feat: Remove default collection initialization for perf reasons

src/Microsoft.OpenApi/Extensions/OpenApiExtensibleExtensions.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT license.
 
 using System;
+using System.Collections.Generic;
 using Microsoft.OpenApi.Exceptions;
 using Microsoft.OpenApi.Interfaces;
 using Microsoft.OpenApi.Models;
@@ -32,10 +33,8 @@ public static void AddExtension<T>(this T element, string name, IOpenApiExtensio
                 throw new OpenApiException(string.Format(SRResource.ExtensionFieldNameMustBeginWithXDash, name));
             }
 
-            if (element.Extensions is not null)
-            {
-                element.Extensions[name] = Utils.CheckArgumentNull(any);
-            }            
+            element.Extensions ??= new Dictionary<string, IOpenApiExtension>();
+            element.Extensions[name] = Utils.CheckArgumentNull(any);
         }
     }
 }

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -173,19 +173,19 @@
         public bool WriteOnly { get; set; }
 
         /// <inheritdoc />
-        public IList<IOpenApiSchema>? AllOf { get; set; } = [];
+        public IList<IOpenApiSchema>? AllOf { get; set; }
 
         /// <inheritdoc />
-        public IList<IOpenApiSchema>? OneOf { get; set; } = [];
+        public IList<IOpenApiSchema>? OneOf { get; set; }
 
         /// <inheritdoc />
-        public IList<IOpenApiSchema>? AnyOf { get; set; } = [];
+        public IList<IOpenApiSchema>? AnyOf { get; set; }
 
         /// <inheritdoc />
         public IOpenApiSchema? Not { get; set; }
 
         /// <inheritdoc />
-        public ISet<string>? Required { get; set; } = new HashSet<string>();
+        public ISet<string>? Required { get; set; }
 
         /// <inheritdoc />
         public IOpenApiSchema? Items { get; set; }
@@ -200,10 +200,10 @@
         public bool? UniqueItems { get; set; }
 
         /// <inheritdoc />
-        public IDictionary<string, IOpenApiSchema>? Properties { get; set; } = new Dictionary<string, IOpenApiSchema>(StringComparer.Ordinal);
+        public IDictionary<string, IOpenApiSchema>? Properties { get; set; }
 
         /// <inheritdoc />
-        public IDictionary<string, IOpenApiSchema>? PatternProperties { get; set; } = new Dictionary<string, IOpenApiSchema>(StringComparer.Ordinal);
+        public IDictionary<string, IOpenApiSchema>? PatternProperties { get; set; }
 
         /// <inheritdoc />
         public int? MaxProperties { get; set; }
@@ -227,10 +227,10 @@
         public IList<JsonNode>? Examples { get; set; }
 
         /// <inheritdoc />
-        public IList<JsonNode>? Enum { get; set; } = new List<JsonNode>();
+        public IList<JsonNode>? Enum { get; set; }
 
         /// <inheritdoc />
-        public bool UnevaluatedProperties { get; set;}
+        public bool UnevaluatedProperties { get; set; }
 
         /// <inheritdoc />
         public OpenApiExternalDocs? ExternalDocs { get; set; }
@@ -242,16 +242,16 @@
         public OpenApiXml? Xml { get; set; }
 
         /// <inheritdoc />
-        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; }
 
         /// <inheritdoc />
-        public IDictionary<string, JsonNode>? UnrecognizedKeywords { get; set; } = new Dictionary<string, JsonNode>();
+        public IDictionary<string, JsonNode>? UnrecognizedKeywords { get; set; }
 
         /// <inheritdoc />
         public IDictionary<string, object>? Annotations { get; set; }
 
         /// <inheritdoc />
-        public IDictionary<string, ISet<string>>? DependentRequired { get; set; } = new Dictionary<string, ISet<string>>();
+        public IDictionary<string, ISet<string>>? DependentRequired { get; set; }
 
