[Feature] Microsoft.Toolkit.Mvvm package (Preview 5) (#3562)

## Follow up to #3428 
<!-- Add the relevant issue number after the "#" mentioned above (for ex: Fixes #1234) which will automatically close the issue once the PR is merged. -->
This PR is for tracking all changes/fixes/improvements to the `Microsoft.Toolkit.Mvvm` package following the Preview 4.

<!-- Add a brief overview here of the feature/bug & fix. -->

## PR Type
What kind of change does this PR introduce?
<!-- Please uncomment one or more that apply to this PR. -->

- Feature
- Improvements
<!-- - Code style update (formatting) -->
<!-- - Refactoring (no functional changes, no api changes) -->
<!-- - Build or CI related changes -->
<!-- - Documentation content changes -->
<!-- - Sample app changes -->
<!-- - Other... Please describe: -->

## Overview

This PR is used to track and implement new features and tweaks for the `Microsoft.Toolkit.Mvvm` package.
See the linked issue for more info, and for a full list of changes included in this PR.


## PR Checklist

Please check if your PR fulfills the following requirements:

- [X] Tested code with current [supported SDKs](../readme.md#supported)
- [ ] ~~Pull Request has been submitted to the documentation repository [instructions](..\contributing.md#docs). Link: <!-- docs PR link -->~~
- [ ] ~~Sample in sample app has been added / updated (for bug fixes / features)~~
    - [ ] ~~Icon has been created (if new sample) following the [Thumbnail Style Guide and templates](https://github.com/windows-toolkit/WindowsCommunityToolkit-design-assets)~~
- [X] Tests for the changes have been added (for bug fixes / features) (if applicable)
- [X] Header has been added to all new source files (run *build/UpdateHeaders.bat*)
- [X] Contains **NO** breaking changes

Microsoft.Toolkit.Mvvm/ComponentModel/ObservableObject.cs

@@ -340,7 +340,7 @@ protected bool SetPropertyAndNotifyOnCompletion(ref TaskNotifier? taskNotifier,
             // instance. This will result in no further allocations after the first time this method is called for a given
             // generic type. We only pay the cost of the virtual call to the delegate, but this is not performance critical
             // code and that overhead would still be much lower than the rest of the method anyway, so that's fine.
-            return SetPropertyAndNotifyOnCompletion(taskNotifier ??= new TaskNotifier(), newValue, _ => { }, propertyName);
+            return SetPropertyAndNotifyOnCompletion(taskNotifier ??= new(), newValue, static _ => { }, propertyName);
         }
 
         /// <summary>
@@ -362,7 +362,7 @@ protected bool SetPropertyAndNotifyOnCompletion(ref TaskNotifier? taskNotifier,
         /// </remarks>
         protected bool SetPropertyAndNotifyOnCompletion(ref TaskNotifier? taskNotifier, Task? newValue, Action<Task?> callback, [CallerMemberName] string? propertyName = null)
         {
-            return SetPropertyAndNotifyOnCompletion(taskNotifier ??= new TaskNotifier(), newValue, callback, propertyName);
+            return SetPropertyAndNotifyOnCompletion(taskNotifier ??= new(), newValue, callback, propertyName);
         }
 
         /// <summary>
@@ -401,7 +401,7 @@ protected bool SetPropertyAndNotifyOnCompletion(ref TaskNotifier? taskNotifier,
         /// </remarks>
         protected bool SetPropertyAndNotifyOnCompletion<T>(ref TaskNotifier<T>? taskNotifier, Task<T>? newValue, [CallerMemberName] string? propertyName = null)
         {
-            return SetPropertyAndNotifyOnCompletion(taskNotifier ??= new TaskNotifier<T>(), newValue, _ => { }, propertyName);
+            return SetPropertyAndNotifyOnCompletion(taskNotifier ??= new(), newValue, static _ => { }, propertyName);
         }
 
         /// <summary>
@@ -424,7 +424,7 @@ protected bool SetPropertyAndNotifyOnCompletion<T>(ref TaskNotifier<T>? taskNoti
         /// </remarks>
         protected bool SetPropertyAndNotifyOnCompletion<T>(ref TaskNotifier<T>? taskNotifier, Task<T>? newValue, Action<Task<T>?> callback, [CallerMemberName] string? propertyName = null)
         {
-            return SetPropertyAndNotifyOnCompletion(taskNotifier ??= new TaskNotifier<T>(), newValue, callback, propertyName);
+            return SetPropertyAndNotifyOnCompletion(taskNotifier ??= new(), newValue, callback, propertyName);
         }
 
