Add ReadOnly{Observable}Collection/Dictionary.Empty (#76764)

* Add ReadOnly{Observable}Collection/Dictionary.Empty

* Revert XamlLoadPermission.cs change to fix downlevel build

src/coreclr/nativeaot/System.Private.CoreLib/src/System/Reflection/Runtime/General/Helpers.cs

@@ -83,7 +83,7 @@ public static RuntimeTypeInfo CastToRuntimeTypeInfo(this Type type)
 
         public static ReadOnlyCollection<T> ToReadOnlyCollection<T>(this IEnumerable<T> enumeration)
         {
-            return new ReadOnlyCollection<T>(enumeration.ToArray());
+            return Array.AsReadOnly(enumeration.ToArray());
         }
 
         public static MethodInfo FilterAccessor(this MethodInfo accessor, bool nonPublic)

src/libraries/System.Collections.Concurrent/src/System/Collections/Concurrent/ConcurrentDictionary.cs

@@ -2068,8 +2068,7 @@ private ReadOnlyCollection<TKey> GetKeys()
 
                 if (count == 0)
                 {
-                    // TODO https://github.com/dotnet/runtime/issues/76028: Replace with ReadOnlyCollection<TKey>.Empty.
-                    return Array.Empty<TKey>().AsReadOnly();
+                    return ReadOnlyCollection<TKey>.Empty;
                 }
 
                 var keys = new TKey[count];
@@ -2110,8 +2109,7 @@ private ReadOnlyCollection<TValue> GetValues()
 
                 if (count == 0)
                 {
-                    // TODO https://github.com/dotnet/runtime/pull/76097:  Replace with ReadOnlyCollection<TValue>.Empty.
-                    return Array.Empty<TValue>().AsReadOnly();
+                    return ReadOnlyCollection<TValue>.Empty;
                 }
 
                 var values = new TValue[count];

src/libraries/System.ComponentModel.Composition/src/Microsoft/Internal/Collections/CollectionServices.cs

@@ -60,7 +60,7 @@ public static ReadOnlyCollection<T> ToReadOnlyCollection<T>(this IEnumerable<T>
         {
             ArgumentNullException.ThrowIfNull(source);
 
-            return new ReadOnlyCollection<T>(source.AsArray());
+            return Array.AsReadOnly(source.AsArray());
         }
 
         public static IEnumerable<T>? ConcatAllowingNull<T>(this IEnumerable<T>? source, IEnumerable<T>? second)

src/libraries/System.ComponentModel.Composition/src/System/ComponentModel/Composition/CompositionException.cs

@@ -92,7 +92,7 @@ internal CompositionException(string? message, Exception? innerException, IEnume
                     : base(message, innerException)
         {
             Requires.NullOrNotNullElements(errors, nameof(errors));
-            _errors = new ReadOnlyCollection<CompositionError>(errors == null ? Array.Empty<CompositionError>() : errors.ToArray<CompositionError>());
+            _errors = Array.AsReadOnly(errors == null ? Array.Empty<CompositionError>() : errors.ToArray<CompositionError>());
         }
 
         /// <summary>

src/libraries/System.ComponentModel.Composition/src/System/ComponentModel/Composition/Hosting/AggregateExportProvider.cs

@@ -62,7 +62,7 @@ public AggregateExportProvider(params ExportProvider[]? providers)
             }
 
             _providers = copiedProviders;
-            _readOnlyProviders = new ReadOnlyCollection<ExportProvider>(_providers);
+            _readOnlyProviders = Array.AsReadOnly(_providers);
         }
 
         /// <summary>

src/libraries/System.ComponentModel.Composition/src/System/ComponentModel/Composition/Hosting/CompositionContainer.cs

@@ -223,7 +223,7 @@ public CompositionContainer(ComposablePartCatalog? catalog, CompositionOptions c
             _rootProvider.ExportsChanged += OnExportsChangedInternal;
             _rootProvider.ExportsChanging += OnExportsChangingInternal;
 
