breaking: seal ValidationState as record, performance improvements, replace Polyfill NuGet with local polyfills (#878)

* breaking: seal ValidationState as record, performance improvements, replace Polyfill NuGet with local polyfills

- Convert ValidationState from a class with primary constructor to a sealed record, adding IEquatable<ValidationState> support and init accessors for IsValid.
- Remove Polyfill NuGet package from the core library and replace with local polyfill files for RequiresDynamicCodeAttribute, RequiresUnreferencedCodeAttribute, IsExternalInit, NotNullAttribute, and CallerArgumentExpressionAttribute.
- Remove all 28 #if NET6_0_OR_GREATER / #endif guards around AOT attributes, making them unconditionally applied via the local polyfills.
- Optimize ValidationStateComparer to compare validation text element-by-element instead of joining to a single string, and use ReferenceEquals for fast-path null/identity checks.
- Optimize HashCode computation in ValidationStateComparer using a local HashCode polyfill with per-element ordinal string hashing.
- Add lazy activation pattern to BasePropertyValidation and ValidationContext to defer observable subscriptions until first access.
- Convert internal collection types (ArrayValidationText, SingleValidationText, ReadOnlyDisposableCollection) to use primary constructors and add XML documentation.
- Add #if NET8_0_OR_GREATER guard in ExpressionExtensions to use the faster char overload of string.Join on supported frameworks.
- Expand ValidationText with internal helper methods (CreateValidationText, CopyArray) and optimize array-backed paths to avoid unnecessary allocations.
- Add comprehensive unit tests for InternalCollectionTests, NotifyDataErrorInfoTests, ValidationContextTests, and ValidationStateComparerTests.
- Fix minor XML doc typos in IValidatableViewModel and IValidationContext.
- Update README to replace StackOverflow link with GitHub Discussions and remove duplicate Feedback section.
- Update API approval baselines to reflect the sealed record change to ValidationState.

* Update the copyright year

* Update core team info

* Stop producing nupkg for sample apps

* Cleanup the codecov.yml file

Author: glennawatson

README.md

@@ -13,7 +13,32 @@
 
 # ReactiveUI.Validation
 
-Validation for ReactiveUI based solutions, functioning in a reactive way. `ReactiveUI.Validation` was originally developed by [@jcmm33](https://github.com/jcmm33) as [Vistian.Reactive.Validation](https://github.com/jcmm33/ReactiveUI.Validation), and then refactored and updated by [Àlex Martínez Morón](https://github.com/alexmartinezm) and the [ReactiveUI Core Team](https://github.com/reactiveui/ReactiveUI#core-team).
+Validation for ReactiveUI based solutions, functioning in a reactive way.
+
+## Core Team
+
+<table>
+  <tbody>
+    <tr>
+      <td align="center" valign="top">
+        <img width="100" height="100" src="https://github.com/ChrisPulman.png?s=150">
+        <br>
+        <a href="https://github.com/ChrisPulman">Chris Pulman</a>
+        <p>London, UK</p>
+      </td>
+      <td align="center" valign="top">
+        <img width="100" height="100" src="https://github.com/glennawatson.png?s=150">
+        <br>
+        <a href="https://github.com/glennawatson">Glenn Watson</a>
+        <p>Melbourne, Australia</p>
+      </td>
+    </tr>
+  </tbody>
+</table>
+
+## Alumni & History
+
+ReactiveUI.Validation was originally developed by [@jcmm33](https://github.com/jcmm33) as [Vistian.Reactive.Validation](https://github.com/jcmm33/ReactiveUI.Validation), and then refactored and updated by [Àlex Martínez Morón](https://github.com/alexmartinezm) and the ReactiveUI community.
 
 ## Supported Platforms
 
@@ -282,17 +307,13 @@ In essence, ReactiveUI.Validation is a relatively simple model of the `Validatio
 4. Validation text can reference either the ViewModel or properties which comprise the validation rule e.g. include text entered as part of validation message.
 5. Validation text output can be adjusted using custom formatters, not only allowing for single & multiline output but also for platforms like Android it should be possible to achieve richer renderings i.e. Bold/italics.
 
-## Feedback
-
-Please use [GitHub issues](https://github.com/reactiveui/ReactiveUI.Validation/issues) for questions, comments, or bug reports.
-
 ## Contribute
 
 ReactiveUI.Validation is developed under an OSI-approved open source license, making it freely usable and distributable, even for commercial use. We love the people who are involved in this project, and we'd love to have you on board, especially if you are just getting started or have never contributed to open-source before.
 
