Add lightweight C# 12 refactoring sample

.github/workflows/build-samples.yml

@@ -20,6 +20,7 @@ on:
       - "cloud-native/dockerize-aspnet-core-clean-images/**"
       - "cloud-native/health-resilience-zero-downtime/**"
       - "cloud-native/polly-resilience/**"
+      - "csharp-language/csharp-12-features/**"
       - ".github/workflows/build-samples.yml"
 
   pull_request:
@@ -40,6 +41,7 @@ on:
       - "cloud-native/dockerize-aspnet-core-clean-images/**"
       - "cloud-native/health-resilience-zero-downtime/**"
       - "cloud-native/polly-resilience/**"
+      - "csharp-language/csharp-12-features/**"
       - ".github/workflows/build-samples.yml"
 
   workflow_dispatch:
@@ -693,3 +695,102 @@ jobs:
           cloud-native/polly-resilience/PollyCatalogResilience.slnx
           --configuration Release
           --no-build
+
+  test-csharp-12-features:
+    name: Test C# 12 refactoring sample
+    runs-on: ubuntu-latest
+
+    permissions:
+      contents: read
+
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v5
+
+      - name: Install .NET 10 SDK
+        uses: actions/setup-dotnet@v5
+        with:
+          dotnet-version: "10.0.x"
+
+      - name: Restore
+        run: >
+          dotnet restore
+          csharp-language/csharp-12-features/CSharp12RefactoringLab.slnx
+
+      - name: Build
+        run: >
+          dotnet build
+          csharp-language/csharp-12-features/CSharp12RefactoringLab.slnx
+          --configuration Release
+          --no-restore
+
+      - name: Test
+        run: >
+          dotnet test
+          csharp-language/csharp-12-features/CSharp12RefactoringLab.slnx
+          --configuration Release
+          --no-build
+
+      - name: Verify explicit C# language version
+        shell: bash
+        run: |
+          app_version="$(
+            dotnet msbuild \
+              csharp-language/csharp-12-features/src/CSharp12RefactoringLab/CSharp12RefactoringLab.csproj \
+              -getProperty:LangVersion
+          )"
+
+          test_version="$(
+            dotnet msbuild \
+              csharp-language/csharp-12-features/tests/CSharp12RefactoringLab.Tests/CSharp12RefactoringLab.Tests.csproj \
+              -getProperty:LangVersion
+          )"
+
+          echo "Application LangVersion: ${app_version}"
+          echo "Test LangVersion: ${test_version}"
+
+          test "${app_version}" = "12.0"
+          test "${test_version}" = "12.0"
+
+      - name: Run deterministic console sample
+        shell: bash
+        run: |
+          output="$(
+            dotnet run \
+              --project csharp-language/csharp-12-features/src/CSharp12RefactoringLab/CSharp12RefactoringLab.csproj \
+              --configuration Release \
+              --no-build
+          )"
+
+          printf '%s\n' "${output}"
+
+          expected="$(
+            printf '%s\n' \
+              "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]"
+          )"
+
+          if [[ "${output}" != "${expected}" ]]; then
+            echo "Console output did not match the expected output."
+
+            diff \
+              --unified \
+              <(printf '%s\n' "${expected}") \
+              <(printf '%s\n' "${output}") \
+              || true
+
+            exit 1
+          fi
+
+          line_count="$(
+            printf '%s\n' "${output}" |
+              wc --lines |
+              tr --delete ' '
+          )"
+
+          test "${line_count}" = "7"