-            _providers = (providers != null) ? new ReadOnlyCollection<ExportProvider>((ExportProvider[])providers.Clone()) : EmptyProviders;
+            _providers = (providers != null) ? Array.AsReadOnly((ExportProvider[])providers.Clone()) : EmptyProviders;
         }
 
         internal CompositionOptions CompositionOptions

src/libraries/System.ComponentModel.Composition/src/System/ComponentModel/Composition/Hosting/DirectoryCatalog.cs

@@ -632,7 +632,7 @@ public void Refresh()
                             _catalogCollection.Remove(catalogToRemove.Item2);
                         }
 
-                        _loadedFiles = afterFiles.ToReadOnlyCollection();
+                        _loadedFiles = Array.AsReadOnly(afterFiles);
 
                         // Lastly complete any changes added to the atomicComposition during the change event
                         atomicComposition.Complete();
@@ -756,7 +756,7 @@ private void Initialize(string path, string searchPattern)
             _assemblyCatalogs = new Dictionary<string, AssemblyCatalog>();
             _catalogCollection = new ComposablePartCatalogCollection(null, null, null);
 
-            _loadedFiles = GetFiles().ToReadOnlyCollection();
+            _loadedFiles = Array.AsReadOnly(GetFiles());
 
             foreach (string file in _loadedFiles)
             {

src/libraries/System.Composition.AttributedModel/src/System/Composition/SharingBoundaryAttribute.cs

@@ -35,6 +35,6 @@ public SharingBoundaryAttribute(params string[] sharingBoundaryNames)
         /// <summary>
         /// Boundaries implemented by the created ExportLifetimeContext{T}s.
         /// </summary>
-        public ReadOnlyCollection<string> SharingBoundaryNames => new ReadOnlyCollection<string>(_sharingBoundaryNames);
+        public ReadOnlyCollection<string> SharingBoundaryNames => Array.AsReadOnly(_sharingBoundaryNames);
     }
 }

src/libraries/System.Linq.Expressions/src/System.Linq.Expressions.csproj

@@ -103,7 +103,6 @@
     <Compile Include="System\Runtime\CompilerServices\TrueReadOnlyCollection.cs" />
     <Compile Include="System\Dynamic\Utils\CachedReflectionInfo.cs" />
     <Compile Include="System\Dynamic\Utils\CollectionExtensions.cs" />
-    <Compile Include="System\Dynamic\Utils\EmptyReadOnlyCollection.cs" />
     <Compile Include="System\Dynamic\UpdateDelegates.Generated.cs">
       <AutoGen>True</AutoGen>
       <DesignTime>True</DesignTime>

src/libraries/System.Linq.Expressions/src/System/Dynamic/Utils/CollectionExtensions.cs

@@ -57,25 +57,26 @@ public static T[] RemoveLast<T>(this T[] array)
         /// </summary>
         public static ReadOnlyCollection<T> ToReadOnly<T>(this IEnumerable<T>? enumerable)
         {
-            if (enumerable == null)
+            if (enumerable != null && enumerable != ReadOnlyCollection<T>.Empty)
             {
-                return EmptyReadOnlyCollection<T>.Instance;
-            }
+                if (enumerable is TrueReadOnlyCollection<T> troc)
+                {
+                    return troc;
+                }
 
-            if (enumerable is TrueReadOnlyCollection<T> troc)
-            {
-                return troc;
-            }
+                if (enumerable is ReadOnlyCollectionBuilder<T> builder)
+                {
+                    return builder.ToReadOnlyCollection();
+                }
 
-            if (enumerable is ReadOnlyCollectionBuilder<T> builder)
-            {
-                return builder.ToReadOnlyCollection();
+                T[] array = enumerable.ToArray();
+                if (array.Length != 0)
+                {
+                    return new TrueReadOnlyCollection<T>(array);
+                }
             }
 