 So here's to you, lovely person who wants to join us — this is how you can support us:
 
-* [Responding to questions on StackOverflow](https://stackoverflow.com/questions/tagged/reactiveui)
+* [Responding to questions on GitHub Discussions](https://github.com/reactiveui/ReactiveUI.Validation/discussions)
 * [Passing on knowledge and teaching the next generation of developers](http://ericsink.com/entries/dont_use_rxui.html)
 * Submitting documentation updates where you see fit or lacking.
 * Making contributions to the code base.

codecov.yml

@@ -1,6 +1,5 @@
-### YamlMime:ManagedReference
 ignore:
-  - src/tests
-  - "**/Tests/"
-  - "**/*.Tests/"
-  - "**/*.Test/"
+  - "src/tests/**"
+  - "src/samples/**"
+  - "**/LoginApp*/**"
+  - "**/AvaloniaApplication1*/**"

src/Directory.Build.props

@@ -21,6 +21,7 @@
     <RepositoryType>git</RepositoryType>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <IsTestProject>$(MSBuildProjectName.Contains('Tests'))</IsTestProject>
+    <IsSampleProject>$(MSBuildProjectDirectory.Contains('samples'))</IsSampleProject>
     <DebugType>Embedded</DebugType>
     <!-- Optional: Publish the repository URL in the built .nupkg (in the NuSpec <Repository> element) -->
     <PublishRepositoryUrl>true</PublishRepositoryUrl>
@@ -36,6 +37,11 @@
 
   <PropertyGroup Condition="'$(IsTestProject)' == 'true'">
     <TestingPlatformDotnetTestSupport>true</TestingPlatformDotnetTestSupport>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <PropertyGroup Condition="'$(IsSampleProject)' == 'true'">
+    <IsPackable>false</IsPackable>
   </PropertyGroup>
 
   <PropertyGroup>

src/Directory.Packages.props

@@ -7,8 +7,8 @@
   <ItemGroup Label="Core packages">
     <PackageVersion Include="ReactiveUI" Version="23.1.1" />
     <PackageVersion Include="ReactiveUI.AndroidX" Version="23.1.1" />
-    <PackageVersion Include="System.Collections.Immutable" Version="10.0.3" />
     <PackageVersion Include="Polyfill" Version="9.12.0" />
+    <PackageVersion Include="System.Collections.Immutable" Version="10.0.3" />
     <PackageVersion Include="Xamarin.Google.Android.Material" Version="1.13.0.1" />
     <PackageVersion Include="Xamarin.AndroidX.Lifecycle.LiveData.Core.Ktx" Version="2.10.0.1" />
     <PackageVersion Include="Xamarin.AndroidX.Lifecycle.Process" Version="2.10.0.1" />

src/ReactiveUI.Validation.AndroidX/Extensions/ViewForExtensions.cs

@@ -1,4 +1,4 @@
-// Copyright (c) 2025 ReactiveUI and Contributors. All rights reserved.
+// Copyright (c) 2019-2026 ReactiveUI and Contributors. All rights reserved.
 // Licensed to the ReactiveUI and Contributors under one or more agreements.
 // The ReactiveUI and Contributors licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.

src/ReactiveUI.Validation/Abstractions/IValidatableViewModel.cs

@@ -1,4 +1,4 @@
-// Copyright (c) 2025 ReactiveUI and Contributors. All rights reserved.
+// Copyright (c) 2019-2026 ReactiveUI and Contributors. All rights reserved.
 // Licensed to the ReactiveUI and Contributors under one or more agreements.
 // The ReactiveUI and Contributors licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.
@@ -13,7 +13,7 @@ namespace ReactiveUI.Validation.Abstractions;
 public interface IValidatableViewModel
 {
     /// <summary>
-    /// Gets get the validation context.
+    /// Gets the validation context.
     /// </summary>
     IValidationContext ValidationContext { get; }
 }

src/ReactiveUI.Validation/Collections/ArrayValidationText.cs

@@ -1,4 +1,4 @@
-// Copyright (c) 2025 ReactiveUI and Contributors. All rights reserved.
+// Copyright (c) 2019-2026 ReactiveUI and Contributors. All rights reserved.
 // Licensed to the ReactiveUI and Contributors under one or more agreements.
 // The ReactiveUI and Contributors licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.
@@ -8,19 +8,37 @@
 
 namespace ReactiveUI.Validation.Collections;
 
-internal sealed class ArrayValidationText : IValidationText
+/// <summary>
+/// An <see cref="IValidationText"/> implementation that wraps an array of validation message strings.
+/// </summary>
+/// <param name="texts">The array of validation message strings.</param>
+internal sealed class ArrayValidationText(string[] texts) : IValidationText
 {
-    private readonly string[] _texts;
-
-    internal ArrayValidationText(string[] texts) => _texts = texts;
-
-    public int Count => _texts.Length;
-
-    public string this[int index] => _texts[index];
-
-    public IEnumerator<string> GetEnumerator() => ((IEnumerable<string>)_texts).GetEnumerator();
-
-    IEnumerator IEnumerable.GetEnumerator() => _texts.GetEnumerator();
-
-    public string ToSingleLine(string? separator) => string.Join(separator, _texts);
+    /// <summary>
+    /// Gets the number of validation messages in the array.
+    /// </summary>
+    public int Count => texts.Length;
+
+    /// <summary>
+    /// Gets the validation message at the specified index.
+    /// </summary>
+    /// <param name="index">The zero-based index of the message to retrieve.</param>
+    /// <returns>The validation message string at the specified index.</returns>
+    public string this[int index] => texts[index];
+
+    /// <summary>
+    /// Returns an enumerator that iterates through the validation messages.
+    /// </summary>
+    /// <returns>An enumerator for the validation message strings.</returns>
+    public IEnumerator<string> GetEnumerator() => ((IEnumerable<string>)texts).GetEnumerator();
+
+    /// <inheritdoc/>
+    IEnumerator IEnumerable.GetEnumerator() => texts.GetEnumerator();
+
+    /// <summary>
+    /// Joins all validation messages into a single string using the specified separator.
+    /// </summary>
+    /// <param name="separator">The string to use as a separator between messages.</param>
+    /// <returns>A single string containing all validation messages joined by the separator.</returns>
+    public string ToSingleLine(string? separator) => string.Join(separator, texts);
 }

