10 Reasons to Upgrade to .NET 9: A Friendly Guide for Developers

Imagine your family has an old scooter. It still runs. But the new model uses less petrol, starts faster on cold mornings, and the engine runs cooler. You did not change how you ride. You just got a better machine under you. Upgrading from .NET 8 to .NET 9 feels a lot like that.

In this post we will look at 10 real reasons to make the jump. I will keep the language simple. I will show small code samples. And I will draw a few pictures so the ideas stick.

One quick fact before we start: .NET 9 is a Standard Term Support (STS) release. That used to mean only 18 months of support. Microsoft has now extended it to 24 months. If you want the very longest support, .NET 10 is the LTS release. But .NET 9 is still a great, safe step up from .NET 8.

Let us begin.

Reason 1: Your app gets faster for free

This is the best part. A lot of .NET 9 speed comes from the runtime itself — the engine that runs your code. You do not rewrite anything. You just rebuild on .NET 9.

What got faster?

• The JIT compiler (the part that turns your code into machine code) got better at inlining and loop tricks.
• Dynamic PGO (Profile-Guided Optimization) watches how your app really runs and optimizes the hot paths.
• Startup time is about 15% faster in many apps.
• Kestrel (the web server) handles requests up to 20% faster with about 25% less latency on average.

The picture above shows the journey. Your C# becomes IL (a kind of in-between language). The JIT turns IL into real machine code. In .NET 9 the last two steps got smarter, so the final machine code runs quicker.

You do not see this. You just feel it. The same app, the same hardware, but more requests served per second.

Reason 2: A smarter garbage collector (less memory waste)

The garbage collector (GC) is the cleaner inside .NET. It frees memory you no longer use, so you do not have to do it by hand.

.NET 9 ships a feature called DATAS — Dynamically Adapting To Application Sizes. It is now on by default for Server GC.

Here is the simple idea. When your app is busy, GC uses more memory to keep up. When your app is quiet, GC scales down and uses less memory. It breathes with your traffic.

Real apps report around 8–12% less memory overhead and smoother GC cycles. Less memory means cheaper cloud bills and fewer "out of memory" surprises during a traffic spike.

There is also a faster sorting trick called vxsort that used to work only on Windows. In .NET 9 it works on Linux too. So Linux containers (the most common way to host apps) get the speed-up as well.

Reason 3: Minimal APIs use far less memory

If you build web APIs, this one is big. Minimal APIs in .NET 9 can handle about 15% more requests per second than in .NET 8 — while using around 93% less memory for the routing work.

Here is a tiny Minimal API so you can see how small the code is:

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();
 
app.MapGet("/hello/{name}", (string name) =>
{
    return Results.Ok($"Hello, {name}!");
});
 
app.MapGet("/health", () => Results.Ok("Healthy"));
 
app.Run();

That whole file is a working web service. On .NET 9, the same code is lighter and faster than on .NET 8. You write less, and it runs better.

Reason 4: New LINQ methods that save allocations

LINQ is how we query lists and collections in C#. .NET 9 adds two handy methods: CountBy and AggregateBy.

Before, if you wanted to count items by a key (say, orders per city), you had to use GroupBy. That created extra groups in memory just to count them. Wasteful.

Look at the old way and the new way:

// Old way (.NET 8): GroupBy makes intermediate groups
var oldCounts = orders
    .GroupBy(o => o.City)
    .Select(g => new { City = g.Key, Count = g.Count() });
 
// New way (.NET 9): CountBy is direct and cheaper
var newCounts = orders.CountBy(o => o.City);
// newCounts is a sequence of KeyValuePair<City, Count>

The new way skips the wasteful middle step. Less memory used, cleaner code. AggregateBy does the same idea when you want to sum or combine values by key, not just count them.

Under the hood, .NET 9 also rebuilt LINQ around a base class called Iterator<TSource>. The result is less overhead when you chain Where, Select, and OrderBy. You get the win just by upgrading.

Reason 5: Native AOT is ready for ASP.NET Core

Native AOT (Ahead-Of-Time compilation) turns your app into native machine code at build time instead of compiling at run time. In .NET 9 it is production-ready for many ASP.NET Core apps.

What do you get?

• Faster startup — the app is already native code, so it boots almost instantly.
• Smaller memory footprint.
• No JIT at runtime, and no need to ship the full .NET runtime with the app.

This is great for serverless functions and small containers, where a quick cold start really matters. A function that wakes up faster costs less and feels snappier to your users.

Reason 6: Smaller, cheaper Docker images

If you ship your app in containers, .NET 9 gives you smaller base images — up to around 30% smaller in some cases.

Smaller images mean:

• Faster pulls and deploys.
• Less storage in your registry.
• Quicker scaling when traffic jumps.