         /// <summary>

Microsoft.Toolkit.Mvvm/ComponentModel/ObservableRecipient.cs

@@ -116,7 +116,7 @@ protected virtual void OnDeactivated()
         /// </remarks>
         protected virtual void Broadcast<T>(T oldValue, T newValue, string? propertyName)
         {
-            var message = new PropertyChangedMessage<T>(this, propertyName, oldValue, newValue);
+            PropertyChangedMessage<T> message = new(this, propertyName, oldValue, newValue);
 
             Messenger.Send(message);
         }

Microsoft.Toolkit.Mvvm/DependencyInjection/Ioc.cs

@@ -47,7 +47,7 @@ public sealed class Ioc : IServiceProvider
         /// <summary>
         /// Gets the default <see cref="Ioc"/> instance.
         /// </summary>
-        public static Ioc Default { get; } = new Ioc();
+        public static Ioc Default { get; } = new();
 
         /// <summary>
         /// The <see cref="IServiceProvider"/> instance to use, if initialized.
@@ -134,7 +134,7 @@ public void ConfigureServices(IServiceProvider serviceProvider)
         {
             IServiceProvider? oldServices = Interlocked.CompareExchange(ref this.serviceProvider, serviceProvider, null);
 
-            if (!(oldServices is null))
+            if (oldServices is not null)
             {
                 ThrowInvalidOperationExceptionForRepeatedConfiguration();
             }

Microsoft.Toolkit.Mvvm/Input/AsyncRelayCommand.cs

@@ -22,17 +22,17 @@ public sealed class AsyncRelayCommand : ObservableObject, IAsyncRelayCommand
         /// <summary>
         /// The cached <see cref="PropertyChangedEventArgs"/> for <see cref="CanBeCanceled"/>.
         /// </summary>
-        internal static readonly PropertyChangedEventArgs CanBeCanceledChangedEventArgs = new PropertyChangedEventArgs(nameof(CanBeCanceled));
+        internal static readonly PropertyChangedEventArgs CanBeCanceledChangedEventArgs = new(nameof(CanBeCanceled));
 
         /// <summary>
         /// The cached <see cref="PropertyChangedEventArgs"/> for <see cref="IsCancellationRequested"/>.
         /// </summary>
-        internal static readonly PropertyChangedEventArgs IsCancellationRequestedChangedEventArgs = new PropertyChangedEventArgs(nameof(IsCancellationRequested));
+        internal static readonly PropertyChangedEventArgs IsCancellationRequestedChangedEventArgs = new(nameof(IsCancellationRequested));
 
         /// <summary>
         /// The cached <see cref="PropertyChangedEventArgs"/> for <see cref="IsRunning"/>.
         /// </summary>
-        internal static readonly PropertyChangedEventArgs IsRunningChangedEventArgs = new PropertyChangedEventArgs(nameof(IsRunning));
+        internal static readonly PropertyChangedEventArgs IsRunningChangedEventArgs = new(nameof(IsRunning));
 
         /// <summary>
         /// The <see cref="Func{TResult}"/> to invoke when <see cref="Execute"/> is used.
@@ -122,7 +122,7 @@ private set
         }
 
         /// <inheritdoc/>
-        public bool CanBeCanceled => !(this.cancelableExecute is null) && IsRunning;
+        public bool CanBeCanceled => this.cancelableExecute is not null && IsRunning;
 
         /// <inheritdoc/>
         public bool IsCancellationRequested => this.cancellationTokenSource?.IsCancellationRequested == true;
@@ -155,15 +155,15 @@ public Task ExecuteAsync(object? parameter)
             if (CanExecute(parameter))
             {
                 // Non cancelable command delegate
-                if (!(this.execute is null))
+                if (this.execute is not null)
                 {
                     return ExecutionTask = this.execute();
                 }
 
                 // Cancel the previous operation, if one is pending
                 this.cancellationTokenSource?.Cancel();
 
-                var cancellationTokenSource = this.cancellationTokenSource = new CancellationTokenSource();
+                CancellationTokenSource cancellationTokenSource = this.cancellationTokenSource = new();
 
                 OnPropertyChanged(IsCancellationRequestedChangedEventArgs);
 

