breaking: Remove .NET Standard 2.0, modernize AOT compatibility (#4257)

BREAKING CHANGES:

1. Removed .NET Standard 2.0 support - minimum target is now net8.0 or
net462
  2. Removed RxApp.cs - replaced with RxAppBuilder pattern
3. Removed PlatformRegistrationManager.cs - replaced with modern builder
pattern
4. Solution file renamed: ReactiveUI.sln → reactiveui.slnx (SLNX format)
  5. Removed polyfill attributes now built into modern .NET:
     - CallerArgumentExpressionAttribute
     - DoesNotReturnIfAttribute - NotNullAttribute - IsExternalInit
6. Removed platform-specific ComponentModelTypeConverter implementations
  7. Removed legacy DefaultViewLocator.AOT.cs

  ## Migration Guide

  ### .NET Standard 2.0 Removal
Projects targeting .NET Standard 2.0 must upgrade to at least .NET 8.0.
ReactiveUI
now requires modern .NET with built-in nullable reference types, init
properties,
  and AOT attributes.

  **Before:**
  - Supported: netstandard2.0, net462+, net8+
  - Used polyfill attributes for modern C# features

  **After:**
- Minimum: net8.0 for cross-platform, net462 for Windows-only legacy,
net9.0 for maui platforms
  - Uses built-in .NET attributes for nullable/AOT support

  ### RxApp Removal
The static RxApp class has been removed in favor of the builder pattern.

  **Before:**
  ```csharp
  RxApp.MainThreadScheduler.Schedule(() => { });
  RxApp.TaskpoolScheduler.Schedule(() => { });

  After:
  // Use RxSchedulers for AOT-safe scheduler access
  RxSchedulers.MainThreadScheduler.Schedule(() => { });
  RxSchedulers.TaskpoolScheduler.Schedule(() => { });

  // Or use builder pattern for initialization
  var builder = RxAppBuilder.CreateReactiveUIBuilder(resolver)
      .WithCoreServices()
      .BuildApp();
```

  Solution File Renamed

  Before: src/ReactiveUI.sln (legacy text format)
  After: src/reactiveui.slnx (XML-based format)

  Impact:
  - Update CI/CD scripts referencing ReactiveUI.sln → reactiveui.slnx
  - Requires Visual Studio 2022 17.10+ or JetBrains Rider 2024.1+ for IDE support
  - All dotnet CLI commands work identically (no syntax changes)

  Major Enhancements

  Test Coverage (80%+ achieved)

  - Reorganized test projects for better coverage analysis
  - Removed duplicate/obsolete test projects:
    - ReactiveUI.AOTTests (consolidated into main tests)
    - ReactiveUI.Builder.Tests (consolidated)
    - ReactiveUI.Splat.Tests (functionality moved to core)
    - ReactiveUI.Testing.Tests (consolidated)
  - Fixed intermittent test failures with proper Locator scoping
  - Added comprehensive MAUI activation tests
  - Enhanced builder API tests with WithInstance coverage

  AOT Compatibility Improvements

  This branch addresses numerous AOT warnings and improves trimming compatibility:
  - Removed reflection-heavy PlatformRegistrationManager
  - Removed ComponentModelTypeConverter (used reflection)
  - Streamlined view locator implementation (removed AOT-specific version)
  - All polyfill attributes removed (used UnconditionalSuppressMessage)
  - Improved DynamicallyAccessedMembers usage throughout codebase

  Bug Fixes

  - Nested property binding: Fixed redundant setter calls that caused performance issues and unexpected behavior when binding to nested properties
  - MAUI activation: Resolved activation lifecycle issues in MAUI controls
  - Test flakiness: Fixed race conditions in Locator-dependent tests by introducing LocatorScope pattern