-            T[] array = enumerable.ToArray();
-            return array.Length == 0 ?
-                EmptyReadOnlyCollection<T>.Instance :
-                new TrueReadOnlyCollection<T>(array);
+            return ReadOnlyCollection<T>.Empty;
         }
 
         // We could probably improve the hashing here

src/libraries/System.Linq.Expressions/src/System/Linq/Expressions/BlockExpression.cs

@@ -135,7 +135,7 @@ internal virtual ReadOnlyCollection<Expression> GetOrMakeExpressions()
 
         internal virtual ReadOnlyCollection<ParameterExpression> GetOrMakeVariables()
         {
-            return EmptyReadOnlyCollection<ParameterExpression>.Instance;
+            return ReadOnlyCollection<ParameterExpression>.Empty;
         }
 
         /// <summary>
@@ -913,7 +913,7 @@ public static BlockExpression Block(params Expression[] expressions)
         /// <returns>The created <see cref="BlockExpression"/>.</returns>
         public static BlockExpression Block(IEnumerable<Expression> expressions)
         {
-            return Block(EmptyReadOnlyCollection<ParameterExpression>.Instance, expressions);
+            return Block(ReadOnlyCollection<ParameterExpression>.Empty, expressions);
         }
 
         /// <summary>
@@ -936,7 +936,7 @@ public static BlockExpression Block(Type type, params Expression[] expressions)
         /// <returns>The created <see cref="BlockExpression"/>.</returns>
         public static BlockExpression Block(Type type, IEnumerable<Expression> expressions)
         {
-            return Block(type, EmptyReadOnlyCollection<ParameterExpression>.Instance, expressions);
+            return Block(type, ReadOnlyCollection<ParameterExpression>.Empty, expressions);
         }
 
         /// <summary>
@@ -1089,7 +1089,7 @@ private static BlockExpression GetOptimizedBlockExpression(IReadOnlyList<Express
         {
             return expressions.Count switch
             {
-                0 => BlockCore(typeof(void), EmptyReadOnlyCollection<ParameterExpression>.Instance, EmptyReadOnlyCollection<Expression>.Instance),
+                0 => BlockCore(typeof(void), ReadOnlyCollection<ParameterExpression>.Empty, ReadOnlyCollection<Expression>.Empty),
                 2 => new Block2(expressions[0], expressions[1]),
                 3 => new Block3(expressions[0], expressions[1], expressions[2]),
                 4 => new Block4(expressions[0], expressions[1], expressions[2], expressions[3]),

src/libraries/System.Linq.Expressions/src/System/Linq/Expressions/Compiler/HoistedLocals.cs

@@ -64,16 +64,24 @@ internal HoistedLocals(HoistedLocals? parent, ReadOnlyCollection<ParameterExpres
                 vars = vars.AddFirst(parent.SelfVariable);
             }
 
-            Dictionary<Expression, int> indexes = new Dictionary<Expression, int>(vars.Count);
-            for (int i = 0; i < vars.Count; i++)
+            if (vars.Count != 0)
             {
-                indexes.Add(vars[i], i);
+                Dictionary<Expression, int> indexes = new Dictionary<Expression, int>(vars.Count);
+                for (int i = 0; i < vars.Count; i++)
+                {
+                    indexes.Add(vars[i], i);
+                }
+
+                Indexes = new ReadOnlyDictionary<Expression, int>(indexes);
+            }
+            else
+            {
+                Indexes = ReadOnlyDictionary<Expression, int>.Empty;
             }
 
-            SelfVariable = Expression.Variable(typeof(object[]), name: null);
             Parent = parent;
             Variables = vars;
-            Indexes = new ReadOnlyDictionary<Expression, int>(indexes);
+            SelfVariable = Expression.Variable(typeof(object[]), name: null);
         }
 
         internal ParameterExpression? ParentVariable => Parent?.SelfVariable;

src/libraries/System.Linq.Expressions/src/System/Linq/Expressions/InvocationExpression.cs

