Razor Syntax Overview & Data Passing Techniques

1. Description

Razor is the view engine used by ASP.NET Core MVC to combine C# code with HTML. It uses the @ symbol to switch into C# and provides concise syntax for expressions, loops, conditionals, and layouts. Data passing techniques are ways to send data from controllers to views: strongly-typed models, ViewData, ViewBag, and TempData.

2. Why It Is Important

Razor keeps server-side logic readable and compact inside HTML views, enabling dynamic pages. Knowing data-passing techniques lets you choose the right scope and lifetime for values (request-only vs next-request, typed vs untyped).

3. Real-World Examples

• Render a product list by passing a List<Product> model to the view.
• Use TempData to show a success message after redirecting following a POST.
• Use ViewData/ViewBag for small UI flags (e.g., IsAdmin) that do not require a full model.

4. Syntax & Explanation

Razor basics (in a .cshtml view):

@model List<MyApp.Models.Product>
@{
    ViewData["Title"] = "Products";
}
<h1>@ViewData["Title"]</h1>
<ul>
@foreach (var p in Model)
{
    <li>@p.Name - @p.Price.ToString("C")</li>
}
</ul>

Controller passing a model and TempData:

using Microsoft.AspNetCore.Mvc;
using MyApp.Models;
using System.Collections.Generic;

public class ProductsController : Controller
{
    public IActionResult Index()
    {
        var products = new List<Product> {
            new Product { Id = 1, Name = "Pen", Price = 1.20m }
        };
        return View(products); // strongly-typed model
    }

    [HttpPost]
    public IActionResult Create(Product p)
    {
        // save logic...
        TempData["Success"] = "Product created"; // survives redirect
        return RedirectToAction("Index");
    }
}

Accessing TempData in the redirected view:

@if (TempData["Success"] != null)
{
    <div class="alert alert-success">@TempData["Success"]</div>
}

Notes:

• Model is strongly typed when view declares @model.
• ViewData is a dictionary: ViewData["key"] (object typed).
• ViewBag is dynamic: ViewBag.Message = "hi".
• TempData persists until it's read or the session ends (good for post-redirect-get messages).

5. Use Cases

• Sending page data (lists, single objects) via model binding.
• Showing one-time messages after redirects using TempData.
• Small UI flags or helper values via ViewData/ViewBag.

6. Mini Practice Task

1. Create an Index view that lists Product models and shows a TempData success message after a redirect.
2. Use ViewBag to pass a bool IsAdmin to the view and render admin-only markup conditionally.