  API Enhancements

  - Builder API: Added BuilderMixins with WithInstance pattern for better testability
  - XML Documentation: Comprehensive docs added to:
    - All public interfaces (IActivatableView, IViewFor, etc.)
    - Suspension APIs (ISuspensionHost, ISuspensionDriver)
    - Interaction APIs (IInteraction, IInteractionContext)
    - View locator and activation APIs
  - Usage Examples: Added code examples to public API documentation

  Documentation Improvements

  Added comprehensive educational documentation (CLAUDE.md, copilot-instructions.md):

  SLNX Format Documentation

  - What SLNX is (XML-based solution format, VS 2022 17.10+)
  - Key differences from .sln (structured XML vs proprietary text)
  - IDE compatibility requirements
  - CLI usage (identical to .sln files)

  Microsoft Testing Platform (MTP) Documentation

  - What MTP is and why ReactiveUI uses it (modern test platform replacing VSTest)
  - How MTP differs from VSTest (argument syntax, configuration)
  - Configuration files explained (global.json, testconfig.json, Directory.Build.props)
  - Best practices:
    - Never use --no-build flag (causes stale binary issues)
    - Correct argument placement: dotnet test flags BEFORE --, TUnit flags AFTER --
    - How to see Console.WriteLine output (--output Detailed)
    - Non-parallel test execution rationale

  Command Reference Improvements

  - Fixed coverage argument placement (--coverage goes BEFORE --)
  - Reorganized command-line flags by tool (dotnet test vs TUnit)
  - Removed contradictory examples (--no-build)
  - All solution references updated to reactiveui.slnx

  Key Deletions

  - src/ReactiveUI.sln → migrated to reactiveui.slnx
  - src/ReactiveUI/RxApp.cs → replaced with RxAppBuilder pattern
  - src/ReactiveUI/PlatformRegistrationManager.cs → replaced with builder pattern
  - src/ReactiveUI/RegistrationNamespace.cs → obsolete with new registration approach
  - src/ReactiveUI/Helpers/*.cs → polyfill attributes removed (4 files)
  - src/ReactiveUI/IsExternalInit.cs → built into modern .NET
  - src/ReactiveUI.*/GlobalUsings.cs → removed from Maui/WinUI (2 files)
  - src/ReactiveUI/Platforms/*/ComponentModelTypeConverter.cs → reflection-based, removed (3 files)
  - src/ReactiveUI/View/DefaultViewLocator.AOT.cs → consolidated with main implementation
  - Multiple test projects consolidated (ReactiveUI.AOTTests, Builder.Tests, Splat.Tests, etc.)

  Platform Support Matrix

  Before:
  - netstandard2.0 (cross-platform)
  - net462, net472, net481 (Windows legacy)
  - net8.0, net9.0, net10.0 (modern)

  After:
  - net462, net472, net481 (Windows legacy - minimum for Windows-only projects)
  - net8.0, net9.0, net10.0 (cross-platform minimum)
  - net8.0/9.0/10.0-windows10.0.19041.0 (Windows-specific features)
  - iOS, tvOS, macOS, Android, MAUI targets unchanged

  Co-authored-by: Christian Fischerauer [email protected]

Author: glennawatson

.claude/settings.local.json

@@ -41,9 +41,21 @@ Invoke-WebRequest -Uri https://dot.net/v1/dotnet-install.ps1 -OutFile dotnet-ins
 
 ### Solution Files
 
-* Main solution: `src/ReactiveUI.sln` (repository `root/src` directory)
+* Main solution: `src/reactiveui.slnx` (repository `root/src` directory)
 * Integration tests: `integrationtests/` directory contains platform-specific solutions. These are not required for most tasks to compile.
 