Microsoft.Toolkit.Mvvm/Input/AsyncRelayCommand{T}.cs

@@ -19,17 +19,17 @@ public sealed class AsyncRelayCommand<T> : ObservableObject, IAsyncRelayCommand<
         /// <summary>
         /// The <see cref="Func{TResult}"/> to invoke when <see cref="Execute(T)"/> is used.
         /// </summary>
-        private readonly Func<T, Task>? execute;
+        private readonly Func<T?, Task>? execute;
 
         /// <summary>
         /// The cancelable <see cref="Func{T1,T2,TResult}"/> to invoke when <see cref="Execute(object?)"/> is used.
         /// </summary>
-        private readonly Func<T, CancellationToken, Task>? cancelableExecute;
+        private readonly Func<T?, CancellationToken, Task>? cancelableExecute;
 
         /// <summary>
         /// The optional action to invoke when <see cref="CanExecute(T)"/> is used.
         /// </summary>
-        private readonly Func<T, bool>? canExecute;
+        private readonly Predicate<T?>? canExecute;
 
         /// <summary>
         /// The <see cref="CancellationTokenSource"/> instance to use to cancel <see cref="cancelableExecute"/>.
@@ -44,7 +44,7 @@ public sealed class AsyncRelayCommand<T> : ObservableObject, IAsyncRelayCommand<
         /// </summary>
         /// <param name="execute">The execution logic.</param>
         /// <remarks>See notes in <see cref="RelayCommand{T}(Action{T})"/>.</remarks>
-        public AsyncRelayCommand(Func<T, Task> execute)
+        public AsyncRelayCommand(Func<T?, Task> execute)
         {
             this.execute = execute;
         }
@@ -54,7 +54,7 @@ public AsyncRelayCommand(Func<T, Task> execute)
         /// </summary>
         /// <param name="cancelableExecute">The cancelable execution logic.</param>
         /// <remarks>See notes in <see cref="RelayCommand{T}(Action{T})"/>.</remarks>
-        public AsyncRelayCommand(Func<T, CancellationToken, Task> cancelableExecute)
+        public AsyncRelayCommand(Func<T?, CancellationToken, Task> cancelableExecute)
         {
             this.cancelableExecute = cancelableExecute;
         }
@@ -65,7 +65,7 @@ public AsyncRelayCommand(Func<T, CancellationToken, Task> cancelableExecute)
         /// <param name="execute">The execution logic.</param>
         /// <param name="canExecute">The execution status logic.</param>
         /// <remarks>See notes in <see cref="RelayCommand{T}(Action{T})"/>.</remarks>
-        public AsyncRelayCommand(Func<T, Task> execute, Func<T, bool> canExecute)
+        public AsyncRelayCommand(Func<T?, Task> execute, Predicate<T?> canExecute)
         {
             this.execute = execute;
             this.canExecute = canExecute;
@@ -77,7 +77,7 @@ public AsyncRelayCommand(Func<T, Task> execute, Func<T, bool> canExecute)
         /// <param name="cancelableExecute">The cancelable execution logic.</param>
         /// <param name="canExecute">The execution status logic.</param>
         /// <remarks>See notes in <see cref="RelayCommand{T}(Action{T})"/>.</remarks>
-        public AsyncRelayCommand(Func<T, CancellationToken, Task> cancelableExecute, Func<T, bool> canExecute)
+        public AsyncRelayCommand(Func<T?, CancellationToken, Task> cancelableExecute, Predicate<T?> canExecute)
         {
             this.cancelableExecute = cancelableExecute;
             this.canExecute = canExecute;
@@ -106,7 +106,7 @@ private set
         }
 
         /// <inheritdoc/>
-        public bool CanBeCanceled => !(this.cancelableExecute is null) && IsRunning;
+        public bool CanBeCanceled => this.cancelableExecute is not null && IsRunning;
 
         /// <inheritdoc/>
         public bool IsCancellationRequested => this.cancellationTokenSource?.IsCancellationRequested == true;
@@ -122,7 +122,7 @@ public void NotifyCanExecuteChanged()
 
         /// <inheritdoc/>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public bool CanExecute(T parameter)
+        public bool CanExecute(T? parameter)
         {
             return this.canExecute?.Invoke(parameter) != false;
         }
@@ -131,44 +131,43 @@ public bool CanExecute(T parameter)
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public bool CanExecute(object? parameter)
         {
-            if (typeof(T).IsValueType &&
-                parameter is null &&
-                this.canExecute is null)
+            if (default(T) is not null &&
+                parameter is null)
             {
-                return true;
+                return false;
             }
 
