Fixes ReactiveValidationObject NullReferenceException and enhances performance and thread-safety of ValidationText

src/ReactiveUI.Validation.Tests/API/ApiApprovalTests.ValidationProject.net472.approved.txt

@@ -10,15 +10,29 @@ namespace ReactiveUI.Validation.Collections
 {
     public class ValidationText : System.Collections.Generic.IEnumerable<string>, System.Collections.IEnumerable
     {
+        public static readonly ReactiveUI.Validation.Collections.ValidationText Empty;
+        [System.Obsolete("Calling the constructor is deprecated, please use ValidationText.Create() overloa" +
+            "d instead.")]
         public ValidationText() { }
+        [System.Obsolete("Calling the constructor is deprecated, please use ValidationText.Create(IEnumerab" +
+            "le<ValidationText>) overload instead.")]
         public ValidationText(System.Collections.Generic.IEnumerable<ReactiveUI.Validation.Collections.ValidationText> validationTexts) { }
+        [System.Obsolete("Calling the constructor is deprecated, please use ValidationText.Create(string) o" +
+            "verload instead.")]
         public ValidationText(string text) { }
         public int Count { get; }
         public string this[int index] { get; }
+        [System.Obsolete("ValidationText will be made immutable in future versions, please do not use the A" +
+            "dd(string) method.")]
         public void Add(string text) { }
+        [System.Obsolete("ValidationText will be made immutable in future versions, please do not use the C" +
+            "lear() method.")]
         public void Clear() { }
         public System.Collections.Generic.IEnumerator<string> GetEnumerator() { }
         public string ToSingleLine(string? separator = ",") { }
+        public static ReactiveUI.Validation.Collections.ValidationText Create(System.Collections.Generic.IEnumerable<ReactiveUI.Validation.Collections.ValidationText> validationTexts) { }
+        public static ReactiveUI.Validation.Collections.ValidationText Create(System.Collections.Generic.IEnumerable<string> validationTexts) { }
+        public static ReactiveUI.Validation.Collections.ValidationText Create(params string[] validationTexts) { }
     }
 }
 namespace ReactiveUI.Validation.Comparators

src/ReactiveUI.Validation.Tests/API/ApiApprovalTests.ValidationProject.netcoreapp3.1.approved.txt

@@ -10,15 +10,29 @@ namespace ReactiveUI.Validation.Collections
 {
     public class ValidationText : System.Collections.Generic.IEnumerable<string>, System.Collections.IEnumerable
     {
+        public static readonly ReactiveUI.Validation.Collections.ValidationText Empty;
+        [System.Obsolete("Calling the constructor is deprecated, please use ValidationText.Create() overloa" +
+            "d instead.")]
         public ValidationText() { }
+        [System.Obsolete("Calling the constructor is deprecated, please use ValidationText.Create(IEnumerab" +
+            "le<ValidationText>) overload instead.")]
         public ValidationText(System.Collections.Generic.IEnumerable<ReactiveUI.Validation.Collections.ValidationText> validationTexts) { }
+        [System.Obsolete("Calling the constructor is deprecated, please use ValidationText.Create(string) o" +
+            "verload instead.")]
         public ValidationText(string text) { }
         public int Count { get; }
         public string this[int index] { get; }
+        [System.Obsolete("ValidationText will be made immutable in future versions, please do not use the A" +
+            "dd(string) method.")]
         public void Add(string text) { }
+        [System.Obsolete("ValidationText will be made immutable in future versions, please do not use the C" +
+            "lear() method.")]
         public void Clear() { }
         public System.Collections.Generic.IEnumerator<string> GetEnumerator() { }
         public string ToSingleLine(string? separator = ",") { }
+        public static ReactiveUI.Validation.Collections.ValidationText Create(System.Collections.Generic.IEnumerable<ReactiveUI.Validation.Collections.ValidationText> validationTexts) { }
+        public static ReactiveUI.Validation.Collections.ValidationText Create(System.Collections.Generic.IEnumerable<string> validationTexts) { }
+        public static ReactiveUI.Validation.Collections.ValidationText Create(params string[] validationTexts) { }
     }
 }
 namespace ReactiveUI.Validation.Comparators

src/ReactiveUI.Validation.Tests/ValidationBindingTests.cs

@@ -706,7 +706,7 @@ private class CustomValidationState : IValidationState
             public CustomValidationState(bool isValid, string message)
             {
                 IsValid = isValid;
-                Text = new ValidationText(isValid ? string.Empty : message);
+                Text = isValid ? ValidationText.Empty : ValidationText.Create(message);
             }
 
             public ValidationText Text { get; }
@@ -723,4 +723,4 @@ private class ConstFormatter : IValidationTextFormatter<string>
             public string Format(ValidationText validationText) => _text;
         }
     }
-}
+}

src/ReactiveUI.Validation/Collections/ValidationText.cs

@@ -3,8 +3,10 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.
 