+**About SLNX Format:**
+
+This repository uses **SLNX** (XML-based solution format) introduced in Visual Studio 2022 17.10+, instead of the legacy `.sln` format.
+
+**Key characteristics:**
+- Structured XML format vs. proprietary text format
+- Better support for complex multi-platform projects like ReactiveUI
+- Full compatibility with `dotnet` CLI (works identically to .sln)
+- Requires Visual Studio 2022 17.10+ or JetBrains Rider 2024.1+ for IDE support
+
+All commands in this document reference `src/reactiveui.slnx`.
+
 ---
 
 ## 🛠️ Build & Test Commands
@@ -67,24 +79,42 @@ dotnet workload restore
 cd ..
 
 # Restore NuGet packages
-dotnet restore src/ReactiveUI.sln
+dotnet restore src/reactiveui.slnx
 
 # Build the solution (requires Windows for platform-specific targets)
-dotnet build src/ReactiveUI.sln -c Release -warnaserror
+dotnet build src/reactiveui.slnx -c Release -warnaserror
 ```
 
 ### Test Commands (Microsoft Testing Platform)
 
 **CRITICAL:** This repository uses MTP configured in `src/global.json`. All TUnit-specific arguments must be passed after `--`.
 
+**Microsoft Testing Platform (MTP) Overview:**
+
+MTP is the modern test execution platform replacing VSTest, providing:
+- Native integration with `dotnet test` command
+- Better performance and modern architecture for .NET 6.0+
+- Enhanced test filtering and parallel execution control
+- Required for TUnit framework (ReactiveUI's chosen test framework)
+
+**Key Differences from VSTest:**
+- Arguments passed AFTER `--`: `dotnet test -- --treenode-filter "..."`
+- Configured via `global.json` (specifies "Microsoft.Testing.Platform")
+- Test settings in `testconfig.json` (parallel execution, coverage format)
+
+**Best Practices:**
+- **Never use `--no-build`** - always build before testing to avoid stale binaries
+- Use `--output Detailed` BEFORE `--` to see Console.WriteLine output
+- TUnit runs non-parallel (`"parallel": false`) to prevent test interference
+
 **Note:** Commands below assume repository root as working directory. Use `src/` prefix for paths.
 
 ```powershell
 # Run all tests
-dotnet test src/ReactiveUI.sln -c Release --no-build
+dotnet test src/reactiveui.slnx -c Release
 
 # Run tests with code coverage (Microsoft Code Coverage)
-dotnet test src/ReactiveUI.sln -- --coverage --coverage-output-format cobertura
+dotnet test src/reactiveui.slnx --coverage --coverage-output-format cobertura
 
 # Run specific test project
 dotnet test --project src/tests/ReactiveUI.Tests/ReactiveUI.Tests.csproj
@@ -96,16 +126,16 @@ dotnet test --project src/tests/ReactiveUI.Tests/ReactiveUI.Tests.csproj -- --tr
 dotnet test --project src/tests/ReactiveUI.Tests/ReactiveUI.Tests.csproj -- --treenode-filter "/*/*/MyClassName/*"
 
 # Filter by test property (e.g., Category)
-dotnet test src/ReactiveUI.sln -- --treenode-filter "/*/*/*/*[Category=Integration]"
+dotnet test src/reactiveui.slnx -- --treenode-filter "/*/*/*/*[Category=Integration]"
 
 # List all available tests
 dotnet test --project src/tests/ReactiveUI.Tests/ReactiveUI.Tests.csproj -- --list-tests
 
 # Fail fast (stop on first failure)
-dotnet test src/ReactiveUI.sln -- --fail-fast
+dotnet test src/reactiveui.slnx -- --fail-fast
 
 # Generate TRX report with coverage
-dotnet test src/ReactiveUI.sln -- --coverage --coverage-output-format cobertura --report-trx --output Detailed
+dotnet test src/reactiveui.slnx --coverage --coverage-output-format cobertura -- --report-trx --output Detailed
 ```
 
 **TUnit Treenode-Filter Syntax:**
@@ -118,10 +148,14 @@ Examples:
 - All tests in namespace: `--treenode-filter "/*/MyNamespace/*/*"`
 - Filter by property: `--treenode-filter "/*/*/*/*[Category=Integration]"`
 
-**Key TUnit Command-Line Flags:**
-- `--treenode-filter` - Filter tests by path or properties
+**Key Command-Line Flags:**
+
+**dotnet test flags (BEFORE `--`):**
 - `--coverage` - Enable Microsoft Code Coverage
 - `--coverage-output-format` - Set format (cobertura, xml, coverage)
+
+**TUnit flags (AFTER `--`):**
+- `--treenode-filter` - Filter tests by path or properties
 - `--report-trx` - Generate TRX reports
 - `--output` - Verbosity (Normal or Detailed)
 - `--list-tests` - Display tests without running
@@ -528,11 +562,66 @@ _total = this.WhenAnyValue(
 