-            return CanExecute((T)parameter!);
+            return CanExecute((T?)parameter);
         }
 
         /// <inheritdoc/>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public void Execute(T parameter)
+        public void Execute(T? parameter)
         {
             ExecuteAsync(parameter);
         }
 
         /// <inheritdoc/>
         public void Execute(object? parameter)
         {
-            ExecuteAsync((T)parameter!);
+            ExecuteAsync((T?)parameter);
         }
 
         /// <inheritdoc/>
-        public Task ExecuteAsync(T parameter)
+        public Task ExecuteAsync(T? parameter)
         {
             if (CanExecute(parameter))
             {
                 // Non cancelable command delegate
-                if (!(this.execute is null))
+                if (this.execute is not null)
                 {
                     return ExecutionTask = this.execute(parameter);
                 }
 
                 // Cancel the previous operation, if one is pending
                 this.cancellationTokenSource?.Cancel();
 
-                var cancellationTokenSource = this.cancellationTokenSource = new CancellationTokenSource();
+                CancellationTokenSource cancellationTokenSource = this.cancellationTokenSource = new();
 
                 OnPropertyChanged(AsyncRelayCommand.IsCancellationRequestedChangedEventArgs);
 
@@ -182,7 +181,7 @@ public Task ExecuteAsync(T parameter)
         /// <inheritdoc/>
         public Task ExecuteAsync(object? parameter)
         {
-            return ExecuteAsync((T)parameter!);
+            return ExecuteAsync((T?)parameter);
         }
 
         /// <inheritdoc/>

Microsoft.Toolkit.Mvvm/Input/Interfaces/IAsyncRelayCommand{T}.cs

@@ -18,6 +18,6 @@ public interface IAsyncRelayCommand<in T> : IAsyncRelayCommand, IRelayCommand<T>
         /// </summary>
         /// <param name="parameter">The input parameter.</param>
         /// <returns>The <see cref="Task"/> representing the async operation being executed.</returns>
-        Task ExecuteAsync(T parameter);
+        Task ExecuteAsync(T? parameter);
     }
 }

Microsoft.Toolkit.Mvvm/Input/Interfaces/IRelayCommand{T}.cs

@@ -18,13 +18,13 @@ public interface IRelayCommand<in T> : IRelayCommand
         /// <param name="parameter">The input parameter.</param>
         /// <returns>Whether or not the current command can be executed.</returns>
         /// <remarks>Use this overload to avoid boxing, if <typeparamref name="T"/> is a value type.</remarks>
-        bool CanExecute(T parameter);
+        bool CanExecute(T? parameter);
 
         /// <summary>
         /// Provides a strongly-typed variant of <see cref="ICommand.Execute(object)"/>.
         /// </summary>
         /// <param name="parameter">The input parameter.</param>
         /// <remarks>Use this overload to avoid boxing, if <typeparamref name="T"/> is a value type.</remarks>
-        void Execute(T parameter);
+        void Execute(T? parameter);
     }
 }

Microsoft.Toolkit.Mvvm/Input/RelayCommand{T}.cs

@@ -24,12 +24,12 @@ public sealed class RelayCommand<T> : IRelayCommand<T>
         /// <summary>
         /// The <see cref="Action"/> to invoke when <see cref="Execute(T)"/> is used.
         /// </summary>
-        private readonly Action<T> execute;
+        private readonly Action<T?> execute;
 
         /// <summary>
         /// The optional action to invoke when <see cref="CanExecute(T)"/> is used.
         /// </summary>
-        private readonly Func<T, bool>? canExecute;
+        private readonly Predicate<T?>? canExecute;
 
         /// <inheritdoc/>
         public event EventHandler? CanExecuteChanged;
@@ -43,7 +43,7 @@ public sealed class RelayCommand<T> : IRelayCommand<T>
         /// nullable <see cref="object"/> parameter, it is recommended that if <typeparamref name="T"/> is a reference type,
         /// you should always declare it as nullable, and to always perform checks within <paramref name="execute"/>.
         /// </remarks>
-        public RelayCommand(Action<T> execute)
+        public RelayCommand(Action<T?> execute)
         {
             this.execute = execute;
         }
@@ -54,7 +54,7 @@ public RelayCommand(Action<T> execute)
         /// <param name="execute">The execution logic.</param>
         /// <param name="canExecute">The execution status logic.</param>
         /// <remarks>See notes in <see cref="RelayCommand{T}(Action{T})"/>.</remarks>
-        public RelayCommand(Action<T> execute, Func<T, bool> canExecute)
+        public RelayCommand(Action<T?> execute, Predicate<T?> canExecute)
         {
             this.execute = execute;
             this.canExecute = canExecute;
@@ -68,27 +68,26 @@ public void NotifyCanExecuteChanged()
 