+using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Linq;
 
 namespace ReactiveUI.Validation.Collections
 {
@@ -13,79 +15,124 @@ namespace ReactiveUI.Validation.Collections
     /// </summary>
     public class ValidationText : IEnumerable<string>
     {
-        private readonly List<string> _texts = new List<string>();
+        /// <summary>
+        /// The empty validation text singleton instance, contains a single empty string.
+        /// </summary>
+        public static readonly ValidationText Empty = new ValidationText(Array.Empty<string>());
+
+        // TODO When Add() & Clear() are obsolesced this should be made read-only.
+        private /* readonly */ string[] _texts;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ValidationText"/> class.
         /// </summary>
+        [Obsolete("Calling the constructor is deprecated, please use ValidationText.Create() overload instead.")]
         public ValidationText()
+            : this(Array.Empty<string>())
         {
         }
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ValidationText"/> class.
         /// </summary>
         /// <param name="text">Text to be added in the collection.</param>
+        [Obsolete("Calling the constructor is deprecated, please use ValidationText.Create(string) overload instead.")]
         public ValidationText(string text)
+        : this(new[] { text })
         {
-            _texts.Add(text);
         }
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ValidationText"/> class.
         /// </summary>
         /// <param name="validationTexts"><see cref="ValidationText"/> collection to be added into the text collection.</param>
+        [Obsolete("Calling the constructor is deprecated, please use ValidationText.Create(IEnumerable<ValidationText>) overload instead.")]
         public ValidationText(IEnumerable<ValidationText> validationTexts)
+            : this((validationTexts ?? throw new ArgumentNullException(nameof(validationTexts))).SelectMany(vt => vt._texts).ToArray())
         {
-            if (validationTexts is null)
-            {
-                throw new System.ArgumentNullException(nameof(validationTexts));
-            }
+        }
 
-            foreach (var text in validationTexts)
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ValidationText"/> class with the array of texts.
+        /// </summary>
+        /// <param name="texts">The array of texts.</param>
+        private ValidationText(string[] texts)
+        {
+            if (texts is null)
             {
-                _texts.AddRange(text._texts);
+                throw new ArgumentNullException(nameof(texts));
             }
+
+            // TODO Can remove this check when public constructors are obsolesced as Create method already checks this.
+            _texts = texts.Length < 1
+                ? Array.Empty<string>()
+                : texts;
         }
 
         /// <summary>
         /// Gets returns the number of elements in the collection.
         /// </summary>
-        public int Count => _texts.Count;
+        public int Count => _texts.Length;
 
         /// <summary>
         /// Indexer.
         /// </summary>
         /// <param name="index">Position.</param>
         public string this[int index] => _texts[index];
 
+        /// <summary>
+        /// Combines multiple <see cref="ValidationText"/> instances into a single instance, or returns <see cref="Empty"/> if the enumeration is empty, or contains a single empty element.
+        /// </summary>
+        /// <param name="validationTexts">An enumeration of <see cref="ValidationText"/>.</param>
+        /// <returns>A <see cref="ValidationText"/>.</returns>
+        public static ValidationText Create(IEnumerable<ValidationText> validationTexts) =>
+            Create(validationTexts.SelectMany(vt => vt._texts).ToArray());
+
+        /// <summary>
+        /// Combines multiple validation messages into a single instance, or returns <see cref="Empty"/> if the enumeration is empty, or contains a single empty element.
+        /// </summary>
+        /// <param name="validationTexts">An enumeration of validation messages.</param>
+        /// <returns>A <see cref="ValidationText"/>.</returns>
+        public static ValidationText Create(IEnumerable<string> validationTexts) => Create(validationTexts.ToArray());
+
+        /// <summary>
+        /// Combines multiple validation messages into a single instance, or returns <see cref="Empty"/> if the enumeration is empty, or contains a single empty element.
+        /// </summary>
+        /// <param name="validationTexts">An array of validation messages.</param>
+        /// <returns>A <see cref="ValidationText"/>.</returns>
+        public static ValidationText Create(params string[] validationTexts) => validationTexts.Length < 1
+                ? Empty
+                : new ValidationText(validationTexts);
+
         /// <inheritdoc/>
         public IEnumerator<string> GetEnumerator()
         {
-            return _texts.GetEnumerator();
+            return ((IEnumerable<string>)_texts).GetEnumerator();
         }
 
