Revert "make factory struct"

This reverts commit 37c39894c115ed73a69d9ef222e81974d948e9b8.

Author: twsouthwick

src/DocumentFormat.OpenXml.Framework/Framework/Metadata/ElementFactory.cs

@@ -4,12 +4,29 @@
 using System;
 using System.Diagnostics;
 
-namespace DocumentFormat.OpenXml.Framework.Metadata;
-
-[DebuggerDisplay("{QName,nq}")]
-internal readonly struct ElementFactory(OpenXmlSchemaType type, Func<OpenXmlElement> factory)
+namespace DocumentFormat.OpenXml.Framework.Metadata
 {
-    public OpenXmlSchemaType Type => type;
+    [DebuggerDisplay("{QName,nq}")]
+    internal sealed class ElementFactory
+    {
+        private readonly Func<OpenXmlElement> _factory;
+
+        public ElementFactory(in OpenXmlSchemaType type, Func<OpenXmlElement> factory)
+        {
+            Type = type;
+            _factory = factory;
+        }
+
+        public OpenXmlSchemaType Type { get; }
+
+        public OpenXmlElement Create() => _factory();
+
+        public static ElementFactory Create<T>()
+            where T : OpenXmlElement, new()
+        {
+            var instance = new T();
 
-    public OpenXmlElement Create() => factory();
+            return new ElementFactory(instance.Metadata.Type, static () => new T());
+        }
+    }
 }

src/DocumentFormat.OpenXml.Framework/Framework/Metadata/ElementFactoryCollection.cs

@@ -1,49 +1,81 @@
 // 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;
+using System.Linq;
 
-namespace DocumentFormat.OpenXml.Framework.Metadata;
-
-/// <summary>
-/// A lookup that identifies properties on an <see cref="OpenXmlElement"/> and caches the schema information
-/// from those elements.
-/// </summary>
-internal class ElementFactoryCollection
+namespace DocumentFormat.OpenXml.Framework.Metadata
 {
-    public static readonly ElementFactoryCollection Empty = new([]);
-
-    private readonly List<ElementFactory> _data;
-
-    public ElementFactoryCollection(List<ElementFactory> lookup)
+    /// <summary>
+    /// A lookup that identifies properties on an <see cref="OpenXmlElement"/> and caches the schema information
+    /// from those elements.
+    /// </summary>
+    internal class ElementFactoryCollection
     {
-        lookup.Sort(ElementChildNameComparer.Instance);
-        _data = lookup;
-    }
+        public static readonly ElementFactoryCollection Empty = new(Enumerable.Empty<ElementFactory>());
 
-    public OpenXmlElement? Create(in OpenXmlQualifiedName qname)
-    {
-        if (_data.Count == 0)
+        private readonly ElementFactory[] _data;
+
+        public ElementFactoryCollection(IEnumerable<ElementFactory> lookup)
         {
-            return null;
+            var array = lookup.ToArray();
+
+            Array.Sort(array, ElementChildNameComparer.Instance);
+
+            _data = array;
         }
 
-        // This is on a hot-path and using a dictionary adds substantial time to the lookup. Most child lists are small, so using a sorted
-        // list to store them with a binary search improves overall performance.
-        var idx = _data.BinarySearch(new ElementFactory(new(qname, default), null!), ElementChildNameComparer.Instance);
+        public int Count => _data.Length;
 
-        if (idx < 0)
+        public IEnumerable<ElementFactory> Elements => _data;
+
+        public OpenXmlElement? Create(in OpenXmlQualifiedName qname)
         {
-            return null;
+            if (_data.Length == 0)
+            {
+                return null;
+            }
+
+            // This is on a hot-path and using a dictionary adds substantial time to the lookup. Most child lists are small, so using a sorted
+            // list to store them with a binary search improves overall performance.
+            var idx = Array.BinarySearch(_data, new ElementFactory(new(qname, default), null!), ElementChildNameComparer.Instance);
+
+            if (idx < 0)
+            {
+                return null;
+            }
+
+            return _data[idx].Create();
         }
 
-        return _data[idx].Create();
-    }
+        private class ElementChildNameComparer : IComparer<ElementFactory>
+        {
+            public static IComparer<ElementFactory> Instance { get; } = new ElementChildNameComparer();
 
-    private sealed class ElementChildNameComparer : IComparer<ElementFactory>
-    {
-        public static IComparer<ElementFactory> Instance { get; } = new ElementChildNameComparer();
+            private ElementChildNameComparer()
+            {
+            }
 
-        public int Compare(ElementFactory x, ElementFactory y) => x.Type.Name.CompareTo(y.Type.Name);
+            public int Compare(ElementFactory? x, ElementFactory? y)
+            {
+                if (x is null && y is null)
+                {
+                    return 0;
+                }
+
+                if (x is null)
+                {
+                    return -1;
+                }
+
+                if (y is null)
+                {
+                    return 1;
+                }
+
+                return x.Type.Name.CompareTo(y.Type.Name);
+            }
+        }
     }
 }