@@ -159,7 +159,7 @@ public InvocationExpression0(Expression lambda, Type returnType)
 
         internal override ReadOnlyCollection<Expression> GetOrMakeArguments()
         {
-            return EmptyReadOnlyCollection<Expression>.Instance;
+            return ReadOnlyCollection<Expression>.Empty;
         }
 
         public override Expression GetArgument(int index)

src/libraries/System.Linq.Expressions/src/System/Linq/Expressions/LambdaExpression.cs

@@ -370,7 +370,7 @@ internal override ParameterExpression GetParameter(int index)
             throw Error.ArgumentOutOfRange(nameof(index));
         }
 
-        internal override ReadOnlyCollection<ParameterExpression> GetOrMakeParameters() => EmptyReadOnlyCollection<ParameterExpression>.Instance;
+        internal override ReadOnlyCollection<ParameterExpression> GetOrMakeParameters() => ReadOnlyCollection<ParameterExpression>.Empty;
 
         internal override Expression<TDelegate> Rewrite(Expression body, ParameterExpression[]? parameters)
         {

src/libraries/System.Linq.Expressions/src/System/Linq/Expressions/ListInitExpression.cs

@@ -125,7 +125,7 @@ public static ListInitExpression ListInit(NewExpression newExpression, IEnumerab
             ReadOnlyCollection<Expression> initializerlist = initializers.ToReadOnly();
             if (initializerlist.Count == 0)
             {
-                return new ListInitExpression(newExpression, EmptyReadOnlyCollection<ElementInit>.Instance);
+                return new ListInitExpression(newExpression, ReadOnlyCollection<ElementInit>.Empty);
             }
 
             MethodInfo? addMethod = FindMethod(newExpression.Type, "Add", null, new Expression[] { initializerlist[0] }, BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);

src/libraries/System.Linq.Expressions/src/System/Linq/Expressions/MethodCallExpression.cs

@@ -242,7 +242,7 @@ public override Expression GetArgument(int index)
 
         internal override ReadOnlyCollection<Expression> GetOrMakeArguments()
         {
-            return EmptyReadOnlyCollection<Expression>.Instance;
+            return ReadOnlyCollection<Expression>.Empty;
         }
 
         internal override bool SameArguments(ICollection<Expression>? arguments) =>
@@ -622,7 +622,7 @@ public override Expression GetArgument(int index)
 
         internal override ReadOnlyCollection<Expression> GetOrMakeArguments()
         {
-            return EmptyReadOnlyCollection<Expression>.Instance;
+            return ReadOnlyCollection<Expression>.Empty;
         }
 
         internal override bool SameArguments(ICollection<Expression>? arguments) =>

src/libraries/System.Linq.Expressions/src/System/Linq/Expressions/NewExpression.cs

@@ -213,7 +213,7 @@ public static NewExpression New(ConstructorInfo constructor, IEnumerable<Express
             {
                 throw Error.TypeMissingDefaultConstructor(type, nameof(type));
             }
-            return new NewValueTypeExpression(type, EmptyReadOnlyCollection<Expression>.Instance, null);
+            return new NewValueTypeExpression(type, ReadOnlyCollection<Expression>.Empty, null);
         }
 
         [RequiresUnreferencedCode(PropertyFromAccessorRequiresUnreferencedCode)]

src/libraries/System.Linq.Expressions/src/System/Runtime/CompilerServices/ReadOnlyCollectionBuilder.cs

@@ -412,6 +412,11 @@ public T[] ToArray()
         /// <returns>A new instance of <see cref="ReadOnlyCollection{T}"/>.</returns>
         public ReadOnlyCollection<T> ToReadOnlyCollection()
         {
+            if (_size == 0)
+            {
+                return ReadOnlyCollection<T>.Empty;
+            }
+
             // Can we use the stored array?
             T[] items;
             if (_size == _items.Length)

src/libraries/System.Linq/tests/ToListTests.cs

@@ -158,19 +158,16 @@ public void ToList_IListWhereSelect(int[] sourceIntegers, string[] convertedStri
             var sourceList = new ReadOnlyCollection<int>(sourceIntegers);
             var convertedList = new ReadOnlyCollection<string>(convertedStrings);
 
