What’s New in C# 12 and 13? A Quick Guide for Modern Backend Developers

C# is evolving fast—not just for desktop or mobile, but to help us write more elegant, high-performance cloud and microservice backends. Here’s a focused summary on what really matters in C# 12 (now released) and the most exciting proposals in C# 13 (coming soon). I’ll highlight features with direct impact for backend work, along with bite-size code samples.

C# 12: What’s New (Released with .NET 8)

1. Primary Constructors for Non-Records

One of the most impactful: primary constructors are now allowed in classes and structs, not just records. Makes DTOs, services, and many microservice models cleaner.

public class Invoice(string id, decimal amount)
{
    public string Id { get; } = id;
    public decimal Amount { get; } = amount;
}

Why it matters:
Less boilerplate, better immutability for models across APIs.

2. Collection Expressions

Quickly create arrays and lists without explicit constructors or new keyword (inspired by JavaScript, Python).

int[] a = [1, 2, 3];
List<string> b = ["A", "B", "C"];

Why it matters:
Faster and cleaner initialization of configs, DTOs, test data.

3. Default Lambda Parameters

Set default values right in your lambda arguments.

Func<int, int, int> adder = (a, b = 1) => a + b;
Console.WriteLine(adder(10)); // 11

Why it matters:
Lambdas are more flexible for utilities, especially in LINQ or functional code.

4. Alias Any Type (using Aliases for Tuples, Arrays, Spans, etc.)

Use the using keyword as an alias for complex types, not just classes.

using Person = (string Name, int Age);

Person p = ("Alice", 30);

Why it matters:
Makes working with complex tuple or generic types readable and maintainable.

5. Optional Parameters in Struct Constructors

You can now use optional and default values in struct constructors.

public struct Point(int x = 0, int y = 0)
{
    public int X = x;
    public int Y = y;
}

6. Inline Arrays

Better support for fixed-length buffers and serialization—useful in high-perf and interop code.

[System.Runtime.CompilerServices.InlineArray(10)]
public struct InlineByteArray
{
    private byte _element0;
}

C# 13: What’s Coming? (Preview/Proposal)

C# 13 features are still being discussed, but several are hot topics and some are appearing in early previews. Here’s what backend engineers should keep an eye on:

1. Params Collections in Method Signatures

Write APIs that take variable-sized arrays or lists but remain type-safe and concise.

void PrintNames(params string[] names) { }
void PrintNumbers(params List<int> nums) { } // Collection params in broader usage

2. Lambda Expressions with Attributes

Soon you’ll be able to decorate lambdas with attributes, enhancing diagnostics and policies for advanced function pipelining or middleware.

[MyLog]
var add = ([LogArg] int a, int b) => a + b;

3. Primary Constructors for Abstract and Static Classes

Primary constructors are expected to be extended for even more class types, such as abstract/service-oriented classes.

public abstract class BaseHandler(string type) { }

4. Pattern Matching Enhancements

Pattern matching keeps improving (properties, collections, discriminated unions) to write safer, more concise microservice logic.

if (obj is Order { Status: "Pending", Amount: > 100 }) { ... }

5. Improved Intellisense/Diagnostics

Not a code feature, but C# 13 + tooling is expected to deliver enhanced code analyzers, error messages, and fix suggestions, reducing daily friction for all engineers.

Should You Upgrade?

If you’re on .NET 8:
Start using C# 12 features today—especially primary constructors, collection expressions, and type aliasing. They pay off in model clarity and fewer bugs.

For C# 13:
Follow its preview if you love cutting-edge—big gains are coming for API ergonomics and pattern-based architecture.

Final Thoughts

C# remains a modern, rapidly evolving language. Features like primary constructors, collection expressions, and smarter defaults matter a lot as we build bigger, more reliable distributed systems. Adopt them early—you’ll save code, cut bugs, and keep your toolkit sharp.

Have a favorite new feature or one you’re waiting for? Share below!

Nguyen Thanh Tuyen – Technical Lead, practical cloud architecture and modern .NET from the trenches