chore: avoid double enumeration of map nodes in V2

Signed-off-by: Vincent Biret <vibiret@microsoft.com>

Author: baywet

src/Microsoft.OpenApi/Reader/ParseNodes/MapNode.cs

@@ -37,19 +37,6 @@ public MapNode(ParsingContext context, JsonNode node) : base(
             _nodes = _node.Select(p => GetPropertyNodeFromJsonNode(p.Key, p.Value)).ToList();
         }
 
-        public PropertyNode? this[string key]
-        {
-            get
-            {
-                if (_node.TryGetPropertyValue(key, out var value))
-				{
-					return GetPropertyNodeFromJsonNode(key, value);
-				}
-
-                return null;
-            }
-        }
-
         public override Dictionary<string, T> CreateMap<T>(Func<MapNode, OpenApiDocument, T> map, OpenApiDocument hostDocument)
         {
             var jsonMap = _node ?? throw new OpenApiReaderException($"Expected map while parsing {typeof(T).Name}", Context);

src/Microsoft.OpenApi/Reader/V2/OpenApiV2Deserializer.cs

@@ -2,7 +2,6 @@
 // Licensed under the MIT license.
 
 using System;
-using System.Collections.Generic;
 using System.Linq;
 using System.Text.Json.Nodes;
 
@@ -19,19 +18,22 @@ private static void ParseMap<T>(
             T domainObject,
             FixedFieldMap<T> fixedFieldMap,
             PatternFieldMap<T> patternFieldMap,
-            OpenApiDocument doc,
-            List<string>? requiredFields = null)
+            OpenApiDocument doc)
         {
             if (mapNode == null)
             {
                 return;
             }
 
-            var allFields = fixedFieldMap.Keys.Union(mapNode.Select(static x => x.Name));
-            foreach (var propertyNode in allFields)
+            var mapNodeFields = mapNode.ToDictionary(static x => x.Name, static x => x);
+            var allFields = fixedFieldMap.Keys.Union(mapNodeFields.Keys);
+            foreach (var propertyNodeName in allFields)
             {
-                mapNode[propertyNode]?.ParseField(domainObject, fixedFieldMap, patternFieldMap, doc);
-                requiredFields?.Remove(propertyNode);
+                if (!mapNodeFields.TryGetValue(propertyNodeName, out var propertyNode))
+                {
+                    continue;
+                }
+                propertyNode.ParseField(domainObject, fixedFieldMap, patternFieldMap, doc);
             }
         }
 

test/Microsoft.OpenApi.Tests/Reader/MapNodeTests.cs

@@ -1,4 +1,5 @@
-using System.Text.Json.Nodes;
+using System.Linq;
+using System.Text.Json.Nodes;
 using Microsoft.OpenApi.Reader;
 using Xunit;
 
@@ -14,6 +15,6 @@ public void DoesNotFailOnNullValue()
 
         Assert.NotNull(mapNode);
         Assert.Single(mapNode);
-        Assert.True(mapNode["key"].Value.JsonNode.IsJsonNullSentinel());
+        Assert.True(mapNode.First().Value.JsonNode.IsJsonNullSentinel());
     }
 }