-            var emptyIntegersList = new ReadOnlyCollection<int>(Array.Empty<int>());
-            var emptyStringsList = new ReadOnlyCollection<string>(Array.Empty<string>());
-
             Assert.Equal(convertedList, sourceList.Select(i => i.ToString()).ToList());
 
             Assert.Equal(sourceList, sourceList.Where(i => true).ToList());
-            Assert.Equal(emptyIntegersList, sourceList.Where(i => false).ToList());
+            Assert.Equal(ReadOnlyCollection<int>.Empty, sourceList.Where(i => false).ToList());
 
             Assert.Equal(convertedList, sourceList.Where(i => true).Select(i => i.ToString()).ToList());
-            Assert.Equal(emptyStringsList, sourceList.Where(i => false).Select(i => i.ToString()).ToList());
+            Assert.Equal(ReadOnlyCollection<string>.Empty, sourceList.Where(i => false).Select(i => i.ToString()).ToList());
 
             Assert.Equal(convertedList, sourceList.Select(i => i.ToString()).Where(s => s != null).ToList());
-            Assert.Equal(emptyStringsList, sourceList.Select(i => i.ToString()).Where(s => s == null).ToList());
+            Assert.Equal(ReadOnlyCollection<string>.Empty, sourceList.Select(i => i.ToString()).Where(s => s == null).ToList());
         }
 
         [Fact]

src/libraries/System.ObjectModel/ref/System.ObjectModel.cs

@@ -46,6 +46,7 @@ public partial class ObservableCollection<T> : System.Collections.ObjectModel.Co
     public partial class ReadOnlyObservableCollection<T> : System.Collections.ObjectModel.ReadOnlyCollection<T>, System.Collections.Specialized.INotifyCollectionChanged, System.ComponentModel.INotifyPropertyChanged
     {
         public ReadOnlyObservableCollection(System.Collections.ObjectModel.ObservableCollection<T> list) : base (default(System.Collections.Generic.IList<T>)) { }
+        public static new System.Collections.ObjectModel.ReadOnlyObservableCollection<T> Empty { get { throw null; } }
         protected virtual event System.Collections.Specialized.NotifyCollectionChangedEventHandler? CollectionChanged { add { } remove { } }
         protected virtual event System.ComponentModel.PropertyChangedEventHandler? PropertyChanged { add { } remove { } }
         event System.Collections.Specialized.NotifyCollectionChangedEventHandler? System.Collections.Specialized.INotifyCollectionChanged.CollectionChanged { add { } remove { } }

src/libraries/System.ObjectModel/src/System/Collections/ObjectModel/ReadOnlyObservableCollection.cs

@@ -28,6 +28,11 @@ public ReadOnlyObservableCollection(ObservableCollection<T> list) : base(list)
             ((INotifyPropertyChanged)Items).PropertyChanged += new PropertyChangedEventHandler(HandlePropertyChanged);
         }
 
+        /// <summary>Gets an empty <see cref="ReadOnlyObservableCollection{T}"/>.</summary>
+        /// <value>An empty <see cref="ReadOnlyObservableCollection{T}"/>.</value>
+        /// <remarks>The returned instance is immutable and will always be empty.</remarks>
+        public static new ReadOnlyObservableCollection<T> Empty { get; } = new ReadOnlyObservableCollection<T>(new ObservableCollection<T>());
+
         /// <summary>
         /// CollectionChanged event (per <see cref="INotifyCollectionChanged" />).
         /// </summary>

src/libraries/System.ObjectModel/tests/ReadOnlyDictionary/ReadOnlyDictionaryTests.cs

