Use a central cache of empty collections (#356)

Author: twsouthwick

DocumentFormat.OpenXml.Tests/OpenXmlDomTest/PackageReflector.cs

@@ -444,7 +444,7 @@ internal void ReflectPart(OpenXmlPart srcPart, OpenXmlPart destPart, OpenXmlPart
 
                 // Load source part into source root element
                 var loader = rootType.GetMethod("Load", new Type[] { srcPartType });
-                var ctor = rootType.GetConstructor(new Type[] { });
+                var ctor = rootType.GetConstructor(Cached.Array<Type>());
                 OpenXmlElement srcRoot = ctor.Invoke(null) as OpenXmlElement;
                 loader.Invoke(srcRoot, new Object[] { srcPart });
 

DocumentFormat.OpenXml.Tests/ofapiTest/OpenXmlElementTest.cs

@@ -703,14 +703,9 @@ public void ChildElementsTest()
             var unknown2 = unknown1.AppendChild(new OpenXmlUnknownElement("tmp:node2", "http://temp"));
             var miscNode = p.AppendChild(new OpenXmlMiscNode(System.Xml.XmlNodeType.Comment, "<!-- comment -->"));
 
-            var emptyListSingleton = EmptyElementList.EmptyElementListSingleton;
-            var emptyEnumeratorSingleton = EmptyEnumerator<OpenXmlElement>.EmptyEnumeratorSingleton;
-
-            Assert.NotSame(emptyListSingleton, p.ChildElements);
             Assert.Equal(4, p.ChildElements.Count);
 
             var enumerator = p.ChildElements.GetEnumerator();
-            Assert.NotSame(emptyEnumeratorSingleton, enumerator);
             Assert.True(enumerator.MoveNext());
             Assert.Same(pPr, enumerator.Current);
 
@@ -721,60 +716,47 @@ public void ChildElementsTest()
             Assert.False(enumerator.MoveNext()); // the end
 
             // pPr, empty children
-            Assert.Same(emptyListSingleton, pPr.ChildElements);
-            Assert.Equal(0, pPr.ChildElements.Count);
+            Assert.Empty(pPr.ChildElements);
+            Assert.Same(OpenXmlElementList.Empty, pPr.ChildElements);
             enumerator = pPr.ChildElements.GetEnumerator();
-            Assert.Same(emptyEnumeratorSingleton, enumerator);
             Assert.False(enumerator.MoveNext());
 
             // r, two child
-            Assert.NotSame(emptyListSingleton, r.ChildElements);
             Assert.Equal(2, r.ChildElements.Count);
             enumerator = r.ChildElements.GetEnumerator();
-            Assert.NotSame(emptyEnumeratorSingleton, enumerator);
             Assert.True(enumerator.MoveNext());
             Assert.Same(rPr, enumerator.Current);
 
             // rPr, one child
-            Assert.NotSame(emptyListSingleton, rPr.ChildElements);
             Assert.Equal(1, rPr.ChildElements.Count);
             enumerator = rPr.ChildElements.GetEnumerator();
-            Assert.NotSame(emptyEnumeratorSingleton, enumerator);
             Assert.True(enumerator.MoveNext());
 
             // b, leaf element
-            Assert.Same(emptyListSingleton, b.ChildElements);
             Assert.Equal(0, b.ChildElements.Count);
             enumerator = b.ChildElements.GetEnumerator();
-            Assert.Same(emptyEnumeratorSingleton, enumerator);
             Assert.False(enumerator.MoveNext());
 
             // t, leaf text element
-            Assert.Same(emptyListSingleton, t.ChildElements);
             Assert.Equal(0, t.ChildElements.Count);
             enumerator = t.ChildElements.GetEnumerator();
-            Assert.Same(emptyEnumeratorSingleton, enumerator);
             Assert.False(enumerator.MoveNext());
 
             // unknown1, one child
-            Assert.NotSame(emptyListSingleton, unknown1.ChildElements);
             Assert.Equal(1, unknown1.ChildElements.Count);
             enumerator = unknown1.ChildElements.GetEnumerator();
-            Assert.NotSame(emptyEnumeratorSingleton, enumerator);
             Assert.True(enumerator.MoveNext());
 
             // unknown2, empty children
-            Assert.Same(emptyListSingleton, unknown2.ChildElements);
+            Assert.Same(OpenXmlElementList.Empty, unknown2.ChildElements);
             Assert.Equal(0, unknown2.ChildElements.Count);
             enumerator = unknown2.ChildElements.GetEnumerator();
-            Assert.Same(emptyEnumeratorSingleton, enumerator);
             Assert.False(enumerator.MoveNext());
 
             // miscNode
-            Assert.Same(emptyListSingleton, miscNode.ChildElements);
+            Assert.Same(OpenXmlElementList.Empty, miscNode.ChildElements);
             Assert.Equal(0, miscNode.ChildElements.Count);
             enumerator = miscNode.ChildElements.GetEnumerator();
-            Assert.Same(emptyEnumeratorSingleton, enumerator);
             Assert.False(enumerator.MoveNext());
         }
 

