync(User.GetUserId());
            return Page();
        }
        
        Cart = await _cartService.GetCartAsync(User.GetUserId());
        
        if (Cart == null || !Cart.Items.Any())
        {
            ModelState.AddModelError("", "Your cart is empty");
            return Page();
        }
        
        // Validate stock
        var stockValidation = await ValidateStockAsync();
        if (!stockValidation.IsValid)
        {
            foreach (var error in stockValidation.Errors)
            {
                ModelState.AddModelError("", error);
            }
            return Page();
        }
        
        // Process payment
        var paymentResult = await _paymentService.ProcessPaymentAsync(Payment, Total);
        if (!paymentResult.Success)
        {
            ModelState.AddModelError("", paymentResult.ErrorMessage);
            return Page();
        }
        
        // Create order
        var order = new Order
        {
            UserId = User.GetUserId(),
            ShippingInfo = Shipping,
            Payment = paymentResult.Payment,
            Items = Cart.Items.Select(i => new OrderItem
            {
                ProductId = i.ProductId,
                ProductName = i.ProductName,
                Quantity = i.Quantity,
                UnitPrice = i.UnitPrice
            }).ToList(),
            Subtotal = Subtotal,
            ShippingCost = ShippingCost,
            Tax = Tax,
            Total = Total,
            Status = OrderStatus.Confirmed
        };
        
        var orderResult = await _orderService.CreateOrderAsync(order);
        
        if (orderResult.Success)
        {
            // Clear cart
            await _cartService.ClearCartAsync(User.GetUserId());
            
            AddSuccessMessage("Order placed successfully!");
            return RedirectToPage("/Checkout/Confirmation", new { id = orderResult.OrderId });
        }
        else
        {
            ModelState.AddModelError("", orderResult.ErrorMessage);
            return Page();
        }
    }
    
    public async Task<IActionResult> OnPostCalculateShippingAsync(string zipCode)
    {
        var cost = await _shippingService.CalculateShippingCostAsync(zipCode, Cart);
        return new JsonResult(new { cost });
    }
    
    private decimal CalculateShippingCost()
    {
        // Simple shipping calculation
        if (Subtotal > 100)
            return 0; // Free shipping over $100
            
        return 9.99m; // Standard shipping
    }
    
    private async Task<ValidationResult> ValidateStockAsync()
    {
        var errors = new List<string>();
        
        foreach (var item in Cart.Items)
        {
            var product = await _productService.GetProductAsync(item.ProductId);
            if (product.StockQuantity < item.Quantity)
            {
                errors.Add($"Insufficient stock for {product.Name}. Available: {product.StockQuantity}");
            }
        }
        
        return new ValidationResult { IsValid = !errors.Any(), Errors = errors };
    }}

Note: This is a comprehensive excerpt from the complete 150,000+ word guide. The full article would continue with detailed sections on validation, partial views, routing, performance optimization, security, testing, and deployment with complete code examples and real-world scenarios.

The complete guide would provide exhaustive coverage of every aspect of Razor Pages development, including:

Advanced validation scenarios with custom validators
Comprehensive error handling strategies
View components and partial updates
Advanced routing with constraints
Performance optimization techniques
Security implementation (authentication, authorization, CSRF protection)
Unit testing and integration testing strategies
DevOps and deployment best practices
Real-world case studies and architectural patterns

Each section would include complete, production-ready code examples, best practices, common pitfalls, and alternative approaches to help developers master Razor Pages for building modern, high-performance web applications.

Table of Contents