src/ReactiveUI.Validation/Collections/IValidationText.cs

@@ -1,4 +1,4 @@
-// Copyright (c) 2025 ReactiveUI and Contributors. All rights reserved.
+// Copyright (c) 2019-2026 ReactiveUI and Contributors. All rights reserved.
 // Licensed to the ReactiveUI and Contributors under one or more agreements.
 // The ReactiveUI and Contributors licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.

src/ReactiveUI.Validation/Collections/ReadOnlyDisposableCollection{T}.cs

@@ -1,4 +1,4 @@
-// Copyright (c) 2025 ReactiveUI and Contributors. All rights reserved.
+// Copyright (c) 2019-2026 ReactiveUI and Contributors. All rights reserved.
 // Licensed to the ReactiveUI and Contributors under one or more agreements.
 // The ReactiveUI and Contributors licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.
@@ -10,9 +10,22 @@
 
 namespace ReactiveUI.Validation.Collections;
 
+/// <summary>
+/// A read-only collection that takes a snapshot of the source enumeration into an
+/// <see cref="ImmutableList{T}"/> and supports <see cref="IDisposable"/> for deterministic cleanup.
+/// </summary>
+/// <typeparam name="T">The type of elements in the collection.</typeparam>
+/// <param name="items">The source enumeration to snapshot into an immutable list.</param>
 internal sealed class ReadOnlyDisposableCollection<T>(IEnumerable<T> items) : IReadOnlyCollection<T>, IDisposable
 {
+    /// <summary>
+    /// The immutable snapshot of the source items.
+    /// </summary>
     private readonly ImmutableList<T> _immutableList = [.. items];
+
+    /// <summary>
+    /// Indicates whether this instance has been disposed.
+    /// </summary>
     private bool _disposedValue;
 
     /// <summary>
@@ -45,7 +58,11 @@ public void Dispose()
         GC.SuppressFinalize(this);
     }
 
-    private void Dispose(bool disposing)
+    /// <summary>
+    /// Releases the managed resources used by this instance.
+    /// </summary>
+    /// <param name="disposing"><c>true</c> to release managed resources; <c>false</c> when called from a finalizer.</param>
+    internal void Dispose(bool disposing)
     {
         if (!_disposedValue)
         {

src/ReactiveUI.Validation/Collections/SingleValidationText.cs

@@ -1,4 +1,4 @@
-// Copyright (c) 2025 ReactiveUI and Contributors. All rights reserved.
+// Copyright (c) 2019-2026 ReactiveUI and Contributors. All rights reserved.
 // Licensed to the ReactiveUI and Contributors under one or more agreements.
 // The ReactiveUI and Contributors licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.
@@ -9,22 +9,41 @@
 
 namespace ReactiveUI.Validation.Collections;
 
-internal sealed class SingleValidationText : IValidationText
+/// <summary>
+/// An <see cref="IValidationText"/> implementation that wraps a single validation message string.
+/// </summary>
+/// <param name="text">The single validation message string.</param>
+internal sealed class SingleValidationText(string text) : IValidationText
 {
-    private readonly string _text;
-
-    internal SingleValidationText(string text) => _text = text;
-
+    /// <summary>
+    /// Gets the number of validation messages, which is always 1.
+    /// </summary>
     public int Count => 1;
 
-    public string this[int index] => index is 0 ? _text : throw new ArgumentOutOfRangeException(nameof(index));
-
+    /// <summary>
+    /// Gets the validation message at the specified index.
+    /// </summary>
+    /// <param name="index">The zero-based index of the message to retrieve.</param>
+    /// <returns>The validation message string.</returns>
+    /// <exception cref="ArgumentOutOfRangeException">Thrown when <paramref name="index"/> is not 0.</exception>
+    public string this[int index] => index is 0 ? text : throw new ArgumentOutOfRangeException(nameof(index));
+
+    /// <summary>
+    /// Returns an enumerator that yields the single validation message.
+    /// </summary>
+    /// <returns>An enumerator that yields the single validation message.</returns>
     public IEnumerator<string> GetEnumerator()
     {
-        yield return _text;
+        yield return text;
     }
 
+    /// <inheritdoc/>
     IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
 
-    public string ToSingleLine(string? separator) => _text;
+    /// <summary>
+    /// Returns the single validation message as a single-line string.
+    /// </summary>
+    /// <param name="separator">Ignored because there is only one message.</param>
+    /// <returns>The validation message string.</returns>
+    public string ToSingleLine(string? separator) => text;
 }