Rework the XML configuration provider

The XML configuration provider now builds a simple in-memory model of the XML elements that it encounters
Each element keeps track of its attributes, children and content.
Furthermore, each element also has a reference to its siblings.
Each group of siblings all share the same list.

All of the above makes it possible to intelligently produce configuration keys and values, taking into account repeated XML elements.

src/libraries/Microsoft.Extensions.Configuration.Xml/src/XmlConfigurationElement.cs

@@ -3,8 +3,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Linq;
-using System.Text;
 
 namespace Microsoft.Extensions.Configuration.Xml
 {
@@ -16,55 +14,41 @@ internal class XmlConfigurationElement
 
         public string LineInfo { get; }
 
-        public bool Multiple { get; set; }
-
-        public int Index { get; set; }
+        /// <summary>
+        /// The children of this element
+        /// </summary>
+        public List<XmlConfigurationElement>? Children { get; set; }
 
         /// <summary>
-        /// The parent element, or null if this is the root element
+        /// The siblings of this element, including itself
+        /// Elements are considered siblings if they share the same element name and name attribute
+        /// This list is shared by each sibling
         /// </summary>
-        public XmlConfigurationElement Parent { get; }
+        public List<XmlConfigurationElement>? Siblings { get; set; }
+
+        public XmlConfigurationElementTextContent? TextContent { get; set; }
 
-        public XmlConfigurationElement(XmlConfigurationElement parent, string elementName, string name, string lineInfo)
+        public List<XmlConfigurationElementAttributeValue>? Attributes { get; set; }
+
+        public XmlConfigurationElement(string elementName, string name, string lineInfo)
         {
-            Parent = parent;
             ElementName = elementName ?? throw new ArgumentNullException(nameof(elementName));
             Name = name;
             LineInfo = lineInfo;
+            Children = null;
+            Siblings = null;
+            TextContent = null;
+            Attributes = null;
         }
 
-        public string Key
-        {
-            get
-            {
-                var tokens = new List<string>(3);
-
-                // the root element does not contribute to the prefix
-                if (Parent != null) tokens.Add(ElementName);
-
-                // the name attribute always contributes to the prefix
-                if (Name != null) tokens.Add(Name);
-
-                // the index only contributes to the prefix when there are multiple elements wih the same name
-                if (Multiple) tokens.Add(Index.ToString());
-
-                // the root element without a name attribute does not contribute to prefix at all
-                if (!tokens.Any()) return null;
-
-                return string.Join(ConfigurationPath.KeyDelimiter, tokens);
-            }
-        }
-
-        public bool IsSibling(XmlConfigurationElement xmlConfigurationElement)
+        public bool IsSiblingOf(XmlConfigurationElement xmlConfigurationElement)
         {
             if (xmlConfigurationElement is null)
             {
                 throw new ArgumentNullException(nameof(xmlConfigurationElement));
             }
 
-            return Parent != null
-                && xmlConfigurationElement.Parent == Parent
-                && string.Equals(ElementName, xmlConfigurationElement.ElementName)
+            return string.Equals(ElementName, xmlConfigurationElement.ElementName)
                 && string.Equals(Name, xmlConfigurationElement.Name);
         }
     }

src/libraries/Microsoft.Extensions.Configuration.Xml/src/XmlConfigurationElementAttributeValue.cs

@@ -2,30 +2,19 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
-using System.Collections.Generic;
-using System.Linq;
 
 namespace Microsoft.Extensions.Configuration.Xml
 {
-    internal class XmlConfigurationElementAttributeValue : IXmlConfigurationValue
+    internal class XmlConfigurationElementAttributeValue
     {
-        private readonly XmlConfigurationElement[] _elementPath;
-        private readonly string _attribute;
-
-        public XmlConfigurationElementAttributeValue(Stack<XmlConfigurationElement> elementPath, string attribute, string value, string lineInfo)
+        public XmlConfigurationElementAttributeValue(string attribute, string value, string lineInfo)
         {
-            _elementPath = elementPath?.Reverse()?.ToArray() ?? throw new ArgumentNullException(nameof(elementPath));
-            _attribute = attribute ?? throw new ArgumentNullException(nameof(attribute));
+            Attribute = attribute ?? throw new ArgumentNullException(nameof(attribute));
             Value = value ?? throw new ArgumentNullException(nameof(value));
             LineInfo = lineInfo;
         }
 
-        /// <summary>
-        /// Combines the path to this element with the attribute value to produce a key.
-        /// Note that this property cannot be computed during construction,
-        /// because the keys of the elements along the path may change when multiple elements with the same name are encountered
-        /// </summary>
-        public string Key => ConfigurationPath.Combine(_elementPath.Select(e => e.Key).Concat(new[] { _attribute }).Where(key => key != null));
+        public string Attribute { get; }
 
         public string Value { get; }
 

src/libraries/Microsoft.Extensions.Configuration.Xml/src/XmlConfigurationElementTextContent.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.
+
+using System;
+
+namespace Microsoft.Extensions.Configuration.Xml
+{
+    internal class XmlConfigurationElementTextContent
+    {
+        public XmlConfigurationElementTextContent(string textContent, string lineInfo)
+        {
+            TextContent = textContent ?? throw new ArgumentNullException(nameof(textContent));
+            LineInfo = lineInfo ?? throw new ArgumentNullException(nameof(lineInfo));
+        }
+
+        public string TextContent { get; }
+
+        public string LineInfo { get; }
+    }
+}