         /// <summary>
         /// Parameterless constructor
@@ -262,7 +262,7 @@
         /// Initializes a copy of <see cref="IOpenApiSchema"/> object
         /// </summary>
         /// <param name="schema">The schema object to copy from.</param>
         internal OpenApiSchema(IOpenApiSchema schema)
         {
             Utils.CheckArgumentNull(schema);
             Title = schema.Title ?? Title;
@@ -309,7 +309,7 @@
             MinProperties = schema.MinProperties ?? MinProperties;
             AdditionalPropertiesAllowed = schema.AdditionalPropertiesAllowed;
             AdditionalProperties = schema.AdditionalProperties?.CreateShallowCopy();
-            Discriminator = schema.Discriminator != null ? new(schema.Discriminator) : null; 
+            Discriminator = schema.Discriminator != null ? new(schema.Discriminator) : null;
             Example = schema.Example != null ? JsonNodeCloneHelper.Clone(schema.Example) : null;
             Examples = schema.Examples != null ? new List<JsonNode>(schema.Examples) : null;
             Enum = schema.Enum != null ? new List<JsonNode>(schema.Enum) : null;
@@ -334,7 +334,7 @@
             SerializeInternal(writer, OpenApiSpecVersion.OpenApi3_0, (writer, element) => element.SerializeAsV3(writer));
         }
 
         private static void SerializeBounds(IOpenApiWriter writer, OpenApiSpecVersion version, string propertyName, string exclusivePropertyName, string isExclusivePropertyName, decimal? value, decimal? exclusiveValue, bool? isExclusiveValue)
         {
             if (version >= OpenApiSpecVersion.OpenApi3_1)
             {
@@ -421,9 +421,9 @@
             writer.WriteProperty(OpenApiConstants.MinProperties, MinProperties);
 
             // required
-            writer.WriteOptionalCollection(OpenApiConstants.Required, Required, (w, s) => 
+            writer.WriteOptionalCollection(OpenApiConstants.Required, Required, (w, s) =>
             {
                 if (!string.IsNullOrEmpty(s) && s is not null)
                 {
                     w.WriteValue(s);
                 }
@@ -502,7 +502,7 @@
             // Unrecognized keywords
             if (UnrecognizedKeywords is not null && UnrecognizedKeywords.Any())
             {
-                writer.WriteOptionalMap(OpenApiConstants.UnrecognizedKeywords, UnrecognizedKeywords, (w,s) => w.WriteAny(s));
+                writer.WriteOptionalMap(OpenApiConstants.UnrecognizedKeywords, UnrecognizedKeywords, (w, s) => w.WriteAny(s));
             }
 
             writer.WriteEndObject();
@@ -665,9 +665,9 @@
             writer.WriteProperty(OpenApiConstants.MinProperties, MinProperties);
 
             // required
-            writer.WriteOptionalCollection(OpenApiConstants.Required, Required, (w, s) => 
+            writer.WriteOptionalCollection(OpenApiConstants.Required, Required, (w, s) =>
             {
                 if (!string.IsNullOrEmpty(s) && s is not null)
                 {
                     w.WriteValue(s);
                 }
@@ -744,12 +744,12 @@
             writer.WriteEndObject();
         }
 
         private void SerializeTypeProperty(JsonSchemaType? type, IOpenApiWriter writer, OpenApiSpecVersion version)
         {
             // check whether nullable is true for upcasting purposes
             var isNullable = (Type.HasValue && Type.Value.HasFlag(JsonSchemaType.Null)) ||
                                 Extensions is not null &&
-                                Extensions.TryGetValue(OpenApiConstants.NullableExtension, out var nullExtRawValue) && 
+                                Extensions.TryGetValue(OpenApiConstants.NullableExtension, out var nullExtRawValue) &&
                                 nullExtRawValue is OpenApiAny { Node: JsonNode jsonNode } &&
                                 jsonNode.GetValueKind() is JsonValueKind.True;
             if (type is null)
@@ -761,7 +761,7 @@
             }
             else if (!HasMultipleTypes(type.Value))
             {
-                
+
                 switch (version)
                 {
                     case OpenApiSpecVersion.OpenApi3_1 when isNullable:
@@ -794,13 +794,13 @@
                                 select flag).ToList();
                     writer.WriteOptionalCollection(OpenApiConstants.Type, list, (w, s) =>
                     {
-                        foreach(var item in s.ToIdentifiers())
+                        foreach (var item in s.ToIdentifiers())
                         {
                             w.WriteValue(item);
                         }
                     });
                 }
-            } 
+            }
         }
 