Here is a simple Dockerfile using the .NET 9 images:

// Dockerfile (shown as code for highlighting)
// Build stage
// FROM mcr.microsoft.com/dotnet/sdk:9.0 AS build
// WORKDIR /src
// COPY . .
// RUN dotnet publish -c Release -o /app
 
// Runtime stage (small image)
// FROM mcr.microsoft.com/dotnet/aspnet:9.0
// WORKDIR /app
// COPY --from=build /app .
// ENTRYPOINT ["dotnet", "MyApp.dll"]

Pair this with the lower memory use from DATAS and Minimal APIs, and your cloud bill goes down without any clever tricks.

Reason 7: Faster, friendlier System.Text.Json

JSON is everywhere — every API call, every config file. .NET 9 makes System.Text.Json about 35% faster at serialization in many cases, and adds nice quality-of-life features:

• It understands nullable reference type annotations.
• You can export a JSON schema from your types.
• You can set custom indentation for pretty output.
• You can read multiple root-level JSON values from one stream.

Here is the indentation option in action:

using System.Text.Json;
 
var options = new JsonSerializerOptions
{
    WriteIndented = true,
    IndentCharacter = ' ',
    IndentSize = 2
};
 
var person = new { Name = "Asha", City = "Pune", Age = 12 };
string json = JsonSerializer.Serialize(person, options);
Console.WriteLine(json);

Small thing, but when you read logs or debug output, neat JSON saves real time.

Reason 8: C# 13 comes along for the ride

When you move to the .NET 9 SDK, you also get C# 13. Two features stand out for everyday code.

Params collections. The params keyword used to work only with arrays. Now it works with Span<T>, List<T>, and more. That means fewer hidden array allocations.

// C# 13: params works with more than arrays
void Print(params ReadOnlySpan<int> numbers)
{
    foreach (var n in numbers)
        Console.Write($"{n} ");
}
 
Print(1, 2, 3, 4, 5); // no array allocated for this call

Partial properties. You can now split properties across partial types, which helps source generators a lot.

C# 13 also adds ref struct types as generic arguments and overload resolution priority. You do not have to use these. But they are there when you need them.

Reason 9: Built-in AI building blocks

.NET 9 makes it much easier to add AI to your apps without leaving the .NET world. The Microsoft.Extensions.AI libraries give you one common way to talk to language models, embeddings, and more.

The big idea is a shared interface. You write your code once, and you can swap the model behind it.

A quick taste of the shape (provider setup left out for clarity):

// Using the common IChatClient abstraction
IChatClient chat = GetChatClient();
 
var reply = await chat.GetResponseAsync(
    "Explain .NET 9 to a 12 year old in one line.");
 
Console.WriteLine(reply.Text);

Because the interface is shared, you can start with a cloud model and later move to a local one with very little change. One note for teams: some popular libraries like MediatR and MassTransit are now commercially licensed. They are not part of .NET itself — just be aware of their license terms when you pick tools.

Reason 10: Longer support and an easy upgrade

Two practical reasons to finish on.

First, support. .NET 9 now gets 24 months of support (up from 18). That gives your team more breathing room before the next forced move. End of support is currently November 10, 2026. If you want even longer, plan a hop to .NET 10, the LTS release.

Second, the upgrade is easy. .NET 9 stays highly compatible with .NET 8. In most projects you just change the target framework, restore packages, and rebuild.

<!-- In your .csproj file -->
<PropertyGroup>
  <TargetFramework>net9.0</TargetFramework>
</PropertyGroup>

Here is a simple plan to follow:

A side-by-side look

Here is a quick table comparing the two versions on the things we covered.

And here is a short table to help you decide which release to target.

How the reasons connect

All ten reasons feed into the same goal: an app that is faster, cheaper, and easier to run. The diagram below shows how they link together.

Quick recap

• .NET 9 is faster for free. The runtime, JIT, and PGO got smarter, so just recompiling helps.
• The garbage collector is smarter. DATAS scales memory with traffic and uses 8–12% less overhead.
• Minimal APIs are leaner — about 15% more requests per second and far less memory.
• LINQ adds CountBy and AggregateBy to skip wasteful grouping.
• Native AOT is production-ready for many ASP.NET Core apps, giving instant startup.
• Docker images are up to 30% smaller, so deploys are quicker and cheaper.
• System.Text.Json is ~35% faster with handy new options.
• C# 13 ships with it — params collections and partial properties.
• AI is built in through Microsoft.Extensions.AI and one common interface.
• Support is now 24 months, and the upgrade from .NET 8 is usually painless.

If your app runs on .NET 8 today, .NET 9 is a small, friendly step that pays you back in speed and lower cost. And when you are ready for the longest support, .NET 10 (LTS) is waiting.

References and further reading

• What's new in .NET 9 — Microsoft Learn
• .NET STS releases supported for 24 months — .NET Blog
• .NET and .NET Core official support policy
• Performance Improvements in .NET 9 — Microsoft .NET Blog
• 10 Reasons to Upgrade to .NET 9 — antondevtips