Added more nullability annotations to helper methods

Microsoft.Toolkit.Mvvm/Attributes/NotNullAttribute.cs

@@ -0,0 +1,20 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+#if NETSTANDARD2_0
+
+namespace System.Diagnostics.CodeAnalysis
+{
+    /// <summary>
+    /// Specifies that an output will not be null even if the corresponding type allows it.
+    /// Specifies that an input argument was not null when the call returns.
+    /// </summary>
+    /// <remarks>Internal copy from the BCL attribute.</remarks>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)]
+    internal sealed class NotNullAttribute : Attribute
+    {
+    }
+}
+
+#endif

Microsoft.Toolkit.Mvvm/Attributes/NotNullIfNotNullAttribute.cs

@@ -0,0 +1,27 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+#if NETSTANDARD2_0
+
+namespace System.Diagnostics.CodeAnalysis
+{
+    /// <summary>
+    /// Specifies that the output will be non-null if the named parameter is non-null.
+    /// </summary>
+    /// <remarks>Internal copy from the BCL attribute.</remarks>
+    [AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, AllowMultiple = true, Inherited = false)]
+    internal sealed class NotNullIfNotNullAttribute : Attribute
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="NotNullIfNotNullAttribute"/> class.
+        /// </summary>
+        /// <param name="parameterName"> The associated parameter name. The output will be non-null if the argument to the parameter specified is non-null.</param>
+        public NotNullIfNotNullAttribute(string parameterName) => ParameterName = parameterName;
+
+        /// <summary>Gets the associated parameter name.</summary>
+        public string ParameterName { get; }
+    }
+}
+
+#endif

Microsoft.Toolkit.Mvvm/ComponentModel/ObservableObject.cs

@@ -18,6 +18,7 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
 using System.Threading.Tasks;
 
@@ -84,7 +85,7 @@ namespace Microsoft.Toolkit.Mvvm.ComponentModel
         /// The <see cref="PropertyChanging"/> and <see cref="PropertyChanged"/> events are not raised
         /// if the current and new value for the target property are the same.
         /// </remarks>
-        protected bool SetProperty<T>(ref T field, T newValue, [CallerMemberName] string? propertyName = null)
+        protected bool SetProperty<T>([NotNullIfNotNull("newValue")] ref T field, T newValue, [CallerMemberName] string? propertyName = null)
         {
             // We duplicate the code here instead of calling the overload because we can't
             // guarantee that the invoked SetProperty<T> will be inlined, and we need the JIT
@@ -120,7 +121,7 @@ namespace Microsoft.Toolkit.Mvvm.ComponentModel
         /// <param name="comparer">The <see cref="IEqualityComparer{T}"/> instance to use to compare the input values.</param>
         /// <param name="propertyName">(optional) The name of the property that changed.</param>
         /// <returns><see langword="true"/> if the property was changed, <see langword="false"/> otherwise.</returns>
-        protected bool SetProperty<T>(ref T field, T newValue, IEqualityComparer<T> comparer, [CallerMemberName] string? propertyName = null)
+        protected bool SetProperty<T>([NotNullIfNotNull("newValue")] ref T field, T newValue, IEqualityComparer<T> comparer, [CallerMemberName] string? propertyName = null)
         {
             if (comparer.Equals(field, newValue))
             {
@@ -340,7 +341,7 @@ namespace Microsoft.Toolkit.Mvvm.ComponentModel
         /// indicates that the new value being assigned to <paramref name="taskNotifier"/> is different than the previous one,
         /// and it does not mean the new <see cref="Task"/> instance passed as argument is in any particular state.
         /// </remarks>
-        protected bool SetPropertyAndNotifyOnCompletion(ref TaskNotifier? taskNotifier, Task? newValue, [CallerMemberName] string? propertyName = null)
+        protected bool SetPropertyAndNotifyOnCompletion([NotNull] ref TaskNotifier? taskNotifier, Task? newValue, [CallerMemberName] string? propertyName = null)
         {
             // We invoke the overload with a callback here to avoid code duplication, and simply pass an empty callback.
             // The lambda expression here is transformed by the C# compiler into an empty closure class with a
@@ -368,7 +369,7 @@ namespace Microsoft.Toolkit.Mvvm.ComponentModel
         /// The <see cref="PropertyChanging"/> and <see cref="PropertyChanged"/> events are not raised
         /// if the current and new value for the target property are the same.
         /// </remarks>
-        protected bool SetPropertyAndNotifyOnCompletion(ref TaskNotifier? taskNotifier, Task? newValue, Action<Task?> callback, [CallerMemberName] string? propertyName = null)
+        protected bool SetPropertyAndNotifyOnCompletion([NotNull] ref TaskNotifier? taskNotifier, Task? newValue, Action<Task?> callback, [CallerMemberName] string? propertyName = null)
         {
             return SetPropertyAndNotifyOnCompletion(taskNotifier ??= new(), newValue, callback, propertyName);
         }
@@ -407,7 +408,7 @@ namespace Microsoft.Toolkit.Mvvm.ComponentModel
         /// indicates that the new value being assigned to <paramref name="taskNotifier"/> is different than the previous one,
         /// and it does not mean the new <see cref="Task{TResult}"/> instance passed as argument is in any particular state.
         /// </remarks>
-        protected bool SetPropertyAndNotifyOnCompletion<T>(ref TaskNotifier<T>? taskNotifier, Task<T>? newValue, [CallerMemberName] string? propertyName = null)
+        protected bool SetPropertyAndNotifyOnCompletion<T>([NotNull] ref TaskNotifier<T>? taskNotifier, Task<T>? newValue, [CallerMemberName] string? propertyName = null)
         {
             return SetPropertyAndNotifyOnCompletion(taskNotifier ??= new(), newValue, static _ => { }, propertyName);
         }
@@ -430,7 +431,7 @@ namespace Microsoft.Toolkit.Mvvm.ComponentModel
         /// The <see cref="PropertyChanging"/> and <see cref="PropertyChanged"/> events are not raised
         /// if the current and new value for the target property are the same.
         /// </remarks>
-        protected bool SetPropertyAndNotifyOnCompletion<T>(ref TaskNotifier<T>? taskNotifier, Task<T>? newValue, Action<Task<T>?> callback, [CallerMemberName] string? propertyName = null)
+        protected bool SetPropertyAndNotifyOnCompletion<T>([NotNull] ref TaskNotifier<T>? taskNotifier, Task<T>? newValue, Action<Task<T>?> callback, [CallerMemberName] string? propertyName = null)
         {
             return SetPropertyAndNotifyOnCompletion(taskNotifier ??= new(), newValue, callback, propertyName);
         }