         /// <inheritdoc/>
         IEnumerator IEnumerable.GetEnumerator()
         {
-            return GetEnumerator();
+            return _texts.GetEnumerator();
         }
 
         /// <summary>
         /// Adds a text to the collection.
         /// </summary>
         /// <param name="text">Text to be added in the collection.</param>
+        [Obsolete("ValidationText will be made immutable in future versions, please do not use the Add(string) method.")]
         public void Add(string text)
         {
-            _texts.Add(text);
+            _texts = _texts.Concat(new[] { text }).ToArray();
         }
 
         /// <summary>
         /// Clear all texts.
         /// </summary>
+        [Obsolete("ValidationText will be made immutable in future versions, please do not use the Clear() method.")]
         public void Clear()
         {
-            _texts.Clear();
+            _texts = Array.Empty<string>();
         }
 
         /// <summary>

src/ReactiveUI.Validation/Components/BasePropertyValidation.cs

@@ -204,9 +204,7 @@ public BasePropertyValidation(
             Expression<Func<TViewModel, TViewModelProperty>> viewModelProperty,
             Func<TViewModelProperty, bool> isValidFunc,
             string message)
-            : this(viewModel, viewModelProperty, isValidFunc, (p, v) => new ValidationText(v
-                ? string.Empty
-                : message))
+            : this(viewModel, viewModelProperty, isValidFunc, (p, v) => v ? ValidationText.Empty : ValidationText.Create(message))
         {
         }
 
@@ -222,9 +220,8 @@ public BasePropertyValidation(
             Expression<Func<TViewModel, TViewModelProperty>> viewModelProperty,
             Func<TViewModelProperty, bool> isValidFunc,
             Func<TViewModelProperty, string> message)
-            : this(viewModel, viewModelProperty, isValidFunc, (p, v) => new ValidationText(v
-                ? string.Empty
-                : message(p)))
+            : this(viewModel, viewModelProperty, isValidFunc, (p, v) =>
+                v ? ValidationText.Empty : ValidationText.Create(message(p)))
         {
         }
 
@@ -241,7 +238,7 @@ public BasePropertyValidation(
             Func<TViewModelProperty, bool> isValidFunc,
             Func<TViewModelProperty, bool, string> messageFunc)
             : this(viewModel, viewModelProperty, isValidFunc, (prop1, isValid) =>
-                new ValidationText(messageFunc(prop1, isValid)))
+                ValidationText.Create(messageFunc(prop1, isValid)))
         {
         }
 

src/ReactiveUI.Validation/Components/ModelObservableValidation.cs

@@ -47,7 +47,7 @@ public ModelObservableValidation(
             Func<TViewModel, IObservable<bool>> validityObservable,
             string message)
             : this(viewModel, viewModelProperty, validityObservable, (p, isValid) =>
-                new ValidationText(isValid ? string.Empty : message))
+                isValid ? ValidationText.Empty : ValidationText.Create(message))
         {
         }
 
@@ -64,7 +64,7 @@ public ModelObservableValidation(
             Func<TViewModel, IObservable<bool>> validityObservable,
             Func<TViewModel, string> message)
             : this(viewModel, viewModelProperty, validityObservable, (p, isValid) =>
-                new ValidationText(isValid ? string.Empty : message(p)))
+                isValid ? ValidationText.Empty : ValidationText.Create(message(p)))
         {
         }
 
@@ -81,7 +81,7 @@ public ModelObservableValidation(
             Func<TViewModel, IObservable<bool>> validityObservable,
             Func<TViewModel, bool, string> messageFunc)
             : this(viewModel, viewModelProperty, validityObservable, (vm, state) =>
-                new ValidationText(messageFunc(vm, state)))
+                ValidationText.Create(messageFunc(vm, state)))
         {
         }
 
@@ -129,7 +129,7 @@ public ModelObservableValidation(
             Func<TViewModel, IObservable<bool>> validityObservable,
             string message)
             : this(viewModel, validityObservable, (p, isValid) =>
-                new ValidationText(isValid ? string.Empty : message))
+                isValid ? ValidationText.Empty : ValidationText.Create(message))
         {
         }
 
@@ -144,7 +144,7 @@ public ModelObservableValidation(
             Func<TViewModel, IObservable<bool>> validityObservable,
             Func<TViewModel, string> message)
             : this(viewModel, validityObservable, (p, isValid) =>
-                new ValidationText(isValid ? string.Empty : message(p)))
+                isValid ? ValidationText.Empty : ValidationText.Create(message(p)))
         {
         }
 