README.md

@@ -25,6 +25,7 @@ Each sample folder contains a focused implementation of one tutorial topic. The
 | [`cloud-native/dockerize-aspnet-core-clean-images`](cloud-native/dockerize-aspnet-core-clean-images/) | Focused .NET 10 container sample demonstrating a multi-stage Dockerfile, locked restore, runtime-only image, non-root execution, port 8080, runtime configuration, health checks, hardened Compose settings, and CI smoke testing | [Dockerizing ASP.NET Core: Multi-Stage Builds, Clean Images & a Production-Ready Ship Workflow](https://www.dotnet-guide.com/tutorials/cloud-native/dockerize-aspnet-core-clean-images/) |
 | [`cloud-native/health-resilience-zero-downtime`](cloud-native/health-resilience-zero-downtime/) | Focused .NET 10 Orders API demonstrating tagged liveness/readiness checks, structured health output, shutdown readiness draining, typed HttpClient retries, circuit breaking, attempt timeouts, and deterministic integration testing | [.NET 8 Cloud-Native: Health Probes, Polly Resilience & Zero-Downtime Kubernetes Deployments](https://www.dotnet-guide.com/tutorials/cloud-native/health-resilience-zero-downtime/) |
 | [`cloud-native/polly-resilience`](cloud-native/polly-resilience/) | Focused .NET 10 Polly v8 catalog sample demonstrating explicit stale-cache fallback, timeout-driven degradation, outbound concurrency isolation, strategy event counters, and deterministic integration testing | [Polly Resilience (Polly v8): Timeouts, Retries, Circuits, Bulkheads, Hedging](https://www.dotnet-guide.com/tutorials/cloud-native/polly-resilience/) |
+| [`csharp-language/csharp-12-features`](csharp-language/csharp-12-features/) | Focused .NET 10 console lab locked to C# 12.0, demonstrating primary constructors with explicit properties, collection expressions and spreads, default lambda parameters, tuple-type aliases, deterministic output, and unit tests | [C# 12 Language Features: Primary Constructors, Collections & More](https://www.dotnet-guide.com/tutorials/csharp-language/csharp-12-features/) |
 
 ## Companion articles
 - [Common Microsoft.Extensions.AI mistakes](https://www.dotnet-guide.com/articles/dotnet-ai/microsoft-extensions-ai-common-mistakes/)
@@ -267,6 +268,24 @@ tutorials/
 |           `-- PollyCatalogResilience.Tests/
 |               |-- PollyCatalogResilience.Tests.csproj
 |               `-- CatalogResilienceTests.cs
+|-- csharp-language/
+|   `-- csharp-12-features/
+|       |-- 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
 |-- aspnet-core/
 |   |-- api-security-in-practice/
 |   |   |-- ApiSecurityMinimal.slnx

csharp-language/csharp-12-features/CSharp12RefactoringLab.slnx

@@ -0,0 +1,8 @@
+<Solution>
+  <Folder Name="/src/">
+    <Project Path="src/CSharp12RefactoringLab/CSharp12RefactoringLab.csproj" />
+  </Folder>
+  <Folder Name="/tests/">
+    <Project Path="tests/CSharp12RefactoringLab.Tests/CSharp12RefactoringLab.Tests.csproj" />
+  </Folder>
+</Solution>

csharp-language/csharp-12-features/README.md

@@ -0,0 +1,202 @@
+# 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](https://www.dotnet-guide.com/tutorials/csharp-language/csharp-12-features/)
+
+## 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:
+
+```xml
+<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:
+
+```csharp
+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:
+
+```csharp
+TodoItem[] array = [item];
+List<TodoItem> list = [item];
+```
+
+A bare declaration such as:
+
+```csharp
+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
+
+```csharp
+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
+
+```powershell
+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
+
+```text
+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
+
+```powershell
+dotnet msbuild `
+  .\src\CSharp12RefactoringLab\CSharp12RefactoringLab.csproj `
+  -getProperty:LangVersion
+```
+
+Expected:
+
+```text
+12.0
+```
+
+## Project structure
+
+```text
+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.

csharp-language/csharp-12-features/src/CSharp12RefactoringLab/CSharp12RefactoringLab.csproj

@@ -0,0 +1,12 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net10.0</TargetFramework>
+    <LangVersion>12.0</LangVersion>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+  </PropertyGroup>
+
+</Project>