         /// <inheritdoc/>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public bool CanExecute(T parameter)
+        public bool CanExecute(T? parameter)
         {
             return this.canExecute?.Invoke(parameter) != false;
         }
 
         /// <inheritdoc/>
         public bool CanExecute(object? parameter)
         {
-            if (typeof(T).IsValueType &&
-                parameter is null &&
-                this.canExecute is null)
+            if (default(T) is not null &&
+                parameter is null)
             {
-                return true;
+                return false;
             }
 
-            return CanExecute((T)parameter!);
+            return CanExecute((T?)parameter);
         }
 
         /// <inheritdoc/>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public void Execute(T parameter)
+        public void Execute(T? parameter)
         {
             if (CanExecute(parameter))
             {
@@ -99,7 +98,7 @@ public void Execute(T parameter)
         /// <inheritdoc/>
         public void Execute(object? parameter)
         {
-            Execute((T)parameter!);
+            Execute((T?)parameter);
         }
     }
 }

Microsoft.Toolkit.Mvvm/Messaging/IMessengerExtensions.cs

@@ -47,8 +47,7 @@ private static class DiscoveredRecipients<TToken>
             /// <summary>
             /// The <see cref="ConditionalWeakTable{TKey,TValue}"/> instance used to track the preloaded registration actions for each recipient.
             /// </summary>
-            public static readonly ConditionalWeakTable<Type, Action<IMessenger, object, TToken>[]> RegistrationMethods
-                = new ConditionalWeakTable<Type, Action<IMessenger, object, TToken>[]>();
+            public static readonly ConditionalWeakTable<Type, Action<IMessenger, object, TToken>[]> RegistrationMethods = new();
         }
 
         /// <summary>
@@ -139,7 +138,7 @@ static Action<IMessenger, object, TToken> GetRegistrationAction(Type type, Metho
             // For more info on this, see the related issue at https://github.com/dotnet/roslyn/issues/5835.
             Action<IMessenger, object, TToken>[] registrationActions = DiscoveredRecipients<TToken>.RegistrationMethods.GetValue(
                 recipient.GetType(),
-                t => LoadRegistrationMethodsForType(t));
+                static t => LoadRegistrationMethodsForType(t));
 
             foreach (Action<IMessenger, object, TToken> registrationAction in registrationActions)
             {
@@ -158,7 +157,7 @@ static Action<IMessenger, object, TToken> GetRegistrationAction(Type type, Metho
         public static void Register<TMessage>(this IMessenger messenger, IRecipient<TMessage> recipient)
             where TMessage : class
         {
-            messenger.Register<IRecipient<TMessage>, TMessage, Unit>(recipient, default, (r, m) => r.Receive(m));
+            messenger.Register<IRecipient<TMessage>, TMessage, Unit>(recipient, default, static (r, m) => r.Receive(m));
         }
 
         /// <summary>
@@ -175,7 +174,7 @@ public static void Register<TMessage, TToken>(this IMessenger messenger, IRecipi
             where TMessage : class
             where TToken : IEquatable<TToken>
         {
-            messenger.Register<IRecipient<TMessage>, TMessage, TToken>(recipient, token, (r, m) => r.Receive(m));
+            messenger.Register<IRecipient<TMessage>, TMessage, TToken>(recipient, token, static (r, m) => r.Receive(m));
         }
 
         /// <summary>

Microsoft.Toolkit.Mvvm/Messaging/Internals/Microsoft.Collections.Extensions/DictionarySlim{TKey,TValue}.cs

@@ -334,7 +334,7 @@ private Entry[] Resize()
         /// <inheritdoc cref="IEnumerable{T}.GetEnumerator"/>
         [Pure]
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public Enumerator GetEnumerator() => new Enumerator(this);
+        public Enumerator GetEnumerator() => new(this);
 
         /// <summary>
         /// Enumerator for <see cref="DictionarySlim{TKey,TValue}"/>.