csharp-12-features

C# 12 Everyday Refactoring Lab

A focused .NET 10 console companion demonstrating practical C# 12 refactoring with primary constructors, explicit public properties, collection expressions, spread elements, default lambda parameters, and an alias for a tuple type.

Full tutorial

C# 12 Language Features: Primary Constructors, Collections & More

Framework and language note

The tutorial introduces C# 12 with the .NET 8 toolchain.

This companion targets .NET 10 because that is the DOTNET GUIDE repository's current SDK, but both projects explicitly set:

<LangVersion>12.0</LangVersion>

The compiler accepts C# 12 syntax while rejecting newer C# 13 or C# 14 features accidentally introduced during maintenance.

The project doesn't use latest or preview.

What the sample demonstrates

• a class primary constructor;
• explicit properties initialized from constructor parameters;
• validation through a property initializer;
• a service primary constructor;
• collection expressions targeting List<T> and arrays;
• spread elements for copying and composition;
• a tuple alias using C# 12 "alias any type";
• a default lambda parameter;
• an explicit override of the lambda default;
• deterministic output;
• seven tests.

Primary-constructor boundary

Primary-constructor parameters are parameters, not class members.

This class:

public sealed class TodoItem(string title)
{
    public string Title { get; } = title;
}

has a public Title property because the property is explicitly declared.

It doesn't automatically gain a public property named title.

The tests protect this distinction with reflection.

Collection-expression boundary

Collection expressions are target-typed.

These compile because the target type is known:

TodoItem[] array = [item];
List<TodoItem> list = [item];

A bare declaration such as:

var items = [item];

doesn't provide a target collection type and isn't used by this sample.

Spread elements enumerate their source.

They should not be described as allocation-free without measurement.

Alias boundary

using TodoCounts = (int Total, int Active, int Completed);

creates a source-code alias for a tuple type.

It doesn't create a new nominal runtime type.

Use a record or struct when a distinct domain type is required.

Default-lambda boundary

The formatter declares its lambda with var.

The compiler synthesizes a delegate type that preserves the optional parameter.

The sample calls the lambda both with and without the optional prefix.

Prerequisite

• .NET 10 SDK

Restore, build, test, and run

dotnet restore `
  .\CSharp12RefactoringLab.slnx

dotnet build `
  .\CSharp12RefactoringLab.slnx `
  --configuration Release `
  --no-restore

dotnet test `
  .\CSharp12RefactoringLab.slnx `
  --configuration Release `
  --no-build

dotnet run `
  --project .\src\CSharp12RefactoringLab\CSharp12RefactoringLab.csproj `
  --configuration Release `
  --no-build

Expected output

C# 12 Todo Refactoring Lab
Counts: total=4, active=2, completed=2
TODO: Welcome to the C# 12 lab [done]
TODO: Adopt primary constructors [active]
TODO: Use collection expressions [done]
TODO: Try default lambda parameters [active]
TODO: Review explicit properties [done]

Verify the language version

dotnet msbuild `
  .\src\CSharp12RefactoringLab\CSharp12RefactoringLab.csproj `
  -getProperty:LangVersion

Expected:

12.0

Project structure

CSharp12RefactoringLab.slnx
README.md
src/
`-- CSharp12RefactoringLab/
    |-- CSharp12RefactoringLab.csproj
    |-- Program.cs
    |-- Formatting/
    |   `-- TodoFormatter.cs
    |-- Models/
    |   `-- TodoItem.cs
    `-- Services/
        `-- TodoService.cs
tests/
`-- CSharp12RefactoringLab.Tests/
    |-- CSharp12RefactoringLab.Tests.csproj
    `-- CSharp12FeatureTests.cs

Deliberately omitted

• inline arrays;
• stack allocation;
• unsafe code;
• benchmarks;
• allocation claims;
• Minimal APIs;
• C# 11 list patterns;
• preview features;
• external services.

Inline arrays are advanced struct-based contiguous storage. They deserve a separate measured sample rather than a contrived use in this Todo application.

Verification

• Companion target framework: .NET 10
• Explicit language version: C# 12.0
• Tutorial framework: .NET 8
• Application packages: none
• External services required: none
• Expected tests: 7
• Last reviewed: 2026-08-04

This sample demonstrates language semantics. It doesn't claim universal performance improvements.