 ```powershell
 # Build with warnings as errors (includes StyleCop violations)
-dotnet build src/ReactiveUI.sln -c Release -warnaserror
+dotnet build src/reactiveui.slnx -c Release -warnaserror
 ```
 
 **Important:** Style violations will cause build failures. Use an IDE with EditorConfig support (Visual Studio, VS Code, Rider) to automatically format code according to project standards.
 
+### Zero Pragma Policy for Warning Suppression
+
+**CRITICAL: This project enforces a strict zero pragma policy.**
+
+* **NO `#pragma warning disable` statements are allowed** in any code files
+* **All StyleCop analyzer warnings (SA****) MUST be fixed**, never suppressed
+* **Code analyzer warnings (CA****) may ONLY be suppressed as an absolute last resort** using `[SuppressMessage]` attribute with these requirements:
+  1. You have attempted to fix the warning first
+  2. Fixing it would make the code worse or is not technically feasible
+  3. You use `[SuppressMessage]` attribute (NOT pragma)
+  4. You provide a clear, valid justification in the `Justification` parameter
+
+#### Examples
+
+```csharp
+// ❌ WRONG - Never use pragma directives
+#pragma warning disable CA1062
+public void ProcessData(object data)
+{
+    data.ToString();  // CA1062: Validate parameter is non-null
+}
+#pragma warning restore CA1062
+
+// ✅ CORRECT - Fix the issue
+public void ProcessData(object data)
+{
+    ArgumentNullException.ThrowIfNull(data);
+    data.ToString();
+}
+
+// ⚠️ ACCEPTABLE - SuppressMessage with justification (last resort only)
+[SuppressMessage("Microsoft.Design", "CA1062:ValidateArgumentsOfPublicMethods",
+    Justification = "TUnit framework guarantees non-null parameters from test data sources")]
+public async Task ValidateConverter(IBindingTypeConverter converter, int expectedAffinity)
+{
+    var affinity = converter.GetAffinityForObjects();  // converter is never null
+    await Assert.That(affinity).IsEqualTo(expectedAffinity);
+}
+```
+
+#### Common StyleCop Fixes (Must Be Fixed, Never Suppressed)
+
+* **SA1202** (Static members before instance members): Reorder class members
+* **SA1204** (Static members before non-static): Move static members to top
+* **SA1402** (One type per file): Convert file-scoped types to nested private classes
+* **SA1611** (Parameter documentation missing): Add `<param>` XML tags
+* **SA1615** (Return value documentation missing): Add `<returns>` XML tags
+* **SA1600** (Elements should be documented): Add XML documentation to public members
+
+#### Enforcement
+
+* Builds fail with `-warnaserror` if any analyzer warnings exist
+* Any pragma directives will be flagged and rejected during code review
+* `SuppressMessage` usage requires clear justification and approval
+
 ---
 
 ## 📋 Testing Guidelines

.editorconfig

@@ -20,7 +20,6 @@ jobs:
     with:
       configuration: Release
       productNamespacePrefix: "ReactiveUI"
-      solutionFile: "ReactiveUI.sln"
       installWorkloads: true
     secrets:
       CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}

.github/copilot-instructions.md

@@ -450,3 +450,6 @@ src/Tools/
 **/*.GeneratedMSBuildEditorConfig.editorconfig
 /app
 .dotnet/
+
+# Claude Settings
+.claude/