         private static bool IsPowerOfTwo(int x)
@@ -824,9 +824,9 @@
                         select flag.ToFirstIdentifier()).ToList();
             if (list.Count > 1)
             {
-                writer.WriteOptionalCollection(OpenApiConstants.Type, list, (w, s) => 
+                writer.WriteOptionalCollection(OpenApiConstants.Type, list, (w, s) =>
                 {
                     if (!string.IsNullOrEmpty(s) && s is not null)
                     {
                         w.WriteValue(s);
                     }
@@ -844,7 +844,7 @@
         private static readonly Array jsonSchemaTypeValues = System.Enum.GetValues(typeof(JsonSchemaType));
 #endif
 
         private void DowncastTypeArrayToV2OrV3(JsonSchemaType schemaType, IOpenApiWriter writer, OpenApiSpecVersion version)
         {
             /* If the array has one non-null value, emit Type as string
             * If the array has one null value, emit x-nullable as true

src/Microsoft.OpenApi/Reader/V31/OpenApiSchemaDeserializer.cs

@@ -10,12 +10,13 @@
 using System.Collections.Generic;
 using System.Globalization;
 using System.Linq;
+using System.Text.Json.Nodes;
 
 namespace Microsoft.OpenApi.Reader.V31
 {
     internal static partial class OpenApiV31Deserializer
     {
         private static readonly FixedFieldMap<OpenApiSchema> _openApiSchemaFixedFields = new()
         {
             {
                 "title",
@@ -202,7 +203,7 @@
                     {
                         var list = n.CreateSimpleList((n2, p) => n2.GetScalarValue(), doc);
                         JsonSchemaType combinedType = 0;
                         foreach(var type in list)
                         {
                             if (type is not null)
                             {
@@ -382,8 +383,9 @@
                 {
                     propertyNode.ParseField(schema, _openApiSchemaFixedFields, _openApiSchemaPatternFields, hostDocument);
                 }
-                else if (schema.UnrecognizedKeywords is not null && propertyNode.JsonNode is not null)
+                else if (propertyNode.JsonNode is not null)
                 {
+                    schema.UnrecognizedKeywords ??= new Dictionary<string, JsonNode>(StringComparer.Ordinal);
                     schema.UnrecognizedKeywords[propertyNode.Name] = propertyNode.JsonNode;
                 }
             }

src/Microsoft.OpenApi/Validations/Rules/OpenApiSchemaRules.cs

@@ -50,7 +50,7 @@ public static bool ValidateChildSchemaAgainstDiscriminator(IOpenApiSchema schema
         {
             if (discriminatorName is not null)
             {
-                if (!schema.Required?.Contains(discriminatorName) ?? false)
+                if (schema.Required is null || !schema.Required.Contains(discriminatorName))
                 {
                     // recursively check nested schema.OneOf, schema.AnyOf or schema.AllOf and their required fields for the discriminator
                     if (schema.OneOf?.Count != 0)

test/Microsoft.OpenApi.Readers.Tests/V2Tests/OpenApiHeaderTests.cs

@@ -68,11 +68,11 @@ public void ParseHeaderWithEnumShouldSucceed()
                         Type = JsonSchemaType.Number,
                         Format = "float",
                         Enum =
-                        {
+                        [
                             new OpenApiAny(7).Node,
                             new OpenApiAny(8).Node,
                             new OpenApiAny(9).Node
-                        }
+                        ]
                     }
                 }, options => options.IgnoringCyclicReferences()
                                      .Excluding((IMemberInfo memberInfo) =>

