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/OpenApiCallback.cs

@@ -13,17 +13,16 @@ namespace Microsoft.OpenApi.Models
     /// <summary>
     /// Callback Object: A map of possible out-of band callbacks related to the parent operation.
     /// </summary>
-    public class OpenApiCallback : IOpenApiReferenceable, IOpenApiExtensible, IOpenApiCallback
+    public class OpenApiCallback : IOpenApiExtensible, IOpenApiCallback
     {
         /// <inheritdoc/>
         public Dictionary<RuntimeExpression, IOpenApiPathItem>? PathItems { get; set; }
-            = [];
 
 
         /// <summary>
         /// This object MAY be extended with Specification Extensions.
         /// </summary>
-        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; }
 
         /// <summary>
         /// Parameter-less constructor

src/Microsoft.OpenApi/Models/OpenApiComponents.cs

@@ -19,60 +19,57 @@ public class OpenApiComponents : IOpenApiSerializable, IOpenApiExtensible
         /// <summary>
         /// An object to hold reusable <see cref="IOpenApiSchema"/> Objects.
         /// </summary>
-        public IDictionary<string, IOpenApiSchema>? Schemas { get; set; } = new Dictionary<string, IOpenApiSchema>();
+        public IDictionary<string, IOpenApiSchema>? Schemas { get; set; }
 
         /// <summary>
         /// An object to hold reusable <see cref="IOpenApiResponse"/> Objects.
         /// </summary>
-        public IDictionary<string, IOpenApiResponse>? Responses { get; set; } = new Dictionary<string, IOpenApiResponse>();
+        public IDictionary<string, IOpenApiResponse>? Responses { get; set; }
 
         /// <summary>
         /// An object to hold reusable <see cref="IOpenApiParameter"/> Objects.
         /// </summary>
-        public IDictionary<string, IOpenApiParameter>? Parameters { get; set; } =
-            new Dictionary<string, IOpenApiParameter>();
+        public IDictionary<string, IOpenApiParameter>? Parameters { get; set; }
 
         /// <summary>
         /// An object to hold reusable <see cref="OpenApiExample"/> Objects.
         /// </summary>
-        public IDictionary<string, IOpenApiExample>? Examples { get; set; } = new Dictionary<string, IOpenApiExample>();
+        public IDictionary<string, IOpenApiExample>? Examples { get; set; }
 
         /// <summary>
         /// An object to hold reusable <see cref="IOpenApiRequestBody"/> Objects.
         /// </summary>
-        public IDictionary<string, IOpenApiRequestBody>? RequestBodies { get; set; } =
-            new Dictionary<string, IOpenApiRequestBody>();
+        public IDictionary<string, IOpenApiRequestBody>? RequestBodies { get; set; }
 
         /// <summary>
         /// An object to hold reusable <see cref="IOpenApiHeader"/> Objects.
         /// </summary>
-        public IDictionary<string, IOpenApiHeader>? Headers { get; set; } = new Dictionary<string, IOpenApiHeader>();
+        public IDictionary<string, IOpenApiHeader>? Headers { get; set; }
 
         /// <summary>
         /// An object to hold reusable <see cref="IOpenApiSecurityScheme"/> Objects.
         /// </summary>
-        public IDictionary<string, IOpenApiSecurityScheme>? SecuritySchemes { get; set; } =
-            new Dictionary<string, IOpenApiSecurityScheme>();
+        public IDictionary<string, IOpenApiSecurityScheme>? SecuritySchemes { get; set; }
 
         /// <summary>
         /// An object to hold reusable <see cref="IOpenApiLink"/> Objects.
         /// </summary>
-        public IDictionary<string, IOpenApiLink>? Links { get; set; } = new Dictionary<string, IOpenApiLink>();
+        public IDictionary<string, IOpenApiLink>? Links { get; set; }
 
         /// <summary>
         /// An object to hold reusable <see cref="OpenApiCallback"/> Objects.
         /// </summary>
-        public IDictionary<string, IOpenApiCallback>? Callbacks { get; set; } = new Dictionary<string, IOpenApiCallback>();
+        public IDictionary<string, IOpenApiCallback>? Callbacks { get; set; }
 
         /// <summary>
         /// An object to hold reusable <see cref="IOpenApiPathItem"/> Object.
         /// </summary>
-        public IDictionary<string, IOpenApiPathItem>? PathItems { get; set; } = new Dictionary<string, IOpenApiPathItem>();
+        public IDictionary<string, IOpenApiPathItem>? PathItems { get; set; }
 