@@ -158,7 +158,7 @@ public ModelObservableValidation(
             TViewModel viewModel,
             Func<TViewModel, IObservable<bool>> validityObservable,
             Func<TViewModel, bool, string> messageFunc)
-            : this(viewModel, validityObservable, (vm, state) => new ValidationText(messageFunc(vm, state)))
+            : this(viewModel, validityObservable, (vm, state) => ValidationText.Create(messageFunc(vm, state)))
         {
         }
 

src/ReactiveUI.Validation/Components/ObservableValidation.cs

@@ -116,7 +116,7 @@ public ObservableValidation(
             Func<TViewModel, TValue, bool> isValidFunc,
             Func<TViewModel, TValue, bool, string> messageFunc)
             : base(viewModel, observable, isValidFunc, (vm, value, isValid) =>
-                new ValidationText(messageFunc(vm, value, isValid))) =>
+                ValidationText.Create(messageFunc(vm, value, isValid))) =>
             AddProperty(viewModelProperty);
 
         /// <summary>
@@ -217,7 +217,7 @@ public ObservableValidation(
             Func<TViewModel, TValue, bool> isValidFunc,
             Func<TViewModel, TValue, bool, string> messageFunc)
             : base(viewModel, observable, isValidFunc, (vm, value, isValid) =>
-                new ValidationText(messageFunc(vm, value, isValid)))
+                ValidationText.Create(messageFunc(vm, value, isValid)))
         {
         }
 

src/ReactiveUI.Validation/Contexts/ValidationContext.cs

@@ -82,7 +82,7 @@ public ValidationContext(IScheduler? scheduler = null)
             _validationText = _validSubject
                 .StartWith(true)
                 .Select(_ => BuildText())
-                .ToProperty(this, m => m.Text, new ValidationText(), scheduler: scheduler)
+                .ToProperty(this, m => m.Text, ValidationText.Empty, scheduler: scheduler)
                 .DisposeWith(_disposables);
 
             _validSubject
@@ -219,7 +219,7 @@ private void Activate()
         /// Returns the <see cref="ValidationText"/> with all the error messages from the non valid components.
         /// </returns>
         private ValidationText BuildText() =>
-            new ValidationText(_validations
+            ValidationText.Create(_validations
                 .Where(p => !p.IsValid && p.Text != null)
                 .Select(p => p.Text!));
     }

src/ReactiveUI.Validation/Helpers/ReactiveValidationObject.cs

@@ -14,6 +14,7 @@
 using DynamicData;
 using DynamicData.Binding;
 using ReactiveUI.Validation.Abstractions;
+using ReactiveUI.Validation.Collections;
 using ReactiveUI.Validation.Components.Abstractions;
 using ReactiveUI.Validation.Contexts;
 using ReactiveUI.Validation.States;
@@ -90,11 +91,11 @@ public bool HasErrors
         public virtual IEnumerable GetErrors(string propertyName) =>
             string.IsNullOrEmpty(propertyName) ?
                 SelectInvalidPropertyValidations()
-                    .SelectMany(validation => validation.Text)
+                    .SelectMany(validation => validation.Text ?? ValidationText.Empty)
                     .ToArray() :
                 SelectInvalidPropertyValidations()
                     .Where(validation => validation.ContainsPropertyName(propertyName))
-                    .SelectMany(validation => validation.Text)
+                    .SelectMany(validation => validation.Text ?? ValidationText.Empty)
                     .ToArray();
 
         /// <summary>

src/ReactiveUI.Validation/ReactiveUI.Validation.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <TargetFrameworks>MonoAndroid90;Xamarin.iOS10;Xamarin.Mac20;Xamarin.TVOS10;tizen40;netstandard2.0</TargetFrameworks>
-    <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">$(TargetFrameworks);net461;uap10.0.17763</TargetFrameworks>
+    <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">$(TargetFrameworks);net461;</TargetFrameworks>
     <NoWarn>$(NoWarn);CS1591</NoWarn>
     <Nullable>enable</Nullable>
     <LangVersion>latest</LangVersion>

src/ReactiveUI.Validation/States/ValidationState.cs

@@ -26,7 +26,7 @@ public class ValidationState : IValidationState
         /// <param name="isValid">Determines if the property is valid or not.</param>
         /// <param name="text">Validation text.</param>
         public ValidationState(bool isValid, string text)
-            : this(isValid, new ValidationText(text))
+            : this(isValid, ValidationText.Create(text))
         {
         }
 
@@ -50,7 +50,7 @@ public ValidationState(bool isValid, ValidationText text)
         [ExcludeFromCodeCoverage]
         [Obsolete("This constructor overload is going to be removed soon.")]
         public ValidationState(bool isValid, string text, IValidationComponent component)
-            : this(isValid, new ValidationText(text), component)
+            : this(isValid, ValidationText.Create(text), component)
         {
         }