@@ -64,6 +64,14 @@ public static void CtorTests_Negative()
             AssertExtensions.Throws<ArgumentNullException>("dictionary", () => new ReadOnlyDictionary<int, string>(null));
         }
 
+        [Fact]
+        public static void Empty_Idempotent()
+        {
+            Assert.NotNull(ReadOnlyDictionary<string, int>.Empty);
+            Assert.Equal(0, ReadOnlyDictionary<string, int>.Empty.Count);
+            Assert.Same(ReadOnlyDictionary<string, int>.Empty, ReadOnlyDictionary<string, int>.Empty);
+        }
+
         /// <summary>
         /// Tests that true is returned when the key exists in the dictionary
         /// and false otherwise.

src/libraries/System.ObjectModel/tests/ReadOnlyObservableCollection/ReadOnlyObservableCollectionTests.cs

@@ -32,6 +32,14 @@ public static void Ctor_Tests_Negative()
             AssertExtensions.Throws<ArgumentNullException>("list", () => new ReadOnlyObservableCollection<string>(null));
         }
 
+        [Fact]
+        public static void Empty_Idempotent()
+        {
+            Assert.NotNull(ReadOnlyObservableCollection<int>.Empty);
+            Assert.Equal(0, ReadOnlyObservableCollection<int>.Empty.Count);
+            Assert.Same(ReadOnlyObservableCollection<int>.Empty, ReadOnlyObservableCollection<int>.Empty);
+        }
+
         [Fact]
         public static void GetItemTests()
         {

src/libraries/System.Private.CoreLib/src/System/Collections/ObjectModel/ReadOnlyCollection.cs

@@ -23,10 +23,10 @@ public ReadOnlyCollection(IList<T> list)
             this.list = list;
         }
 
-        // TODO https://github.com/dotnet/runtime/issues/76028: Make this public.
         /// <summary>Gets an empty <see cref="ReadOnlyCollection{T}"/>.</summary>
+        /// <value>An empty <see cref="ReadOnlyCollection{T}"/>.</value>
         /// <remarks>The returned instance is immutable and will always be empty.</remarks>
-        internal static ReadOnlyCollection<T> Empty { get; } = new ReadOnlyCollection<T>(Array.Empty<T>());
+        public static ReadOnlyCollection<T> Empty { get; } = new ReadOnlyCollection<T>(Array.Empty<T>());
 
         public int Count => list.Count;
 

src/libraries/System.Private.CoreLib/src/System/Collections/ObjectModel/ReadOnlyDictionary.cs

@@ -27,6 +27,11 @@ public ReadOnlyDictionary(IDictionary<TKey, TValue> dictionary)
             m_dictionary = dictionary;
         }
 
+        /// <summary>Gets an empty <see cref="ReadOnlyDictionary{TKey, TValue}"/>.</summary>
+        /// <value>An empty <see cref="ReadOnlyDictionary{TKey, TValue}"/>.</value>
+        /// <remarks>The returned instance is immutable and will always be empty.</remarks>
+        public static ReadOnlyDictionary<TKey, TValue> Empty { get; } = new ReadOnlyDictionary<TKey, TValue>(new Dictionary<TKey, TValue>());
+
         protected IDictionary<TKey, TValue> Dictionary => m_dictionary;
 
         public KeyCollection Keys => _keys ??= new KeyCollection(m_dictionary.Keys);

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/ClassDataContract.cs

@@ -73,7 +73,7 @@ private void InitClassDataContract()
             set => _helper.Members = value;
         }
 
-        public override ReadOnlyCollection<DataMember> DataMembers => (Members == null) ? DataContract.s_emptyDataMemberList : Members.AsReadOnly();
+        public override ReadOnlyCollection<DataMember> DataMembers => (Members == null) ? ReadOnlyCollection<DataMember>.Empty : Members.AsReadOnly();
 
         internal XmlDictionaryString?[]? ChildElementNamespaces
         {