DocumentFormat.OpenXml/AlternateContent.cs

@@ -109,8 +109,8 @@ internal override byte NamespaceId
             get { return MarkupCompatibilityNamespaceId; }
         }
 
-        private static string[] attributeTagNames = { };
-        private static byte[] attributeNamespaceIds = { };
+        private static string[] attributeTagNames = Cached.Array<string>();
+        private static byte[] attributeNamespaceIds = Cached.Array<byte>();
 
         internal override string[] AttributeTagNames
         {

DocumentFormat.OpenXml/AlternateContentFallback.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
+using System;
 using System.Collections.Generic;
 
 namespace DocumentFormat.OpenXml
@@ -75,8 +76,8 @@ internal override byte NamespaceId
             get { return AlternateContent.MarkupCompatibilityNamespaceId; }
         }
 
-        private static string[] attributeTagNames = { };
-        private static byte[] attributeNamespaceIds = { };
+        private static string[] attributeTagNames = Cached.Array<string>();
+        private static byte[] attributeNamespaceIds = Cached.Array<byte>();
 
         internal override string[] AttributeTagNames
         {

DocumentFormat.OpenXml/Cached.cs

@@ -0,0 +1,28 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System.Collections.ObjectModel;
+
+namespace DocumentFormat.OpenXml
+{
+    internal static class Cached
+    {
+#if FEATURE_ARRAY_EMPTY
+        public static T[] Array<T>() => System.Array.Empty<T>();
+#else
+        public static T[] Array<T>() => EmptyArray<T>.Value;
+
+        private static class EmptyArray<T>
+        {
+            internal static readonly T[] Value = new T[0];
+        }
+#endif
+
+        public static ReadOnlyCollection<T> ReadOnlyCollection<T>() => ReadOnlyCollectionCache<T>.Value;
+
+        private static class ReadOnlyCollectionCache<T>
+        {
+            internal static ReadOnlyCollection<T> Value = new ReadOnlyCollection<T>(Array<T>());
+        }
+    }
+}

DocumentFormat.OpenXml/DocumentFormat.OpenXml.csproj

@@ -11,7 +11,7 @@
     <TargetFrameworks Condition=" '$(ProjectLoadStyle)' == 'DevCore' ">netstandard1.3</TargetFrameworks>
     <TargetFrameworks Condition=" '$(ProjectLoadStyle)' == 'DevFramework' ">net46</TargetFrameworks>
     <TargetFrameworks Condition=" '$(ProjectLoadStyle)' == 'All' ">netstandard1.3;net35;net40;net46</TargetFrameworks>
-    
+
     <NoWarn>$(NoWarn);3003</NoWarn>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
   </PropertyGroup>
@@ -29,6 +29,7 @@
     constants are used to conditionally compile features:
 
     FEATURE_ABSTRACT_XML_CLOSE       : XmlWriter.Close is defined as abstract
+    FEATURE_ARRAY_EMPTY              : Array.Empty exists
     FEATURE_CLOSE                    : Stream.Close is available in the platform
     FEATURE_DCS_SETTINGS             : DataContractSerializer settings class exists
     FEATURE_SERIALIZATION            : Serialization using ISerializable is in the platform
@@ -68,7 +69,7 @@
 
     <When Condition=" '$(TargetFramework)' == 'net46' ">
       <PropertyGroup>
-        <DefineConstants>$(DefineConstants);FEATURE_DCS_SETTINGS;FEATURE_CLOSE;FEATURE_SERIALIZATION;FEATURE_SYSTEMEXCEPTION;FEATURE_XML_QUOTECHAR;FEATURE_XML_SCHEMA;FEATURE_XML_VERIFYTOKEN</DefineConstants>
+        <DefineConstants>$(DefineConstants);FEATURE_ARRAY_EMPTY;FEATURE_DCS_SETTINGS;FEATURE_CLOSE;FEATURE_SERIALIZATION;FEATURE_SYSTEMEXCEPTION;FEATURE_XML_QUOTECHAR;FEATURE_XML_SCHEMA;FEATURE_XML_VERIFYTOKEN</DefineConstants>
       </PropertyGroup>
 
       <ItemGroup Condition=" '$(TargetFramework)' == 'net46' ">
@@ -85,7 +86,7 @@
 
     <When Condition=" '$(TargetFramework)' == 'netstandard1.3' ">
       <PropertyGroup>
-        <DefineConstants> $(DefineConstants);FEATURE_DCS_SETTINGS</DefineConstants>
+        <DefineConstants> $(DefineConstants);FEATURE_DCS_SETTINGS;FEATURE_ARRAY_EMPTY</DefineConstants>
       </PropertyGroup>
 
       <ItemGroup>

DocumentFormat.OpenXml/EmptyElementList.cs

@@ -5,6 +5,7 @@
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Diagnostics;
+using System.Linq;
 
 namespace DocumentFormat.OpenXml
 {
@@ -115,7 +116,7 @@ public override IEnumerable<KeyValuePair<string, string>> NamespaceDeclarations
                 }
                 else
                 {
-                    return EmptyEnumerable<KeyValuePair<string, string>>.EmptyEnumerableSingleton;
+                    return Enumerable.Empty<KeyValuePair<string, string>>();
                 }
             }
         }