         /// <summary>
         /// This object MAY be extended with Specification Extensions.
         /// </summary>
-        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; }
 
         /// <summary>
         /// Parameter-less constructor
@@ -318,7 +315,7 @@ private void RenderComponents(IOpenApiWriter writer, Action<IOpenApiWriter, IOpe
         {
             var loops = writer.GetSettings().LoopDetector.Loops;
             writer.WriteStartObject();
-            if (loops.TryGetValue(typeof(OpenApiSchema), out var schemas))
+            if (loops.TryGetValue(typeof(OpenApiSchema), out _))
             {
                 writer.WriteOptionalMap(OpenApiConstants.Schemas, Schemas, callback);
             }

src/Microsoft.OpenApi/Models/OpenApiContact.cs

@@ -32,7 +32,7 @@ public class OpenApiContact : IOpenApiSerializable, IOpenApiExtensible
         /// <summary>
         /// This object MAY be extended with Specification Extensions.
         /// </summary>
-        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; }
 
         /// <summary>
         /// Parameter-less constructor

src/Microsoft.OpenApi/Models/OpenApiDiscriminator.cs

@@ -21,12 +21,12 @@ public class OpenApiDiscriminator : IOpenApiSerializable, IOpenApiExtensible
         /// <summary>
         /// An object to hold mappings between payload values and schema names or references.
         /// </summary>
-        public IDictionary<string, OpenApiSchemaReference>? Mapping { get; set; } = new Dictionary<string, OpenApiSchemaReference>();
+        public IDictionary<string, OpenApiSchemaReference>? Mapping { get; set; }
 
         /// <summary>
         /// This object MAY be extended with Specification Extensions.
         /// </summary>
-        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; }
 
         /// <summary>
         /// Parameter-less constructor

src/Microsoft.OpenApi/Models/OpenApiDocument.cs

@@ -49,7 +49,7 @@ public void RegisterComponents()
         /// <summary>
         /// An array of Server Objects, which provide connectivity information to a target server.
         /// </summary>
-        public IList<OpenApiServer>? Servers { get; set; } = new List<OpenApiServer>();
+        public IList<OpenApiServer>? Servers { get; set; } = [];
 
         /// <summary>
         /// REQUIRED. The available paths and operations for the API.
@@ -61,7 +61,7 @@ public void RegisterComponents()
         /// A map of requests initiated other than by an API call, for example by an out of band registration. 
         /// The key name is a unique string to refer to each webhook, while the (optionally referenced) Path Item Object describes a request that may be initiated by the API provider and the expected responses
         /// </summary>
-        public IDictionary<string, IOpenApiPathItem>? Webhooks { get; set; } = new Dictionary<string, IOpenApiPathItem>();
+        public IDictionary<string, IOpenApiPathItem>? Webhooks { get; set; }
 
         /// <summary>
         /// An element to hold various schemas for the specification.
@@ -71,8 +71,7 @@ public void RegisterComponents()
         /// <summary>
         /// A declaration of which security mechanisms can be used across the API.
         /// </summary>
-        public IList<OpenApiSecurityRequirement>? Security { get; set; } =
-            new List<OpenApiSecurityRequirement>();
+        public IList<OpenApiSecurityRequirement>? Security { get; set; }
 
         private HashSet<OpenApiTag>? _tags;
         /// <summary>
@@ -104,7 +103,7 @@ public ISet<OpenApiTag>? Tags
         /// <summary>
         /// This object MAY be extended with Specification Extensions.
         /// </summary>
-        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; }
 
         /// <inheritdoc />
         public IDictionary<string, object>? Metadata { get; set; }

src/Microsoft.OpenApi/Models/OpenApiEncoding.cs

@@ -25,7 +25,7 @@ public class OpenApiEncoding : IOpenApiSerializable, IOpenApiExtensible
         /// <summary>
         /// A map allowing additional information to be provided as headers.
         /// </summary>
-        public IDictionary<string, IOpenApiHeader>? Headers { get; set; } = new Dictionary<string, IOpenApiHeader>();
+        public IDictionary<string, IOpenApiHeader>? Headers { get; set; }
 