test/Microsoft.OpenApi.Readers.Tests/V2Tests/OpenApiOperationTests.cs

@@ -543,7 +543,7 @@ public async Task SerializesBodyReferencesWorks()
             openApiDocument.AddComponent("UserSchema", new OpenApiSchema
             {
                 Type = JsonSchemaType.Object,
-                Properties =
+                Properties = new Dictionary<string, IOpenApiSchema>
                 {
                     ["name"] = new OpenApiSchema
                     {

test/Microsoft.OpenApi.Readers.Tests/V2Tests/OpenApiParameterTests.cs

@@ -265,11 +265,11 @@ public void ParseParameterWithEnumShouldSucceed()
                     Type = JsonSchemaType.Number,
                     Format = "float",
                     Enum =
-                        {
-                            new OpenApiAny(7).Node,
-                            new OpenApiAny(8).Node,
-                            new OpenApiAny(9).Node
-                        }
+                    [
+                        new OpenApiAny(7).Node,
+                        new OpenApiAny(8).Node,
+                        new OpenApiAny(9).Node
+                    ]
                 }
             };
 

test/Microsoft.OpenApi.Readers.Tests/V2Tests/OpenApiPathItemTests.cs

@@ -7,8 +7,10 @@
 using System.Linq;
 using System.Net.Http;
 using Microsoft.OpenApi.Models;
+using Microsoft.OpenApi.Models.Interfaces;
 using Microsoft.OpenApi.Reader.ParseNodes;
 using Microsoft.OpenApi.Reader.V2;
+using Microsoft.OpenApi.Writers;
 using Xunit;
 
 namespace Microsoft.OpenApi.Readers.Tests.V2Tests
@@ -69,7 +71,7 @@ public class OpenApiPathItemTests
                                 Schema = new OpenApiSchema()
                                 {
                                     Type = JsonSchemaType.Object,
-                                    Properties =
+                                    Properties = new Dictionary<string, IOpenApiSchema>
                                     {
                                         ["name"] = new OpenApiSchema()
                                         {
@@ -93,7 +95,7 @@ public class OpenApiPathItemTests
                                Schema = new OpenApiSchema()
                                 {
                                     Type = JsonSchemaType.Object,
-                                    Properties =
+                                    Properties = new Dictionary<string, IOpenApiSchema>
                                     {
                                         ["name"] = new OpenApiSchema()
                                         {
@@ -175,7 +177,7 @@ public class OpenApiPathItemTests
                                 Schema = new OpenApiSchema()
                                 {
                                     Type = JsonSchemaType.Object,
-                                    Properties =
+                                    Properties = new Dictionary<string, IOpenApiSchema>
                                     {
                                         ["name"] = new OpenApiSchema()
                                         {
@@ -204,7 +206,7 @@ public class OpenApiPathItemTests
                                 Schema = new OpenApiSchema()
                                 {
                                     Type = JsonSchemaType.Object,
-                                    Properties =
+                                    Properties = new Dictionary<string, IOpenApiSchema>
                                     {
                                         ["name"] = new OpenApiSchema()
                                         {

test/Microsoft.OpenApi.Readers.Tests/V3Tests/OpenApiDocumentTests.cs

@@ -1247,7 +1247,7 @@ public async Task SerializesDoubleHopeReferences()
             {
                 Type = JsonSchemaType.Object,
                 Description = "A pet",
-                Properties =
+                Properties = new Dictionary<string, IOpenApiSchema>
                 {
                     ["id"] = new OpenApiSchema
                     {

test/Microsoft.OpenApi.Readers.Tests/V3Tests/OpenApiParameterTests.cs

@@ -117,12 +117,12 @@ public async Task ParseQueryParameterWithObjectTypeAndContentShouldSucceed()
                            Schema = new OpenApiSchema()
                            {
                                 Type = JsonSchemaType.Object,
-                                Required =
+                                Required = new HashSet<string>
                                 {
                                     "lat",
                                     "long"
                                 },
-                                Properties =
+                                Properties = new Dictionary<string, IOpenApiSchema>
                                 {
                                     ["lat"] = new OpenApiSchema()
                                     {