ASP.NET Core .NET 9 New Features — Complete Guide

Introduction

Graduate ShopNest to .NET 9 — OpenAPI improvements, Blazor updates, SignalR gains, HybridCache, performance wins, and a safe migration checklist from .NET 8.

After this article you will

• Summarize ASP.NET Core 9 highlights
• Migrate ShopNest using dotnet upgrade assistant
• Adopt HybridCache where applicable
• Review breaking changes
• Plan production rollout

Prerequisites

• Article 74 — Output Caching
• Articles 1–64 ShopNest foundations (MVC, EF Core, API, auth, deploy)

Architecture & design

Key .NET 9 / ASP.NET Core 9 highlights

• HybridCache — L1 memory + L2 distributed (Redis) unified API
• OpenAPI — improved built-in document generation
• Blazor — further Blazor United enhancements
• Performance — faster JSON, LINQ, GC improvements (benchmark your app)
• Minimal APIs — new binding and filter improvements

// HybridCache (.NET 9)
builder.Services.AddHybridCache(options =>
{
    options.DefaultEntryOptions = new HybridCacheEntryOptions
    {
        Expiration = TimeSpan.FromMinutes(10),
        LocalCacheExpiration = TimeSpan.FromMinutes(2)
    };
});

// Migration
// dotnet tool install -g upgrade-assistant
// upgrade-assistant upgrade ShopNest.sln

Hands-on build guide — ShopNest Upgrade to .NET 9

1. Retarget all projects to net9.0.
2. Run full test suite; fix obsolete API warnings.
3. Replace IMemoryCache+Redis dual usage with HybridCache on catalog.
4. Deploy to staging slot; smoke test; swap production.
5. Update GitHub Actions dotnet-version to 9.0.x.

Common pitfalls

• Skipping staging validation — breaking changes in third-party packages.
• Mixing net8 and net9 assemblies — retarget all projects together.

Interview & portfolio questions

Q: HybridCache benefit?
A: One API for local + distributed cache tiers — less boilerplate than IMemoryCache + IDistributedCache.

Q: Upgrade strategy?
A: Staging slot → automated tests → swap; keep rollback slot on net8 until stable.

Summary

• Article 75 completes the 75-lesson ShopNest track
• .NET 9 keeps ShopNest current for 2025+ hiring
• HybridCache + OpenAPI upgrades are quick wins
• Always test before production swap

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FAQ