         /// <summary>
         /// Describes how a specific property value will be serialized depending on its type.
@@ -52,7 +52,7 @@ public class OpenApiEncoding : IOpenApiSerializable, IOpenApiExtensible
         /// <summary>
         /// This object MAY be extended with Specification Extensions.
         /// </summary>
-        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; }
 
         /// <summary>
         /// Parameter-less constructor

src/Microsoft.OpenApi/Models/OpenApiExample.cs

@@ -13,7 +13,7 @@ namespace Microsoft.OpenApi.Models
     /// <summary>
     /// Example Object.
     /// </summary>
-    public class OpenApiExample : IOpenApiReferenceable, IOpenApiExtensible, IOpenApiExample
+    public class OpenApiExample : IOpenApiExtensible, IOpenApiExample
     {
         /// <inheritdoc/>
         public string? Summary { get; set; }
@@ -28,7 +28,7 @@ public class OpenApiExample : IOpenApiReferenceable, IOpenApiExtensible, IOpenAp
         public JsonNode? Value { get; set; }
 
         /// <inheritdoc/>
-        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; }
 
         /// <summary>
         /// Parameter-less constructor

src/Microsoft.OpenApi/Models/OpenApiExternalDocs.cs

@@ -26,7 +26,7 @@ public class OpenApiExternalDocs : IOpenApiSerializable, IOpenApiExtensible
         /// <summary>
         /// This object MAY be extended with Specification Extensions.
         /// </summary>
-        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; }
 
         /// <summary>
         /// Parameter-less constructor

src/Microsoft.OpenApi/Models/OpenApiHeader.cs

@@ -47,13 +47,13 @@ public class OpenApiHeader : IOpenApiHeader, IOpenApiExtensible
         public JsonNode? Example { get; set; }
 
         /// <inheritdoc/>
-        public IDictionary<string, IOpenApiExample>? Examples { get; set; } = new Dictionary<string, IOpenApiExample>();
+        public IDictionary<string, IOpenApiExample>? Examples { get; set; }
 
         /// <inheritdoc/>
-        public IDictionary<string, OpenApiMediaType>? Content { get; set; } = new Dictionary<string, OpenApiMediaType>();
+        public IDictionary<string, OpenApiMediaType>? Content { get; set; }
 
         /// <inheritdoc/>
-        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; }
 
         /// <summary>
         /// Parameter-less constructor

src/Microsoft.OpenApi/Models/OpenApiInfo.cs

@@ -51,7 +51,7 @@ public class OpenApiInfo : IOpenApiSerializable, IOpenApiExtensible
         /// <summary>
         /// This object MAY be extended with Specification Extensions.
         /// </summary>
-        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; }
 
         /// <summary>
         /// Parameter-less constructor

src/Microsoft.OpenApi/Models/OpenApiLicense.cs

@@ -31,7 +31,7 @@ public class OpenApiLicense : IOpenApiSerializable, IOpenApiExtensible
         /// <summary>
         /// This object MAY be extended with Specification Extensions.
         /// </summary>
-        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; }
 
         /// <summary>
         /// Parameterless constructor

src/Microsoft.OpenApi/Models/OpenApiLink.cs

@@ -12,7 +12,7 @@ namespace Microsoft.OpenApi.Models
     /// <summary>
     /// Link Object.
     /// </summary>
-    public class OpenApiLink : IOpenApiReferenceable, IOpenApiExtensible, IOpenApiLink
+    public class OpenApiLink : IOpenApiExtensible, IOpenApiLink
     {
         /// <inheritdoc/>
         public string? OperationRef { get; set; }
@@ -21,7 +21,7 @@ public class OpenApiLink : IOpenApiReferenceable, IOpenApiExtensible, IOpenApiLi
         public string? OperationId { get; set; }
 
         /// <inheritdoc/>
-        public IDictionary<string, RuntimeExpressionAnyWrapper>? Parameters { get; set; } = new Dictionary<string, RuntimeExpressionAnyWrapper>();
+        public IDictionary<string, RuntimeExpressionAnyWrapper>? Parameters { get; set; }
 
         /// <inheritdoc/>
         public RuntimeExpressionAnyWrapper? RequestBody { get; set; }
@@ -33,7 +33,7 @@ public class OpenApiLink : IOpenApiReferenceable, IOpenApiExtensible, IOpenApiLi
         public OpenApiServer? Server { get; set; }
 
         /// <inheritdoc/>
-        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; }
 
