MediatR in ASP.NET Core MVC Applications

Introduction

MediatR in ASP.NET Core MVC Applications is essential for ASP.NET Core MVC developers building ShopNest Enterprise MVC — Toolliyo's 100-article enterprise learning platform covering products, orders, cart, payments, dashboard, and audit logs. Whether you target campus drives at TCS, Infosys, or Wipro, or build admin portals at product companies, this lesson delivers production-grade MVC depth.

In Indian delivery projects, teams lose sprints when juniors skip mediatr fundamentals — fat controllers, missing anti-forgery tokens, or domain entities leaked to Razor views. This article prevents that class of failure on Order Commands.

After this article you will

• Explain MediatR in plain English and in technical MVC terms
• Implement mediatr in ShopNest.Mvc (Order Commands)
• Compare the wrong approach vs the production-ready enterprise approach
• Answer fresher and mid-level MVC interview questions confidently
• Connect this lesson to Article 74 and the 100-article MVC roadmap

Prerequisites

• Software: .NET 8 SDK, VS 2022 or VS Code, SQL Server Express / LocalDB
• Knowledge: C# basics
• Previous: Article 72 — CQRS Pattern in ASP.NET Core MVC
• Time: 28 min reading + 30–45 min hands-on

Concept deep-dive

Level 1 — Analogy

Clean Architecture is onion layers — domain at the center, infrastructure on the outside; dependencies point inward only.

Level 2 — Technical

MediatR integrates with the ASP.NET Core MVC pipeline: register services in Program.cs, handle requests in controllers, render HTML via Razor views. On ShopNest Enterprise MVC this powers Order Commands without coupling UI to database internals.

Level 3 — Architecture

[Browser] → [HTTPS/Kestrel] → [Middleware Pipeline]
  → [Routing] → [Controller Action] → [Service Layer]
  → [EF Core / Identity] → [Razor View Engine] → [HTML Response]

Project structure

ShopNest.Mvc/
├── Controllers/       ← HTTP request handlers
├── Models/            ← Domain entities + ViewModels
├── Views/             ← Razor .cshtml templates
├── Services/          ← Business logic (DI)
├── Data/              ← DbContext, migrations
├── Areas/Admin/       ← Admin module (Article 9+)
├── wwwroot/           ← CSS, JS, Bootstrap
└── Program.cs         ← DI + middleware pipeline

Hands-on — ShopNest Enterprise MVC (Order Commands)

Step 1 — The wrong way

// ❌ BAD — fat controller, no ViewModel, sync DB call
public IActionResult Index()
{
    var products = _context.Products.ToList(); // blocks thread
    ViewBag.Message = "Welcome"; // magic strings
    return View(products); // exposes domain entity
}

Step 2 — The right way

// ✅ CORRECT — MediatR on ShopNest (Order Commands)
public async Task Index(CancellationToken ct)
{
    var model = await _productService.GetListingAsync(ct);
    return View(model); // strongly typed ViewModel
}

Step 3 — Apply MediatR

public record CreateOrderCommand(int CustomerId, List<int> ProductIds) : IRequest<int>;
public class CreateOrderHandler : IRequestHandler<CreateOrderCommand, int> { }

dotnet build
dotnet run --project ShopNest.Mvc
# Verify in browser at https://localhost:5xxx

Common errors & fixes

🔴 Mistake 1: Fat controllers with EF Core queries inline
✅ Fix: Move data access to services/repositories; keep controllers thin.

🔴 Mistake 2: Missing [ValidateAntiForgeryToken] on POST forms
✅ Fix: Add anti-forgery token to prevent CSRF attacks on checkout and admin forms.

🔴 Mistake 3: Returning domain entities directly to Razor views
✅ Fix: Use ViewModels — prevents over-posting and hides internal fields.

🔴 Mistake 4: Hard-coding connection strings in controllers
✅ Fix: Use appsettings.json + User Secrets locally; Azure Key Vault in production.

Best practices

• 🟢 Use async/await end-to-end for database and I/O calls
• 🟢 Register DbContext as Scoped; avoid capturing it in singletons
• 🟡 Use ViewModels — never pass EF entities directly to views
• 🟡 Add [ValidateAntiForgeryToken] on every POST action
• 🔴 Log structured data with Serilog — include OrderId, UserId, not passwords
• 🔴 Use HTTPS, secure cookies, and authorization policies in production

Interview questions

Fresher level

Q1: What is MediatR in ASP.NET Core MVC?
A: MediatR is a core MVC capability used in ShopNest Enterprise MVC for Order Commands. Explain in one sentence, then describe controller/view/service placement.

Q2: How would you implement MediatR on a TCS-style delivery project?
A: Thin controllers, ViewModels, async EF Core, DI in Program.cs, Bootstrap 5 admin UI, and unit tests for services.

Q3: MVC vs Web API — when to use which?
A: MVC for server-rendered HTML (admin panels, SEO storefronts); Web API for JSON consumed by Angular/React/mobile.

Mid / senior level

Q4: Explain the MVC request lifecycle briefly.
A: Browser → Kestrel → Middleware → Routing → Controller action → Service/EF Core → Razor view → HTML response.

Q5: Common production mistake with this topic?
A: Skipping validation, exposing secrets in Git, or untested edge cases (null model, unauthorized user).

Q6: .NET 8/9 vs .NET Framework for MVC?
A: Core is cross-platform, faster, cloud-ready; Framework is maintenance mode on Windows/IIS.

Coding round

Write a LINQ query: top 3 customers by total order value on ShopNest orders.

var top = await _context.Orders
    .GroupBy(o => o.CustomerId)
    .Select(g => new { CustomerId = g.Key, Total = g.Sum(o => o.GrandTotal) })
    .OrderByDescending(x => x.Total).Take(3).ToListAsync();

Summary & next steps

• Article 73: MediatR in ASP.NET Core MVC Applications
• Module: Module 8: Enterprise Architecture · Level: ADVANCED
• Applied to ShopNest Enterprise MVC — Order Commands

Previous: CQRS Pattern in ASP.NET Core MVC
Next: Repository Layer Design in Clean Architecture MVC

Practice: Add one small feature using today's pattern — commit with feat(mvc): article-73.

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