Microsoft.Toolkit.Mvvm/ComponentModel/ObservableRecipient.cs

@@ -16,6 +16,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
 using Microsoft.Toolkit.Mvvm.Messaging;
 using Microsoft.Toolkit.Mvvm.Messaging.Messages;
@@ -145,7 +146,7 @@ namespace Microsoft.Toolkit.Mvvm.ComponentModel
         /// the <see cref="ObservableObject.PropertyChanging"/> and <see cref="ObservableObject.PropertyChanged"/> events
         /// are not raised if the current and new value for the target property are the same.
         /// </remarks>
-        protected bool SetProperty<T>(ref T field, T newValue, bool broadcast, [CallerMemberName] string? propertyName = null)
+        protected bool SetProperty<T>([NotNullIfNotNull("newValue")] ref T field, T newValue, bool broadcast, [CallerMemberName] string? propertyName = null)
         {
             T oldValue = field;
 
@@ -174,7 +175,7 @@ namespace Microsoft.Toolkit.Mvvm.ComponentModel
         /// <param name="broadcast">If <see langword="true"/>, <see cref="Broadcast{T}"/> will also be invoked.</param>
         /// <param name="propertyName">(optional) The name of the property that changed.</param>
         /// <returns><see langword="true"/> if the property was changed, <see langword="false"/> otherwise.</returns>
-        protected bool SetProperty<T>(ref T field, T newValue, IEqualityComparer<T> comparer, bool broadcast, [CallerMemberName] string? propertyName = null)
+        protected bool SetProperty<T>([NotNullIfNotNull("newValue")] ref T field, T newValue, IEqualityComparer<T> comparer, bool broadcast, [CallerMemberName] string? propertyName = null)
         {
             T oldValue = field;
 

Microsoft.Toolkit.Mvvm/ComponentModel/ObservableValidator.cs

@@ -7,6 +7,7 @@ using System.Collections;
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.ComponentModel.DataAnnotations;
+using System.Diagnostics.CodeAnalysis;
 using System.Diagnostics.Contracts;
 using System.Linq;
 using System.Linq.Expressions;
@@ -137,7 +138,7 @@ namespace Microsoft.Toolkit.Mvvm.ComponentModel
         /// the <see cref="ObservableObject.PropertyChanging"/> and <see cref="ObservableObject.PropertyChanged"/> events
         /// are not raised if the current and new value for the target property are the same.
         /// </remarks>
-        protected bool SetProperty<T>(ref T field, T newValue, bool validate, [CallerMemberName] string? propertyName = null)
+        protected bool SetProperty<T>([NotNullIfNotNull("newValue")] ref T field, T newValue, bool validate, [CallerMemberName] string? propertyName = null)
         {
             bool propertyChanged = SetProperty(ref field, newValue, propertyName);
 
@@ -162,7 +163,7 @@ namespace Microsoft.Toolkit.Mvvm.ComponentModel
         /// <param name="validate">If <see langword="true"/>, <paramref name="newValue"/> will also be validated.</param>
         /// <param name="propertyName">(optional) The name of the property that changed.</param>
         /// <returns><see langword="true"/> if the property was changed, <see langword="false"/> otherwise.</returns>
-        protected bool SetProperty<T>(ref T field, T newValue, IEqualityComparer<T> comparer, bool validate, [CallerMemberName] string? propertyName = null)
+        protected bool SetProperty<T>([NotNullIfNotNull("newValue")] ref T field, T newValue, IEqualityComparer<T> comparer, bool validate, [CallerMemberName] string? propertyName = null)
         {
             bool propertyChanged = SetProperty(ref field, newValue, comparer, propertyName);