         /// <summary>
         /// Parameterless constructor

src/Microsoft.OpenApi/Models/OpenApiMediaType.cs

@@ -32,20 +32,20 @@ public class OpenApiMediaType : IOpenApiSerializable, IOpenApiExtensible
         /// Examples of the media type.
         /// Each example object SHOULD match the media type and specified schema if present.
         /// </summary>
-        public IDictionary<string, IOpenApiExample>? Examples { get; set; } = new Dictionary<string, IOpenApiExample>();
+        public IDictionary<string, IOpenApiExample>? Examples { get; set; }
 
         /// <summary>
         /// A map between a property name and its encoding information.
         /// The key, being the property name, MUST exist in the schema as a property.
         /// The encoding object SHALL only apply to requestBody objects
         /// when the media type is multipart or application/x-www-form-urlencoded.
         /// </summary>
-        public IDictionary<string, OpenApiEncoding>? Encoding { get; set; } = new Dictionary<string, OpenApiEncoding>();
+        public IDictionary<string, OpenApiEncoding>? Encoding { get; set; }
 
         /// <summary>
         /// Serialize <see cref="OpenApiExternalDocs"/> to Open Api v3.0.
         /// </summary>
-        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; }
 
         /// <summary>
         /// Parameterless constructor

src/Microsoft.OpenApi/Models/OpenApiOAuthFlow.cs

@@ -33,12 +33,12 @@ public class OpenApiOAuthFlow : IOpenApiSerializable, IOpenApiExtensible
         /// <summary>
         /// REQUIRED. A map between the scope name and a short description for it.
         /// </summary>
-        public IDictionary<string, string>? Scopes { get; set; } = new Dictionary<string, string>();
+        public IDictionary<string, string>? Scopes { get; set; }
 
         /// <summary>
         /// Specification Extensions.
         /// </summary>
-        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; }
 
         /// <summary>
         /// Parameterless constructor

src/Microsoft.OpenApi/Models/OpenApiOAuthFlows.cs

@@ -36,7 +36,7 @@ public class OpenApiOAuthFlows : IOpenApiSerializable, IOpenApiExtensible
         /// <summary>
         /// Specification Extensions.
         /// </summary>
-        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; }
 
         /// <summary>
         /// Parameterless constructor

src/Microsoft.OpenApi/Models/OpenApiOperation.cs

@@ -73,7 +73,7 @@ public ISet<OpenApiTagReference>? Tags
         /// The list MUST NOT include duplicated parameters. A unique parameter is defined by a combination of a name and location.
         /// The list can use the Reference Object to link to parameters that are defined at the OpenAPI Object's components/parameters.
         /// </summary>
-        public IList<IOpenApiParameter>? Parameters { get; set; } = [];
+        public IList<IOpenApiParameter>? Parameters { get; set; }
 
         /// <summary>
         /// The request body applicable for this operation.
@@ -96,7 +96,7 @@ public ISet<OpenApiTagReference>? Tags
         /// The key value used to identify the callback object is an expression, evaluated at runtime,
         /// that identifies a URL to use for the callback operation.
         /// </summary>
-        public IDictionary<string, IOpenApiCallback>? Callbacks { get; set; } = new Dictionary<string, IOpenApiCallback>();
+        public IDictionary<string, IOpenApiCallback>? Callbacks { get; set; }
 
         /// <summary>
         /// Declares this operation to be deprecated. Consumers SHOULD refrain from usage of the declared operation.
@@ -110,19 +110,19 @@ public ISet<OpenApiTagReference>? Tags
         /// This definition overrides any declared top-level security.
         /// To remove a top-level security declaration, an empty array can be used.
         /// </summary>
-        public IList<OpenApiSecurityRequirement>? Security { get; set; } = new List<OpenApiSecurityRequirement>();
+        public IList<OpenApiSecurityRequirement>? Security { get; set; }
 
         /// <summary>
         /// An alternative server array to service this operation.
         /// If an alternative server object is specified at the Path Item Object or Root level,
         /// it will be overridden by this value.
         /// </summary>
-        public IList<OpenApiServer>? Servers { get; set; } = new List<OpenApiServer>();
+        public IList<OpenApiServer>? Servers { get; set; }
 
         /// <summary>
         /// This object MAY be extended with Specification Extensions.
         /// </summary>
-        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; }
 
         /// <inheritdoc />
         public IDictionary<string, object>? Metadata { get; set; }